# Patent

## Title
Processes using amino acid dehydrogenases and ketoreductase-based cofactor regenerating system

## Abstract

The present disclosure relates to the use of an amino acid dehydrogenase in combination with a cofactor regenerating system comprising a ketoreductase. In particular embodiments, the process can be used to prepare L-tert-leucine using a leucine dehydrogenase.

## Background
The present application is a Divisional of U.S. patent application Ser. No. 13/577,772, filed Oct. 16, 2012, now U.S. Pat. No. 9,080,192 which is a national stage application filed under 35 USC §371 and claims priority of the international application PCT/US2011/024102, filed Feb. 8, 2011, and U.S. provisional patent application 61/303,179, filed Feb. 10, 2010, each of which is hereby incorporated by reference herein.


1. TECHNICAL FIELD
The present disclosure relates to biocatalysts and processes for preparing chiral amino acids using the biocatalysts.
2. REFERENCE TO SEQUENCE LISTING, TABLE OR COMPUTER PROGRAM
The official copy of the Sequence Listing is submitted concurrently with the specification as an ASCII formatted text file via EFS-Web, with a file name of “CX2-038USP1_ST25.txt”, a creation date of Feb. 9, 2010, and a size of 48 kilobytes. The sequence listing filed via EFS-Web is part of the specification and is hereby incorporated in its entirety by reference herein.
3. BACKGROUND
Amino acid dehydrogenases comprise a group of coenzyme-dependent enzymes that catalyze the reversible oxidative deamination of an amino acid to its keto acid and ammonia with the concomitant reduction of either cofactor NAD+, NADP+ or FAD. The enzyme with dehydrogenase properties is distributed in a number of diverse prokaryotic and eukaryotic organisms. Amino acid dehydrogenases have been studied widely because of their potential applications in biosensors or diagnostic kits, synthesis of L- and D-amino acids for use in production pharmaceutical peptides, herbicides and insecticides (Brunhuber et al., 1994, Crit. Rev. Biochem. Mol. Biol. 29(6):415-467; Hummel et al., 1989, Eur. J. Biochem. 184:1-13; Krix et al., 1997, J. Biotech. 53:29-39; Ohshima et al., 1990, Adv. Biochem. Eng. Biotechnol. 42:181-209; U.S. Pat. No. 7,550,277). For example, the anti-hypertensives ramipril, enalapril, benazapril, and prinivil are prepared using L-homophenylalanine, and certain second generation pril analogs are synthesized from p-substituted-L homophenylalanine. Certain β-lactam antibiotics use substituted D-phenylglycine side chains, and while other antibiotics are based on aminoadipic acid and other unnatural amino acids. The unnatural amino acids L-tert-leucine, L-nor-valine, L-nor-leucine, L-2-amino-5-[1,3]dioxolan-2yl-pentanoic acid have been used as a precursor in the synthesis of a number of different developmental drugs. The enzyme leucine dehydrogenase and mutants thereof have been shown to be capable of catalyzing the reductive amination of the corresponding 2-ketoacids of alkyl and branched-chain amino acids, and L-tert-leucine has been produced commercially with such an enzyme.
Given the industrial utility of L- and D-amino acid dehydrogenases, it is desirable to develop processes and systems that can enhance the biocatalytic reactions carried out by amino acid dehydrogenases.
4. SUMMARY
The present disclosure provides coupled systems for the efficient biosynthesis of chiral amino acid compounds using an L- or D-amino acid dehydrogenase (“AADH”) coupled with a cofactor regenerating system comprising a ketoreductase (“KRED”).
In certain embodiments, the present disclosure provides a process for converting a 2-oxo acid compound of formula I which is a substrate for an amino acid dehydrogenase to a chiral amino acid of formula IIa:




comprising contacting the compound of formula I with a reaction medium comprising an amino acid dehydrogenase, an ammonium ion donor, NAD+/NADH or NADP+/NADPH, and a cofactor regenerating system comprising a ketoreductase and a lower secondary alcohol, under conditions where the compound of formula I is converted to the chiral amino acid of formula IIa and the lower secondary alcohol is converted to a ketone. In certain embodiments of the process, R is a substituted or unsubstituted (C1-C10)alkyl, —(C2-C6)alkenyl, —(C2-C6)alkynyl, —(C3-C8)cycloalkyl, heterocycloalkyl, aryl, or heteroaryl. In certain embodiments, R is a substituted or unsubstituted group selected from those groups shown in either of Table 1 or Table 2 disclosed herein.

In certain embodiments of the process for converting a compound of formula I which is a substrate for an amino acid dehydrogenase to a chiral amino acid of formula IIa, the amino acid dehydrogenase is an L-amino acid dehydrogenase and the chiral amino acid of formula IIa is IIb,




wherein the product of formula IIb is formed in enantiomeric excess. In certain embodiments, the L-amino acid dehydrogenase is from Bacillus, Clostridium, Corynebacterium, Geobacillus, Natronobacterium, Synechocystis, Thermoactinomyces, Thermos, Thermomicrobium, or Carderia. In some embodiments, the L-amino acid dehydrogenase is selected from L-alanine dehydrogenase, L-aspartate dehydrogenase, L-erythro-3,5-diaminohexanoate dehydrogenase, L-leucine dehydrogenase, L-glutamate dehydrogenase, lysine dehydrogenase, L-phenylalanine dehydrogenase, L-serine dehydrogenase, L-valine dehydrogenase, L-2,4-diaminopentanoate dehydrogenase, L-glutamate synthase, L-diaminopimelate dehydrogenase, L-N-methylalanine dehydrogenase, L-lysine 6-dehydrogenase, and L-tryptophan dehydrogenase.

In certain embodiments of the process for converting a compound of formula I which is a substrate for an amino acid dehydrogenase to a chiral amino acid of formula IIa, the amino acid dehydrogenase is a D-amino acid dehydrogenase and the chiral amino acid of formula IIa is IIc,




wherein the compound of formula IIc is formed in enantiomeric excess. In certain embodiments of the process, the D-amino acid dehydrogenase is from Halobacterium, Methanosarcina, Pseudomonas, Pyrobaculum, Salmonella, Corynebacterium, and Escherichia. In certain embodiments, the D-amino acid dehydrogenase is selected from a D-alanine dehydrogenase, D-threonine dehydrogenase, and D-proline dehydrogenase.

In certain embodiments of the process for converting a compound of formula I which is a substrate for an amino acid dehydrogenase to a chiral amino acid of formula IIa, the amino acid dehydrogenase comprises a L-leucine dehydrogenase, the compound of formula I is 3,3-dimethyl-2-oxobutanoic acid, the product of formula IIa is (S)-2-amino-3,3-dimethylbutanoic acid. In some embodiments of the process, the leucine dehydrogenase is a wild type leucine dehydrogenase or an engineered leucine dehydrogenase. In some embodiments, the leucine dehydrogenase is from Bacillus, Clostridium, Corynebacterium, Geobacillus, Natronobacterium, Thermoactinomyces, Thermos, Thermomicrobium, or Carderia. In some embodiments, the leucine dehydrogenase is from Bacillus acidokaludarius, Bacillus brevis, Bacillus caldolyticus, Bacillus cereus, Bacillus megaterium, Bacillus mesentericus, Bacillus mycoides, Bacillus natto, Bacillus pumilus, Bacillus sp., Bacillus sphaericus, Bacillus stearothermophilus, Bacillus subtilis, Clostridium thermoaceticum, Corynebacterium pseudodiphtheriticum, Geobacillus stearothermophilus, Natronobacterium magadii, or Thermoactinomyces intermedius. In some embodiments, the leucine dehydrogenase comprises the amino acid sequence of SEQ ID NO: 26.
In certain embodiments, the present disclosure provides a process for producing (S)-2-amino-3,3-dimethylbutanoic acid, comprising: contacting 3,3-dimethyl-2-oxobutanoic acid with a leucine dehydrogenase in a reaction medium comprising an ammonium ion donor, cofactor NAD+/NADH or NADP+/NADPH, and a cofactor recycling system comprising a ketoreductase and a lower secondary alcohol, under conditions where the 3,3-dimethyl-2-oxobutanoic acid is converted to (S)-2-amino-3,3-dimethylbutanoic acid, wherein the 3,3-dimethyl-2-oxobutanoic acid is at about 75 g/L to 125 g/L, the cofactor is at about 0.30 g/L to 0.70 g/L, and the leucine dehydrogenase and ketoreductase are each independently at about 0.5 to about 1.0 g/L. In certain embodiments of the process, the secondary alcohol is present in at least 1.5 fold stoichiometric excess of substrate. In some embodiments of the process, the secondary alcohol is isopropanol, wherein the isopropanol is at about 7% to 12% volume of the reaction medium by (weight/volume).
In certain embodiments, the present disclosure provides a process for converting a compound mixture of formula IId which comprises a substrate for an amino acid dehydrogenase to a composition of formula I and a chiral amino acid of formula IIa:




where the process comprises contacting the compound mixture of formula IId with an enantioselective amino acid dehydrogenase in a reaction medium comprising NAD+/NADH or NADP+/NADPH and a cofactor recycling system comprising a ketoreductase and a lower alkyl ketone, under conditions where the compound mixture of formula IId is converted to the composition of formula I and a chiral amino acid of formula IIa, and the lower alkyl ketone is converted to a lower secondary alcohol. In some embodiments of the process, the compound mixture of IId is a racemic mixture of formula IIe:





In some embodiments of the process, the amino acid dehydrogenase comprises an L-amino acid dehydrogenase and the chiral amino acid of formula IIa is IIe




wherein the process results in chiral amino acid of formula IIe in enantiomeric excess. In some embodiments of the process, the amino acid dehydrogenase comprises a D-amino acid dehydrogenase and the chiral amino acid of formula IIa is IIb





wherein the process results in a chiral amino acid of formula IIb in enantiomeric excess.

In certain embodiments, the present disclosure provides a process for preparing an N-protected amino acid compound, wherein the method comprises: (i) contacting a compound of formula I with a reaction medium comprising an amino acid dehydrogenase, an ammonium ion donor, NAD+/NADH or NADP+/NADPH, and a cofactor regenerating system comprising a ketoreductase and a lower secondary alcohol under suitable conditions where the compound of formula I is converted to the chiral amino acid compound of formula IIa and the lower secondary alcohol is converted to a ketone; and (ii) contacting the amino acid compound of formula IIa with a compound comprising an N-protecting group under conditions, where the N-protecting group reacts with the compound of formula IIa to form an N-protected amino acid compound.
In certain embodiments of the process for preparing an N-protected amino acid compound, the biocatalytic step comprises contacting 3,3-dimethyl-2-oxobutanoic acid with a leucine dehydrogenase in a reaction medium comprising an ammonium ion donor, cofactor NAD+/NADH or NADP+/NADPH, and a cofactor recycling system comprising a ketoreductase and a lower secondary alcohol, under conditions where the 3,3-dimethyl-2-oxobutanoic acid is converted to (S)-2-amino-3,3-dimethylbutanoic acid. In some embodiments, the 3,3-dimethyl-2-oxobutanoic acid is at about 75 g/L to 125 g/L, the cofactor is at about 0.30 g/L to 0.70 g/L, and the leucine dehydrogenase and ketoreductase are each independently at about 0.5 to about 1.0 g/L. In some embodiments, the secondary alcohol is present in at least 1.5 fold stoichiometric excess of substrate. In some embodiments, the secondary alcohol is isopropanol, and the isopropanol is at about 7% to 12% volume of the reaction medium by (weight/volume). In some embodiments, the N-protecting group is selected from Cbz, FMOC, BOC and MOC.
In certain embodiments of the various processes for preparing chiral amino acid compounds disclosed herein, the process is carried out in a cell free system. In some embodiments of the various processes, the amino acid dehydrogenase is present as a crude extract, and in some embodiments, the amino acid dehydrogenase is substantially purified.
In certain embodiments of the various processes for preparing chiral amino acid compounds disclosed herein, the ketoreductase is a wild type ketoreductase or an engineered ketoreductase. In some embodiments, the ketoreductase is from Lactobacillus, Candida, Novosphingobium, or Saccharomyces, and in some embodiments, the ketoreductase is from an organism selected from Lactobacillus kefir, Lactobacillus brevis, Lactobacillus minor, Candida magnoliae, Saccharomyces cerevisiae, and Novosphingobium aromaticivorans. In some embodiments, the ketoreductase is an engineered ketoreductase derived from the wild-type ketoreductase of Novosphingobium aromaticivorans, wherein the engineered ketoreductase comprising an amino acid sequence having at least 80%, 85%, 90%, 95%, 98%, 99%, or more identity to a sequence selected from SEQ ID NO: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24.
In certain embodiments of the various processes for preparing chiral amino acid compounds disclosed herein, the ketoreductase is characterized by increased thermostability, increased solvent stability, and/or increased enzymatic activity relative to a reference ketoreductase. In some embodiments, the ketoreductase used in the cofactor recycling system has an improved property over a reference ketoreductase of increased activity in the conversion of the lower secondary alcohol (e.g., isopropanol) of the recycling system to the corresponding lower alkyl ketone. In some embodiments, the ketoreductase having the increased activity in the conversion of the lower secondary alcohol is at least 2.0 fold, 2.5 fold, 5.0 fold, 7.5 fold, 10-fold, or more improved relative to a reference ketoreductase (e.g., a reference ketoreductase of SEQ ID NO: 2).
In certain embodiments of the various processes for preparing chiral amino acid compounds disclosed herein, the ketoreductase used in the cofactor recycling system has an improved property over a reference ketoreductase of decreased or no activity with the compound of formula I (e.g., trimethylpyruvic acid) which is a substrate for the amino acid dehydrogenase used in the process. In some embodiments of the process, the activity of the ketoreductase used in the cofactor recycling system with the compound of formula I is less than about 5%, 2%, 1%, 0.5%, 0.1%, 0.05%, 0.01%, or an even smaller %, of the activity of the amino acid dehydrogenase used in the process with the compound of formula I. In some embodiments of the process, the ketoreductase used in the cofactor recycling system has no activity with the compound of formula I.
In certain embodiments of the various processes for preparing chiral amino acid compounds disclosed herein, the ketoreductase used in the cofactor recycling system is an engineered ketoreductase is capable of recycling cofactor by converting isopropanol (IPA) to acetone in a reaction medium of 3 to 20% IPA at a pH of about 9.0 to 10.5 with an activity at least 1.5-fold greater than the reference ketoreductase of SEQ ID NO: 2.
In certain embodiments of the various processes for preparing chiral amino acid compounds disclosed herein, the process further comprises removing from the reaction medium the ketone formed from the lower secondary alcohol, and in certain embodiments the lower secondary alcohol is isopropanol and the ketone removed is acetone. In some embodiments, the secondary alcohol is present in at least 1.5 fold stoichiometric excess of substrate.
In certain embodiments of the various processes for preparing chiral amino acid compounds disclosed herein, the reaction medium is at a pH of about 8.5 to about 10.5, or a pH of about 8.5 to about 9.5, or a pH of about 9.0.
In certain embodiments of the various processes for preparing chiral amino acid compounds disclosed herein, the reaction medium is at a temperature of about 25° C. to about 45° C., or about 35° C. to about 40° C.


5. DETAILED DESCRIPTION
5.1 Definitions
As used herein, the following terms are intended to have the following meanings.
“Protein,” “polypeptide,” “oligopeptide,” and “peptide” are used interchangeably to denote a polymer of at least two amino acids covalently linked by an amide bond, regardless of length or post-translational modification (e.g., glycosylation, phosphorylation, lipidation, myristilation, ubiquitination, etc.). Included within this definition are D- and L-amino acids, and mixtures of D- and L-amino acids.
“Amino acid dehydrogenase” or “AADH” are used interchangeably herein to refer to a polypeptide capable of carrying out the conversion of an amino acid, in the presence of an electron acceptor, to a 2-oxo acid, NH3, and reduced acceptor. In some embodiments, amino acid dehydrogenases are also capable of carrying out the reverse reaction of converting the 2-oxo acid, in the presence of an ammonium ion donor and an electron donor, to an amino acid and oxidized electron donor. L-amino acid dehydrogenase refers to an amino acid dehydrogenase that is stereospecific or stereoselective for an L-amino acid. D-amino acid dehydrogenase refers to an amino acid dehydrogenase that is stereospecific or stereoselective for a D-amino acid. Generally, the electron acceptor/donor for the amino acid dehydrogenase is nicotinamide adenine dinucleotide in oxidized/reduced form (i.e., NAD+/NADH) or nicotinamide adenine dinucleotide phosphate in oxidized/reduced form (i.e., NADP+/NADPH) Amino acid dehydrogenase as used herein include naturally occurring (wild type) amino acid dehydrogenases as well as non-naturally occurring polypeptides generated by human manipulation (e.g., recombinant or engineered enzymes).
“Ketoreductase” and “KRED” are used interchangeably herein to refer to a polypeptide that is capable of reducing a ketone to an alcohol product and/or oxidizing an alcohol to a ketone product. The polypeptide typically utilizes a cofactor reduced nicotinamide adenine dinucleotide (NADH) or reduced nicotinamide adenine dinucleotide phosphate (NADPH) as the reducing agent; or may use the corresponding NAD or NADPH as oxidizing agent. Ketoreductases as used herein include naturally occurring (wild type) ketoreductases as well as non-naturally occurring polypeptides generated by human manipulation (e.g., recombinant or engineered enzymes). They may be used to effect one or more chemical transformations, including the regeneration of cofactors such as NAD(P)H or NAD(P)+.
“Naturally-occurring” or “wild-type” refers to the form found in nature. For example, a naturally occurring or wild-type polypeptide or polynucleotide sequence is a sequence present in an organism that can be isolated from a source in nature and which has not been intentionally modified by human manipulation.
“Recombinant” or “engineered” or “non-naturally occurring” when used with reference to, e.g., a cell, nucleic acid, or polypeptide, refers to a material, or a material corresponding to the natural or native form of the material, that has been modified in a manner that would not otherwise exist in nature, or is identical thereto but produced or derived from synthetic materials and/or by manipulation using recombinant techniques (e.g., genetic engineering). Non-limiting examples include, among others, recombinant cells expressing genes that are not found within the native (non-recombinant) form of the cell or express native genes that are otherwise expressed at a different level.
“Derived from” identifies the originating polypeptide, and/or the gene encoding such polypeptide, upon which the engineering was based.
“Substrate” refers to a substance or compound that is converted or meant to be converted into another compound by the action of an enzyme. The term includes aromatic and aliphatic compounds, and includes not only a single compound, but also combinations of compounds, such as solutions, mixtures and other materials which contain at least one substrate.
“Stereoselectivity” refers to the preferential formation in a chemical or enzymatic reaction of one stereoisomer over another. Stereoselectivity can be partial, where the formation of one stereoisomer is favored over the other, or it may be complete where only one stereoisomer is formed. When the stereoisomers are enantiomers, the stereoselectivity is referred to as enantioselectivity, the fraction (typically reported as a percentage) of one enantiomer in the sum of both. It is commonly reported in the art (typically as a percentage) as the enantiomeric excess calculated therefrom according to the formula [major enantiomer−minor enantiomer]/[major enantiomer+minor enantiomer]. Where the stereoisomers are diastereoisomers, the stereoselectivity is referred to as diastereoselectivity, the fraction (typically reported as a percentage) of one diastereomer in the sum with others.
“Stereospecific” refers to the preferential conversion in a chemical or enzymatic reaction of one stereoisomer over another. Stereospecificity can be partial, where the conversion of one stereoisomer is favored over the other, or it may be complete where only one stereoisomer is converted.
“Increased enzymatic activity” refers to an improved property of an enzyme, which can be represented by an increase in specific activity (e.g., product produced/time/weight protein) or an increase in percent conversion of the substrate to the product (e.g., percent conversion of starting amount of substrate to product in a specified time period using a specified amount of KRED) as compared to a reference enzyme. Exemplary methods to determine enzyme activity are provided in the Examples. Any property relating to enzyme activity may be affected, including the classical enzyme properties of Km, Vmax or kcat, changes of which can lead to increased enzymatic activity. Improvements in enzyme activity can be from about 1.5 times the enzymatic activity of the corresponding wild-type ketoreductase enzyme, to as much as 2 times. 5 times, 10 times, 20 times, 25 times, 50 times, 75 times, 100 times, or more enzymatic activity than the naturally occurring ketoreductase or another engineered ketoreductase from which the ketoreductase polypeptides were derived. In specific embodiments, the engineered enzyme exhibits improved enzymatic activity in the range of 1.5 to 50 times, 1.5 to 100 times greater than that of the parent enzyme. It is understood by the skilled artisan that the activity of any enzyme is diffusion limited such that the catalytic turnover rate cannot exceed the diffusion rate of the substrate, including any required cofactors. The theoretical maximum of the diffusion limit, or kcat/Km, is generally about 108 to 109 (M−1 s−1). Hence, any improvements in the enzyme activity of an enzyme will have an upper limit related to the diffusion rate of the substrates acted on by the enzyme. Comparisons of enzyme activities are made using a defined preparation of enzyme, a defined assay under a set condition, and one or more defined substrates, as further described in detail herein. Generally, when lysates are compared, the numbers of cells and the amount of protein assayed are determined as well as use of identical expression systems and identical host cells to minimize variations in amount of enzyme produced by the host cells and present in the lysates.
“Conversion” refers to the enzymatic transformation of the substrate to the corresponding product. “Percent conversion” refers to the percent of the substrate that is converted to the product within a period of time under specified conditions. Thus, the “enzymatic activity” or “activity” of an enzyme(s) can be expressed as “percent conversion” of the substrate to the product.
“Improved thermostability” and “improved thermal stability” are used interchangeably herein to refer to a property of increased resistance to inactivation when exposed to a set temperature or set of temperatures in defined conditions as compared to the resistance to inactivation of a reference enzyme. Activity of the enzyme pre- and post treatment are measured under the same defined assay condition. Thermostability can also be compared and expressed as the temperature at which half of the initial activity is retained after a defined incubation time after an increase from one temperature to another, i.e., from X° C. to Y° C. “Residual activity” or “residual enzyme activity” refers to the activity that remains following exposure to the set temperature in a defined condition.
“Solvent stable” refers to a polypeptide that maintains similar activity (more than e.g., 60% to 80%) after exposure to varying concentrations (e.g., 5-99%) of solvent (isopropyl alcohol, tetrahydrofuran, 2-methyltetrahydrofuran, acetone, toluene, butylacetate, methyl tert-butylether, etc.) for a period of time (e.g., 0.5-24 hrs) compared to the untreated polypeptide.
“pH stable” refers to a polypeptide that maintains similar activity (more than e.g., 60% to 80%) after exposure to high or low pH (e.g., 4.5-6 or 8 to 12) for a period of time (e.g., 0.5-24 hrs) compared to the untreated polypeptide.
“Thermo- and solvent stable” refers to a polypeptide that is both thermostable and solvent stable, particularly with respect to its biological function, e.g., enzymatic activity.
“Cofactor” refers a substance that is necessary or beneficial to the activity of an enzyme. In the context of an amino acid dehydrogenase, the cofactor is generally a nicotinamide cofactor. “Nicotinamide cofactor” refers to any type of the oxidized and reduced forms of nicotinamide adenine dinucleotide (NAD+ and NADH, respectively) and the oxidized and reduced forms of nicotinamide adenine dinucleotide phosphate (NADP+ and NADPH, respectively) and derivatives and analogs thereof. With regard to a nicotinamide cofactor, the term “derivative” means any compound containing a pyridine structural element, including nicotinamides that have been chemically modified by attachment to soluble or insoluble polymeric materials. Some examples of derivatives of nicotinamide cofactors are described in U.S. Pat. No. 5,106,740, and Mansson and Mosbach, 1987, Methods in Enzymology 136:3 45, the disclosures of which are incorporated herein by reference. The term “analogs,” as used herein, refers to materials that undergo a formal hydride transfer in a redox reaction similar to that undergone by nicotinamide cofactors. Examples of analogs of nicotinamide cofactors useful in the practice of the present process include compounds described in U.S. Pat. No. 5,801,006, the disclosure of which is incorporated herein by reference. Other suitable cofactors, as defined herein, can be used in the practice of the invention, as would be recognized by those skilled in the art.
“Cofactor regenerating system” and “cofactor recycling system” are used interchangeably herein to refer to a set of reactants that participate in a reaction that reduces the oxidized form of the cofactor (e.g., NADP+ to NADPH). In the embodiments herein, cofactors oxidized by the amino acid dehydrogenase-catalyzed reaction are regenerated in reduced form by the cofactor regenerating system. Cofactor regenerating systems comprise a stoichiometric reductant that is a source of reducing hydrogen equivalents and is capable of reducing the oxidized form of the cofactor. The cofactor regenerating system may further comprise a catalyst, for example an enzyme catalyst, that catalyzes the reduction of the oxidized form of the cofactor by the reductant.
“Alkyl” by itself or as part of another substituent refers to a saturated branched or straight hydrocarbon chain derived by the removal of one hydrogen atom from a single carbon atom of a parent alkane. Alkyl groups include, but are not limited to, methyl; ethyl; propyls, such as propan-1-yl, propan-2-yl (isopropyl), etc.; butyls, such as butan-1-yl, butan-2-yl (sec-butyl), 2-methyl propan-1-yl (isobutyl), 2-methyl propan-2-yl (t-butyl), etc.; and the like. In some embodiments, the alkyl groups are (C1-C6) alkyl. “Lower alkyl” refers to a straight-chain or branched saturated aliphatic hydrocarbon having 1 to 6, preferably 1 to 4, carbon atoms. Typical lower alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, t-butyl, 2-butyl, pentyl, hexyl and the like.
“Alkenyl” refers to by itself or as part of another substituent refers to an unsaturated branched, straight chain or cyclic alkyl having at least one carbon-carbon double bond derived by the removal of one hydrogen atom from a single carbon atom of a parent alkene. The group may be in either the cis or trans conformation about the double bond(s). Alkenyl groups include, but are not limited to, ethenyl; propenyls such as prop-1-en-1-yl, prop-1-en-2-yl, prop-2-en-1-yl, prop-2-en-2-yl, cycloprop-1-en-1-yl; cycloprop-2-en-1-yl; butenyls such as but-1-en-1-yl, but-1-en-2-yl, 2-methyl prop-1-en-1-yl, but-2-en-1-yl, but-2-en-2-yl, buta-1,3-dien-1-yl, buta-1,3-dien-2-yl, cyclobut-1-en-1-yl, cyclobut-1-en-3-yl, cyclobuta-1,3-dien-1-yl, etc.; and the like. In some embodiments, the alkenyl group is (C2-C6) alkenyl.
“Alkynyl” by itself or as part of another substituent refers to an unsaturated branched, straight-chain or cyclic alkyl having at least one carbon-carbon triple bond derived by the removal of one hydrogen atom from a single carbon atom of a parent alkyne. Typical alkynyl groups include, but are not limited to, ethynyl; propynyls such as prop-1-yn-1-yl, prop-2-yn-1-yl, etc.; butyryls such as but-1-yn-1-yl, but-1-yn-3-yl, but-3-yn-1-yl, etc.; and the like. In some embodiments, the alkynyl group is (C2-C6) alkynyl.
“Heteroalkyl,” Heteroalkanyl,” Heteroalkenyl,” Heteroalkynyl,” Heteroalkyldiyl” and “Heteroalkyleno” by themselves or as part of another substituent refer to alkyl, alkanyl, alkenyl, alkynyl, alkyldiyl and alkyleno groups, respectively, in which one or more of the carbon atoms are each independently replaced with the same or different heteratoms or heteroatomic groups. Heteroatoms and/or heteroatomic groups which can replace the carbon atoms include, but are not limited to, —O—, —S—, —S—O—, —NR′—, —PH—, —S(O)—, —S(O)2—, —S(O)NR′—, —S(O)2NR′—, and the like, including combinations thereof, where each R′ is independently hydrogen or (C1-C6) alkyl.
“Cycloalkyl” and “Heterocycloalkyl” by themselves or as part of another substituent refer to cyclic versions of “alkyl” and “heteroalkyl” groups, respectively. For heteroalkyl groups, a heteroatom can occupy the position that is attached to the remainder of the molecule. Typical cycloalkyl groups include, but are not limited to, cyclopropyl; cyclobutyls such as cyclobutanyl and cyclobutenyl; cyclopentyls such as cyclopentanyl and cyclopentenyl; cyclohexyls such as cyclohexanyl and cyclohexenyl; and the like. Typical heterocycloalkyl groups include, but are not limited to, tetrahydrofuranyl (e.g., tetrahydrofuran-2-yl, tetrahydrofuran-3-yl, etc.), piperidinyl (e.g., piperidin-1-yl, piperidin-2-yl, etc.), morpholinyl (e.g., morpholin-3-yl, morpholin-4-yl, etc.), piperazinyl (e.g., piperazin-1-yl, piperazin-2-yl, etc.), and the like.
“Aryl” by itself or as part of another substituent refers to a monovalent aromatic hydrocarbon group having the stated number of carbon atoms (i.e., C5-C15 means from 5 to 15 carbon atoms) derived by the removal of one hydrogen atom from a single carbon atom of a parent aromatic ring system. Aryl groups include, but are not limited to, groups derived from aceanthrylene, acenaphthylene, acephenanthrylene, anthracene, azulene, benzene, chrysene, coronene, fluoranthene, fluorene, hexacene, hexaphene, hexalene, as-indacene, s-indacene, indane, indene, naphthalene, octacene, octaphene, octalene, ovalene, penta-2,4-diene, pentacene, pentalene, pentaphene, perylene, phenalene, phenanthrene, picene, pleiadene, pyrene, pyranthrene, rubicene, triphenylene, trinaphthalene, and the like, as well as the various hydro isomers thereof. In some embodiments, the aryl group is (C5-C15) aryl, with (C5-C10) being even more preferred. In some embodiments, the aryls are cyclopentadienyl, phenyl and naphthyl.
“Heteroaryl” by itself or as part of another substituent refers to a monovalent heteroaromatic group having the stated number of ring atoms (e.g., “5-14 membered” means from 5 to 14 ring atoms) derived by the removal of one hydrogen atom from a single atom of a parent heteroaromatic ring system. Typical heteroaryl groups include, but are not limited to, groups derived from acridine, benzimidazole, benzisoxazole, benzodioxan, benzodiaxole, benzofuran, benzopyrone, benzothiadiazole, benzothiazole, benzotriazole, benzoxazine, benzoxazole, benzoxazoline, carbazole, β-carboline, chromane, chromene, cinnoline, furan, imidazole, indazole, indole, indoline, indolizine, isobenzofuran, isochromene, isoindole, isoindoline, isoquinoline, isothiazole, isoxazole, naphthyridine, oxadiazole, oxazole, perimidine, phenanthridine, phenanthroline, phenazine, phthalazine, pteridine, purine, pyran, pyrazine, pyrazole, pyridazine, pyridine, pyrimidine, pyrrole, pyrrolizine, quinazoline, quinoline, quinolizine, quinoxaline, tetrazole, thiadiazole, thiazole, thiophene, triazole, xanthene, and the like, as well as the various hydro isomers thereof. In some embodiments, the heteroaryl group is a 5-14 membered heteroaryl. In some embodiments, the heteroaryl group is a 5-10 membered heteroaryl.
“Substituted” when used to modify a specified group or radical, means that one or more hydrogen atoms of the specified group or radical are each, independently of one another, replaced with the same or different substituent(s). Each substituent can be the same or different. Examples of suitable substituents include, but are not limited to, alkyl, alkenyl, alkynyl, cycloalkyl, aryl, aralkyl, cycloheteroalkyl, heteroaryl, ORa (e.g., hydroxyl, alkoxy (e.g., methoxy, ethoxy, and propoxy), aryloxy, heteroaryloxy, aralkyloxy, ether, ester, carbamate, etc.), hydroxyalkyl, alkoxycarbonyl, alkoxyalkoxy, perhaloalkyl, perfluoroalkyl (e.g., CF3, CF2, CF3), perfluoroalkoxy (e.g., OCF3, OCF2CF3), alkoxyalkyl, SRa (e.g., thiol, alkylthio, arylthio, heteroarylthio, aralkylthio, etc.), S+Ra2, S(O)Ra, SO2Ra, NRaRa (e.g., primary amine (i.e., NH2), secondary amine, tertiary amine, amide, carbamate, urea, etc.), hydrazide, halide, nitrile, nitro, sulfide, sulfoxide, sulfone, sulfonamide, thiol, carboxy, aldehyde, keto, carboxylic acid, ester, amide, imine, and imide, including seleno and thio derivatives thereof, wherein each of the substituents can be optionally further substituted. In embodiments in which a functional group with an aromatic carbon ring is substituted, such substitutions will typically number less than about 10 substitutions, more preferably about 1 to 5, with about 1 or 2 substitutions being preferred.
“Amino acid” refers to a molecule having the general formula NHRb—CHRb′—COOH (wherein R is H, and Rb′ is an amino acid side chain, or R and Rb′ together with the carbon and nitrogen to which they are bonded form a ring, e.g., proline) which is capable of forming a peptide bond with one or more other molecules having the same general formula. The term embraces both L and D amino acids. A “chiral amino acid” refers to an amino acid in which the α-carbon is an asymmetric carbon atom, which is a carbon atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer. In the context of an amino acid, a chiral amino acid of general formula NHR—CHR′—COOH, the R′ group is an amino acid side chain other than H. A chiral amino acid can be an L-amino acid or a D-amino acid.
“Alcohol” refers to an alkyl group in which one or more of the hydrogen atoms has been replaced by an —OH group. A “lower alcohol” refers to an alcohol in which the alkyl group is about 1 to about 6 carbon atoms. A “secondary alcohol” refers to an alcohol in which the —OH group is bonded to a carbon atom that is bonded to one hydrogen atom and to two other carbon atoms, such as in 2-propanol (isopropanol), 2-butanol, 2-hexanol and the like. A “lower secondary alcohol” refers to a secondary alcohol in which the alkyl group is of 3 to about 6 carbon atoms.
“Ketone” refers to a carbonyl compound of general formula R′—C(O)—R′′ in which the carbonyl carbon is bonded to two carbon atoms. In some embodiments, R′ and R′′ are the same and in some embodiments, R′ and R′′ are each independently an optionally substituted alkyl or aryl. A lower alkyl ketone refers to a carbonyl compound of general formula R′—C(O)—R′′ in which R′ and R′′ is each an alkyl of C1 to C5 carbon atoms, where the total number of carbon atoms in the ketone is 3 to 6 carbon atoms.
“Protecting group” refers to a group of atoms that, when attached to a reactive functional group in a molecule, mask, reduce or prevent the reactivity of the functional group. Typically, a protecting group may be selectively removed as desired during the course of a synthesis. Examples of protecting groups can be found in P. G. M. Wuts and T. W. Greene, “Greene's Protective Groups in Organic Synthesis Fourth Edition,” John Wiley and Sons, New York, N.Y., 2007, Chapter 7 (“Greene”) which chapter is hereby incorporated by reference in its entirety, and Harrison et al., Compendium of Synthetic Organic Methods, Vols. 1-8, 1971-1996, John Wiley & Sons, NY.
“N-protecting group” (or nitrogen protecting group) means a substituent commonly employed to block or protect a nitrogen functionality (e.g., the amine nitrogen of an amino acid) while carrying out a reaction with other functional groups on a compound. Accordingly, an “N-protected” compound refers to a modified form of the compound where an N-protecting group is blocking a nitrogen functionality on the compound from undergoing reaction.
“Ammonium source” or “ammonium ion donor” refers to a compound or composition that forms ammonia or ammonium ion in a reaction medium.
“Reaction medium” refers to a solution comprising a mixture of two or more components (e.g., enzyme, substrate, cofactor) which can undergo reaction in the solution. For the enzymatic reactions described herein the reaction medium typically is an at least partially aqueous solution. In some embodiments, the reaction medium comprises aqueous and organic solvents (e.g., isopropanol), one or more phases.
“Isolated polypeptide” refers to a polypeptide which is separated from other contaminants that naturally accompany it, e.g., protein, lipids, and polynucleotides. The term embraces polypeptides which have been removed or purified from their naturally-occurring environment or expression system (e.g., host cell or in vitro synthesis).
“Substantially pure polypeptide” refers to a composition in which the polypeptide species is the predominant species present (i.e., on a molar or weight basis it is more abundant than any other individual macromolecular species in the composition), and is generally a substantially purified composition when the object species comprises at least about 50 percent of the macromolecular species present by mole or % weight. Generally, a substantially pure ketoreductase composition will comprise about 60% or more, about 70% or more, about 80% or more, about 90% or more, about 95% or more, and about 98% or more of all macromolecular species by mole or % weight present in the composition. In some embodiments, the object species is purified to essential homogeneity (i.e., contaminant species cannot be detected in the composition by conventional detection methods) wherein the composition consists essentially of a single macromolecular species. Solvent species, small molecules (<500 Daltons), and elemental ion species are not considered macromolecular species.
5.2 Processes Using Amino Acid Dehydrogenases Coupled to Ketoreductase-Based Cofactor Regenerating System
The present disclosure provides a process for the conversion of a 2-oxo acid (i.e., a keto acid) to an amino acid in the presence of an ammonium source in a reaction mediated by an amino acid dehydrogenase (“AADH”) and a cofactor recycling system comprising a ketoreductase (“KRED”), as generally depicted in Scheme 1.




The stereoselectivity of the amino acid dehydrogenase can be exploited to carry out the reverse reaction, i.e., conversion of an amino acid to the 2-oxo acid, and permit chiral resolution of L- and D-amino acids.
In the conversion of the 2-oxo acid to the amino acid, amino acid dehydrogenases typically use a cofactor, generally nicotinamide adenine dinucleotide (NAD+/NADH) or nicotinamide adenine dinucleotide phosphate (NADP+/NADPH). To enhance the amino acid dehydrogenase-mediated process, a cofactor regenerating system of formate dehydrogenase, glucose dehydrogenase, or phosphite dehydrogenase have been used to convert the oxidized NAD+/NADP+ to the reduced form NADH/NADPH. See, e.g., US patent publication 2009087995; US patent publication 20090117627; EP1925674; Johannes et al., 2005, Appl Environ Microbiol. 71(10):5728-5734; Johannes et al., 2007, Biotechnol Bioeng. 96(1):18-26; McLachan et al., 2008, Biotechnol Bioeng. 99(2):268-274). By continual replenishment of the reduced NADH or NADPH, the equilibrium of the amino acid dehydrogenase mediated process can be shifted towards product formation, thereby increasing the conversion of the oxo acid to the amino acid product. A whole cell-based system for conversion of D-amino acid to L-amino acid using amino oxidase, amino acid dehydrogenase, and a cofactor regenerating system is described in U.S. Pat. No. 7,217,544. The patent publication describes the use of formate dehydrogenase, malate dehydrogenase or alcohol dehydrogenase activities present in whole cells for regeneration of the cofactor. A coupled enzyme system of a phenylalanine dehydrogenase and a cofactor regenerating system using an alcohol dehydrogenase is described in Paradisi et al., 2007, J. Biotech. 128:408-411. However, in Paradisi et al., ethanol was employed as the substrate for the alcohol dehydrogenase, thereby forming acetaldehyde as the product.
As provided in the present disclosure, it has been found that processes using amino acid dehydrogenases when carried out in the presence of a cofactor regenerating system comprising a ketoreductase and an alcohol can be used for the efficient conversion of an oxo acid to its corresponding amino acid. In particular, use of a lower secondary alcohol for the ketoreductase-based cofactor regenerating system can increase the conversion of oxo acid to the amino acid in the amino acid dehydrogenase catalyzed reaction, avoiding the production of acetaldehyde which can react with and inactivate enzymes. Moreover, the ketone product formed by the ketoreductase catalyzed reaction, such as acetone, is less volatile than acetaldehyde, thereby providing greater control over the reaction, particularly in larger scale processes (e.g., reaction medium of 50 L, 100 L, 300 L, 500 L, or even greater volume).
Further, the loss of process control due to the volatility of acetaldehyde would create greater difficulty when using an amino acid dehydrogenase in the reverse reaction (e.g., converting amino acid to the corresponding oxo acid) as described in greater detail herein. In particular, pushing the equilibrium of the reverse reaction would be facilitated with a less volatile lower alkyl ketone such as acetone.
Although advantageously less volatile than acetaldehyde, the ketone product formed by the ketoreductase reaction of a lower secondary alcohol (e.g., acetone) is sufficiently volatile to allow its facile removal from the reaction medium thereby shifting the equilibrium of the ketoreductase mediated process towards further cofactor reduction, and further conversion of oxo acid to the amino acid by the amino acid dehydrogenase. Consequently, the combination of a ketoreductase and lower secondary alcohol provides the advantages greater reaction control (and increased safety) and enhanced ability to drive the desired amino acid dehydrogenase reaction to completion.
Significant benefit can be further obtained when engineered ketoreductases that have improved enzyme properties, including among others, increased enzymatic activity, increased thermostability, increased solvent stability and/or increased pH stability are used in the process. Engineered ketoreductases with such improved properties can allow use of conditions not well tolerated by the naturally occurring enzymes, including conditions such as, for example, high oxo acid concentration, high alcohol concentration, high ammonium ion donor concentration, elevated incubation temperatures, and increased incubation times. Use of engineered ketoreductases also can reduce the amount of enzyme needed in the process.
Accordingly, the present disclosure provides a process for converting a 2-oxo acid compound of formula I that is a substrate for an amino acid dehydrogenase to a chiral amino acid compound of formula IIa,




in a reaction medium comprising NAD+/NADH or NADP+/NADPH, and a cofactor regenerating system, where the cofactor regenerating system comprises a ketoreductase and a secondary alcohol. In particular, the secondary alcohol is a lower secondary alcohol, such as isopropanol, 2-butanol, 3-methyl-2-butanol, 2-pentanol, 3-pentanol, or 3,3-dimethyl-2-butanol.

In some embodiments, the process comprises contacting the 2-oxo acid compound of formula I with a reaction medium comprising an amino acid dehydrogenase, an ammonium ion donor, NAD+/NADH or NADP+/NADPH, and a cofactor regenerating system comprising a ketoreductase and a lower secondary alcohol, under suitable conditions where the compound of formula I is converted to the chiral amino acid compound of formula IIa and the lower secondary alcohol is converted to a ketone.
In the processes herein, the 2-oxo acid compound of formula I is a substrate for the amino acid dehydrogenase. Accordingly, the R group in the compound of formula I can be a substituted or unsubstituted: (C1-C10)alkyl, —(C2-C6)alkenyl, —(C2-C6)alkynyl, heteroalkyl, —(C3-C8)cycloalkyl, heterocycloalkyl, aryl, or heteroaryl. Since amino acid dehydrogenases are known to recognize naturally occurring amino acids, the R group can be any side chain attached to the alpha carbon of an amino acid of a naturally occurring amino acid. These include, among others, the following side chain structures shown in Table 1 (where squiggly line denotes point of connection of R group to rest of molecule):







TABLE 1













 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 















Other R groups recognized by amino acid dehydrogenases include, among others, the following structures shown in Table 2 (where squiggly line denotes point of connection of R group to rest of molecule):







TABLE 2













 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 








 















In some embodiments, the amino acid dehydrogenase can be an L- or D-amino acid dehydrogenase. In other words, in the process for conversion of a pro-chiral 2-oxo acid of formula I to the amino acid of formula IIa, the amino acid dehydrogenase can be enantioselective for the L- or D-amino acid. Thus, by selection of the appropriate amino acid dehydrogenase, the process of the present disclosure can be used to produce the L- or D-amino acid in enantiomeric excess from the prochiral 2-oxo acid. The amino acid dehydrogenase can be a naturally occurring, i.e., wild type, amino acid dehydrogenase or an engineered amino acid dehydrogenase. The engineered amino acid dehydrogenase can be selected for improved properties, such as increased enzymatic activity, thermostability, solvent stability, pH stability, co-factor preference, and/or altered substrate specificity.
In some embodiments of the process, the amino acid dehydrogenase comprises a L-amino acid dehydrogenase and the chiral amino acid compound of formula IIa is IIb,




having the indicated chirality. In this process, the chiral amino acid of formula IIb is formed in enantiomeric excess. In some embodiments, the chiral amino acid of formula IIb can be formed in at least 25%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% or more in enantiomeric excess. In some embodiments, the chiral amino acid of formula IIb is formed in greater than 99% enantiomeric excess.

Various L-amino acid dehydrogenase can be obtained from organisms of the genus Bacillus, Clostridium, Corynebacterium, Geobacillus, Natronobacterium, Synechocystis, Thermoactinomyces, Thermomicrobium, Carderia, Citrobacter, Proteus, and Pseudomonas, as well as from mammalian sources (e.g., beef liver). Specific organisms where the L-amino acid dehydrogenase can be obtained include, among others, Bacillus subtilis, Bacillus sphaericus, Bacillus stearothermophilus, Bacillus thermoproteoliticus, Brevibacterium sp., Clostridium symbiosum, Clostridium difficile, Geobacillus stearothermophilus, Natronobacterium magadii, Synechocystis sp. PCC 6803, Thermoactinomyces intermedius, Citrobacter sp., Proteus sp., and Pseudomonas sp.
L-amino acid dehydrogenases useful in the process of the present disclosure capable of carrying out the conversion of an L-amino acid, in the presence of an electron acceptor, to a 2-oxo acid, NH3, and reduced acceptor. Suitable L-amino acid dehydrogenases have been described in e.g., Oshima et al., International Industrial Biotechnology 9 (1989) 5-11; Ohsima et al., European Journal of Biochemistry 191 (1990) 715-720; Khan et al., Bioscience, Biotechnology and Biochemistry 69 (2005) 1861-1870; Hummel et al., Applied Microbiology and Biotechnology 26 (1987) 409-416 and Bommarius in Enzyme Catalysis in Organic Synthesis, 2nd Edition (2002), ed. Drauz and Waldmann, Wiley-VCH Weinheim.
The polynucleotide and/or amino acid sequences of various L-amino acid dehydrogenases are known in the art and are available from known public databases e.g., the GenBank (located at www.ncbi.nlm.nih.gov). In some embodiments, L-enantioselective amino acid dehydrogenases useful with the process of the present disclosure have been described with respect to the type of amino acid acted upon/formed in the enzyme catalyzed process. Accordingly, in some embodiments, useful L-amino acid dehydrogenase references and sequence information can be obtained by entering into the indexed and searchable public databases any one of the following enzyme classifications: L-alanine dehydrogenase (EC 1.4.1.1) (see e.g., Ohashima et al., 1979, Eur J Biochem. 100(1):29-30; Grimshaw et al., 1981, Biochemistry. September 29; 20(20):5650-5), L-aspartate dehydrogenase (EC 1.4.1.21), L-erythro-3,5-diaminohexanoate dehydrogenase (EC 1.4.1.11), L-leucine dehydrogenase (EC 1.4.1.9), L-glutamate dehydrogenase (EC 1.4.1.2), glutamate dehydrogenase (NAD(P)+) (EC 1.4.1.3), glutamate dehydrogenase (NADP+) (EC 1.4.1.4), glycine dehydrogenase (EC 1.4.1.10), lysine dehydrogenase (EC 1.4.1.15), L-phenylalanine dehydrogenase (EC 1.4.1.20), L-serine dehydrogenase (EC 1.4.1.7), L-valine dehydrogenase (EC 1.4.1.8), L-2,4-diaminopentanoate dehydrogenase, L-glutamate synthase, L-diaminopimelate dehydrogenase (EC 1.4.1.12), L-N-methylalanine dehydrogenase, L-lysine 6-dehydrogenase, and L-tryptophan dehydrogenase, glutamate synthase (NADPH) (EC 1.4.1.13); glutamate synthase (NADH>) (EC 1.4.1.14), diaminopimelate dehydrogenase (EC 1.4.1.16); N-methylalanine dehydrogenase (EC 1.4.1.17), lysine 6-dehydrogenase (EC 1.4.1.18), and tryptophan dehydrogenase (EC 1.4.1.19). The choice of the amino acid dehydrogenase can be based on the type of oxo acid substrate recognized by the enzyme.
In some embodiments, the L-amino acid dehydrogenases are engineered L-amino acid dehydrogenases in which mutations have been introduced into the naturally occurring polypeptide to generate an enzyme with altered properties. Engineered L-amino acid dehydrogenases are described for, by way of example and not limitation, L-phenylalanine dehydrogenase (see e.g., Seah et al., 1995 FEBS Lett. 370(1-2):93-96; Busca et al., 2004, Org. Biomol. Chem. 2, 2684-2691.)
In some embodiments, the amino acid dehydrogenase comprises a D-amino acid dehydrogenase, and the chiral amino acid of formula IIa is IIc,




having the indicated chirality. In this process, the chiral amino acid of formula IIc is formed in enantiomeric excess. In some embodiments, the chiral amino acid of formula IIc can be formed in at least 25%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% or more in enantiomeric excess. In some embodiments, the chiral amino acid of formula IIc is formed in greater than 99% enantiomeric excess.

D-amino acid dehydrogenases can be obtained from Halobacterium, Methanosarcina, Pseudomonas, Pyrobaculum, Salmonella, Corynebacterium, and Escherichia. Specific species where the D-amino acid dehydrogenase can be obtained include, among others, Pseudomonas aeruginosa, Pseudomonas fluorescens, Pyrobaculum islandicum, Salmonella typhimurium, Corynebacterium glutamicum, and Escherichia coli. 
Like the L-amino acid dehydrogenases, references and sequences of wild-type D-amino acid dehydrogenases are publicly available, for example from the GenBank database available at the NCBI web-site. A few exemplary wild-type D-amino acid dehydrogenase sequences listed by GenBank accession include: AAC36880 D-amino acid dehydrogenase [Escherichia coli] gi|145703|gb|AAC36880.1|[145703]; AAC36881 catabolic alanine racemase [Escherichia coli] gi|145704|gb|AAC36881.1|[145704]; AAC38139 D-amino acid dehydrogenase [Klebsiella aerogenes] gi|2360965|gb|AAC38139.1|[2360965]; AAC74273 D-amino acid dehydrogenase [Escherichia coli str. K-12 substr. MG1655] gi|1787438|gb|AAC74273.1|[1787438]; AAD06449 D-Amino acid dehydrogenase [Helicobacter pylori J99] gi|4155445|gb|AAD06449.1|[4155445]; AAF40633 D-amino acid dehydrogenase, small subunit [Neisseria meningitidis MC58] gi|7225395|gnl|tigr|NMB0176|gb|AAF40633.1|[7225395]; AAF83661 D-amino acid dehydrogenase subunit [Xylella fastidiosa 9a5c] gi|9105759|gb|AAF83661.1|AE003925_1[9105759]; AAF93951 D-amino acid dehydrogenase, small subunit [Vibrio cholerae O1 biovar El Tor str. N16961]gi|9655235|gb|AAF93951.1∥gnl|TIGR|VC0786[9655235]; AAF95141 2,4-dienoyl-CoA reductase [Vibrio cholerae O1 biovar El Tor str. N16961] gi|9656535|gb|AAF95141.1∥gnl|TIGR|VC1993[9656535]; AAG08469 probable oxidoreductase [Pseudomonas aeruginosa PAO1] gi|9951378|gb|AAG08469.1|AE004921_61gnl|PseudoCAP|PA5084[9951378]; AAG08689 D-amino acid dehydrogenase, small subunit [Pseudomonas aeruginosa PAO1] gi|9951620|gb|AAG08689.1|AE004943_51gnl|PseudoCAP|PA5304[9951620]; AAG56040 D-amino acid dehydrogenase subunit [Escherichia coli O157:H7 EDL933] gi|12514889|gb|AAG56040.1|AE005336_1[12514889]; AAK90026 D-amino acid dehydrogenase [Agrobacterium tumefaciens str. C58] gi|15159999|gb|AAK90026.1|[15159999]; AAK90097 D-amino acid dehydrogenase, small subunit [Agrobacterium tumefaciens str. C58] gi|15160086|gb|AAK90097.1|[15160086]; AAL20718 D-amino acid dehydrogenase subunit [Salmonella enterica subsp. enterica serovar Typhimurium str. LT2] gi|16420334|gb|AAL20718.1|[16420334]; AAL53615 d-amino acid dehydrogenase small subunit [Brucella melitensis bv. 1 str. 16M] gi|17984529|gb|AAL53615.1∥gnl|integgen|BMEII0373[17984529]; AAL73201 putative D-amino acid dehydrogenase [Agrobacterium sp. IP I-671] gi|18478564|gb|AAL73201.1|AF335479_5[18478564]; AAM38531 D-amino acid dehydrogenase subunit [Xanthomonas axonopodis pv. citri str. 306] gi|21110075|gb|AAM38531.1∥gnl|unicamp|XAC3688[21110075]; AAM42918 D-amino acid dehydrogenase subunit [Xanthomonas campestris pv. campestris str. ATCC 33913] gi|21114929|gb|AAM42918.1∥gnl|unicamp|XCC3648[21114929]; AAM85736 D-amino acid dehydrogenase subunit [Yersinia pestis KIM 10] gi|21959016|gb|AAM85736.1|AE013821_2[21959016]; AAN34096 D-alanine dehydrogenase, small subunit [Brucella suis 1330] gi|23464296|gnl|tigr|BRA0924|gb|AAN34096.1|[23464296]; AAN42793 D-amino acid dehydrogenase subunit [Shigella flexneri 2a str. 301] gi|56383395|gb|AAN42793.2∥gnl|mgcchina|SF0001178[56383395]; and AAN69891 D-amino acid dehydrogenase, small subunit, putative [Pseudomonas putida KT2440] gi|24986030|gb|AAN69891.1|AE016628_4|gnl|tigr|PP4311[24986030].
In some embodiments, the D-amino acid dehydrogenase is selected from D-alanine dehydrogenase (e.g., gi|23464296|gb|AAN34096.11D-alanine dehydrogenase, small subunit [Brucella suis 1330]), D-threonine dehydrogenase (e.g., gi|3845577|dbj|BAA34184.11D-threonine dehydrogenase [Pseudomonas cruciviae]), D-proline dehydrogenase (e.g., gi|145283977|gb|ABP51559.11D-proline dehydrogenase [Pyrobaculum arsenaticum DSM 13514]).
In some embodiments, the D-amino acid dehydrogenases are engineered D-amino acid dehydrogenases in which mutations have been introduced into the naturally occurring polypeptide to generate an enzyme with altered enzyme properties. Engineered D-amino acid dehydrogenases are described in e.g., Vedha-Peters et al., “Creation of a Broad-Range and Highly Stereoselective D-Amino Acid Dehydrogenase for the One-Step Synthesis of D-Amino Acids” J. Am. Chem. Soc. 2006, 128, 10923-10929, or in U.S. Pat. No. 7,550,277, which is hereby incorporated by reference herein.
In the process mediated by the amino acid dehydrogenases, the reaction medium contains an ammonium source, which provides the NH3 group for formation of the amino acid of formula IIa from the 2-oxo acid of formula I. Any compound which is suitable for this purpose can be used as the ammonium source. Exemplary compounds include, among others, ammonium salts, such as ammonium halide (e.g., ammonium chloride), ammonium formate, ammonium sulfate, ammonium phosphate, ammonium nitrate, ammonium tartrate, and ammonium acetate.
While the process described herein is generally used for the preparation of L- or D-amino acids, the amino acid dehydrogenase can also carry out the reverse reaction, i.e., conversion of an amino acid to its corresponding 2-oxo acid. When the substrate is an enantiomerically pure L- or D-amino acid, an appropriate amino acid dehydrogenase can be used for the conversion of the amino acid to the 2-oxo acid. For instance, an L-amino acid dehydrogenase is selected for conversion of L-amino acid preparations to the corresponding 2-oxo acid.
In some embodiments, the stereospecificity of amino acid dehydrogenases can be exploited for the chiral resolution of mixtures of L- and D-amino acids. For example, an L-amino acid dehydrogenase can be used to stereospecifically convert the L-amino acid in a mixture of L- and D-amino acids to the corresponding 2-oxo acid, thereby resulting in a composition that has the D-amino acid in enantiomeric excess. Similarly, a D-amino acid dehydrogenase can be used to stereospecifically convert the D-amino acid in a mixture of L- and D-amino acids to the corresponding 2-oxo acid, thereby resulting in a composition that has the L-amino acid in enantiomeric excess. When desired, the amino acid present in enantiomeric excess can be isolated from the product mixture.
Accordingly, in some embodiments, the present disclosure provides a process for converting a compound mixture of formula IId, which comprises a substrate for an amino acid dehydrogenase, to a composition of a 2 oxo acid compound of formula I and a chiral amino acid of formula IIa:




having a chiral carbon marked with an *, where the compound mixture of formula IId comprises L- and D-amino acid compounds of formula IIb and IIe and the chiral amino acid of formula IIa is an L- or D-amino acid. In these embodiments, the process for chiral resolution of a mixture of L- and D-amino acids can comprise contacting the compound mixture of formula IId with a reaction medium comprising an enantioselective amino acid dehydrogenase, NAD+/NADH or NADP+/NADPH, and a cofactor recycling system comprising a ketoreductase and a lower alkyl ketone, under conditions where either the L-amino acid or D-amino acid of the compound mixture of formula IId is converted to a compound of formula I thereby resulting in an enantiomeric excess of the amino acid of formula IIa (which the chiral amino acid not converted by the amino acid dehydrogenase), and the lower alkyl ketone is converted to a lower secondary alcohol.

In some embodiments of the process, the compound mixture of IId is a racemic mixture of L- and D-amino acid compounds of formulas IIb and IIc, as represented by formula IIe:




In some embodiments, the process can be used for the chiral resolution of a mixture of L- and D-amino acids to form a chiral amino acid of formula IIc:




having the indicated chirality in enantiomeric excess. In these embodiments, the process can comprise contacting the compound mixture of formula IId (which can include a racemic mixture of formula IIe) with a reaction medium comprising an L-amino acid dehydrogenase, NAD+/NADH or NADP+/NADPH and a cofactor recycling system comprising a ketoreductase and a lower alkyl ketone, under conditions where the chiral amino acid compound of formula IIb in the compound mixture of formula IId is converted to the 2-oxo acid compound of formula I thereby resulting in an enantiomeric excess of the chiral amino acid of formula IIc, and the lower alkyl ketone is converted to the corresponding lower secondary alcohol. In some embodiments, the chiral amino acid of formula IIc can be formed in at least 25%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% or more in enantiomeric excess. In some embodiments, the chiral amino acid of formula IIc is formed in greater than 99% enantiomeric excess.

In the process for chiral resolution of a mixture of L- and D-amino acids to form the D-amino acid in enantiomeric excess, the L-amino acid dehydrogenase can be from Bacillus, Clostridium, Corynebacterium, Geobacillus, Natronobacterium, Synechocystis, Thermoactinomyces, Thermos, Thermomicrobium, or Carderia. 
In some embodiments, the L-amino acid dehydrogenase can be selected from L-alanine dehydrogenase, L-aspartate dehydrogenase, L-erythro-3,5-diaminohexanoate dehydrogenase, L-leucine dehydrogenase, L-glutamate dehydrogenase, lysine dehydrogenase, L-phenylalanine dehydrogenase, L-serine dehydrogenase. L-valine dehydrogenase, L-2,4-diaminopentanoate dehydrogenase, L-glutamate synthase, L-diaminopimelate dehydrogenase, L-N-methylalanine dehydrogenase, L-lysine 6-dehydrogenase, and L-tryptophan dehydrogenase, as described above.
In some embodiments, the process can be used for the chiral resolution of a mixture of L- and D-amino acids to form the chiral amino acid of formula IIb,




having the indicated chirality in enantiomeric excess. In these embodiments, the process can comprise contacting the compound mixture of formula IId (which can include a racemic mixture of formula IIe) with a reaction medium comprising a D-amino acid dehydrogenase, NAD+/NADH or NADP+/NADPH and a cofactor recycling system comprising a ketoreductase and a lower alkyl ketone, under conditions where the chiral amino acid of formula IIc in the compound mixture of formula IId is converted to the 2-oxo acid compound of formula I thereby resulting in an enantiomeric excess of the chiral amino acid of formula IIb, and the lower alkyl ketone is converted to the corresponding lower secondary alcohol. In some embodiments, the chiral amino acid of formula IIb can be formed in at least 25%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% or more in enantiomeric excess. In some embodiments, the chiral amino acid of formula IIb is formed in greater than 99% enantiomeric excess.

In the process for chiral resolution of a mixture of L- and D-amino acids to form the L-amino acid in enantiomeric excess, the D-amino acid dehydrogenase can be from Halobacterium, Methanosarcina, Pseudomonas, Pyrobaculum, Salmonella, Corynebacterium, or Escherichia. 
In some embodiments of the process herein, the amino acid dehydrogenase can be present in the form of whole cells, including whole cells transformed with polynucleotide constructs expressing wild type or engineered amino acid dehydrogenases. In some embodiments, the amino acid dehydrogenase can be present in the form of cell extracts and/or lysates thereof, and may be employed in a variety of different forms, including solid (e.g., lyophilized, spray-dried, and the like) or semisolid (e.g., a crude paste). In some embodiments, the amino acid dehydrogenase is isolated, and can be in a substantially purified form. In some embodiments of the process, both the amino acid dehydrogenase and the ketoreductase of the regenerating system can be present in the form of whole cells, including whole cells transformed with polynucleotide constructs such that the whole cells express both the amino acid dehydrogenase and the ketoreductase.
In the embodiments of the processes disclosure herein, the amino acid dehydrogenase is used in combination with cofactor regenerating system comprising: a ketoreductase capable of reducing NAD and/or NADP to NADH and NADPH, respectively, and an alcohol (e.g., a lower secondary alcohol) that is a substrate for the ketoreductase. In the reduction of the cofactor, the ketoreductase converts the alcohol to the corresponding carbonyl compound (e.g., a lower alkyl ketone).
In the embodiments herein, the ketoreductase can be a wild type ketoreductase, or an engineered ketoreductase, in particular an engineered ketoreductase with an improved enzyme property. As used herein, a ketoreductase enzyme that has an “improved enzyme property” refers to a ketoreductase enzyme that exhibits an improvement in any enzyme property as compared to a reference ketoreductase enzyme. For the engineered ketoreductase enzymes described herein, the comparison is generally made to the wild-type ketoreductase enzyme, although in some embodiments, the reference ketoreductase can be an improved engineered ketoreductase. In some embodiments, the ketoreductase is an engineered ketoreductase characterized by increased thermostability, increased solvent stability, increased pH stability, and/or increased enzymatic activity relative to the wild type ketoreductase.
In some embodiments of the process, the ketoreductase used in the cofactor recycling system has an improved property over a reference ketoreductase of increased activity in the conversion of the lower secondary alcohol (e.g., isopropanol) of the recycling system to the corresponding lower alkyl ketone. In some embodiments, the ketoreductase having the increased activity in the conversion of the lower secondary alcohol is at least 2.0 fold, 2.5 fold, 5.0 fold, 7.5 fold, 10-fold, or more improved relative to a reference ketoreductase. In some embodiments, the ketoreductase is an engineered ketoreductase derived from the wild-type ketoreductase of Novosphingobium aromaticivorans (e.g., gi|145322460|gb|ABP64403.1|[145322460]).
The improved activity of engineered ketoreductases (derived from Novosphingobium aromaticivorans ketoreductase of SEQ ID NO: 2) for the conversion of the secondary alcohol, isopropanol (IPA) to its corresponding product, acetone was determined relative to the same activity for the reference ketoreductase of SEQ ID NO: 2. Relative IPA activity was determined using an assay with the following reaction conditions: 100 μl 10× diluted engineered KRED lysate, 10% IPA (v/v), 0.5 g/L NAD+, 100 mM TEA, pH 7.5. Exemplary engineered ketoreductases exhibiting at least 2-fold increased activity with IPA relative to SEQ ID NO: 2 are listed in Table 3. The fold-improvement in IPA activity relative to SEQ ID NO: 2 was quantified as follows: “+” indicates at least 200% to 250% improvement; “++” indicates <250% to 500% improvement; and “+++” indicates >500% to 1000% improvement; and “++++” indicates >1000% to 2000% improvement.










TABLE 3







SEQ ID
FIOP in IPA



NO:
activity



















2
[control]



4
+



6
++



8
++



10
++



12
+++



14
++



16
+



18
++++



20
++



24
+++










In some embodiments, the ketoreductase having the increased activity in the conversion of the lower secondary alcohol is an engineered ketoreductase comprising an amino acid sequence having at least 80%, 85%, 90%, 95%, 98%, 99%, or more identity to a sequence selected from SEQ ID NO: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24. In some embodiments, the ketoreductase having the increased activity in the conversion of the lower secondary alcohol is an engineered ketoreductase comprising an amino acid sequence selected from SEQ ID NO: 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24.
In some embodiments of the process, the ketoreductase used in the cofactor recycling system has an improved property over a reference ketoreductase of decreased or no activity with the 2-oxo acid compound of formula I (e.g., trimethylpyruvic acid) which is a substrate for the amino acid dehydrogenase used in the process. In some embodiments of the process, the activity of the ketoreductase used in the cofactor recycling system with the compound of formula I is less than about 5%, 2%, 1%, 0.5%, 0.1%, 0.05%, 0.01%, or an even smaller percentage of the activity of the amino acid dehydrogenase used in the process with the compound of formula I. In some embodiments of the process, the ketoreductase used in the cofactor recycling system has no detectable activity with the 2-oxo acid compound of formula I.
The relative IPA activity of each of the engineered ketoreductases shown in Table 3 was measured under the same conditions in the presence of the compound of formula I, trimethylpyruvic acid (10% IPA (v/v), 0.5 g/L NAD+, 1 mM TMP, 100 mM TEA pH 7.5, 100 μl 10× KRED diluted lysate). Less than 5% increase or decrease was seen in the presence of the TMP, indicating that the engineered ketoreductases listed in Table 3 did not use TMP as a substrate.
Various ketoreductases that can be used in the cofactor regenerating system include ketoreductases from, by way of example and not limitation, bacteria, such as the genus Escherichia, the genus Bacillus, the genus Pseudomonas, the genus Serratia, the genus Brevibacterium, the genus Corynebacterium, the genus Streptococcus, the genus Lactobacillus, the genus Novosphingobium; actinomycetes such as the genus Rhodococcus, the genus Streptomyces; yeasts such as the genus Saccharomyces, the genus Kluyveromyces, the genus Thermoanerobium, the genus Schizosaccharomyces, the genus Sporobolomyces, the genus Zygosaccharomyces, the genus Yarrowia, the genus Trichosporon, the genus Rhodosporidium, the genus Pichia, the genus Candida; and fungi such as the genus Neurospora, the genus Aspergillus, the genus Cephalosporium, the genus Trichoderma. 
In some embodiments, the ketoreductase can be derived from Lactobacillus kefir, Lactobacillus brevis, Lactobacillus minor, Candida sonorensis, Candida boidini, Candida guilliermondi, Candida magnoliae, Candida utilis, Candida maltosa, Candida kefir, Candida parapslosis, Geotrichum candidum, Rhodococcus erythropolis, Rhodotorula glutinis, Hansenula fabianii, Hansenula polymorpha, Hansenula saturnus, Nocardia salmonicolor, Novosphingobium aromaticivorans, Pichia anomala, Pichia capsulata, Pichia membranafaciens, Pichia methanolica, Pichia pinus, Pichia silvicola, Pichia stipitis, Sphingomonas paucimobilis, Sporobolomyces salmonicolor, Streptomyces coelicolor, Thermoanerobium brockii, or Saccharomyces cerevisiae. 
In some embodiments, the ketoreductase is a wild-type ketoreductase listed in Table 4 or an engineered ketoreductase derived from a wild-type ketoreductase listed in Table 4.







TABLE 4







Wild type KRED from various microorganisms










Microorganism
Genbank Acc. No.
GI No.
Reference













Candida magnoliae
AB036927.1
12657576
SEQ ID No 2 in US publ. no.





20060195947A1


Saccharomyces cerevisiae
NP_010159.1
6320079
SEQ ID NO: 110 in US publ. no.





20090191605A1


Lactobacillus brevis
1NXQ_A
30749782
SEQ ID NO: 2 in US publ. no.





20090191605A1


Rhodococcus erythropolis
AAN73270.1
34776951
SEQ ID NO: 112 in US publ. no.





20090191605A1


Saccharomyces cerevisiae
NP_011476
6321399
SEQ ID NO: 114 in US publ. no.





20090191605A1


Saccharomyces cerevisiae
NP_010656.1
6320576
SEQ ID NO: 116 in US publ. no.





20090191605A1


Saccharomyces cerevisiae
NP_014490.1
6324421
SEQ ID NO: 118 in US publ. no.





20090191605A1


Lactobacillus kefir
AAP94029.1
33112056
SEQ ID NO: 4 in US publ. no.





20090191605A1


Sporobolomyces salmonicolor
Q9UUN9
30315955
SEQ ID No 104 in US publ. no.





20090191605A1


Streptomyces coelicolor
NP_631415.1
21225636
SEQ ID No 102 in US publ. no.





20090191605A1


Thermoanaerobium brockii
X64841.1
1771790
SEQ ID No 108 in US publ. no.





20090191605A1


Candida parapsilosis
BAA24528
2815409
Julich Chiral Solutions Cat. No. 03.11


Lactobacillus brevis
ABJ63353.1
116098204
Julich Chiral Solutions Cat. No. 8.1


Candida boidinii
CAD66648
28400789
Julich Chiral Solutions Cat. No. 02.10


Lactobacillus leichmannii


Fluka Cat. No. 61306









In some embodiments, the ketoreductase is from Lactobacillus, such as, among others, Lactobacillus kefir, Lactobacillus brevis, or Lactobacillus minor. Wild type ketoreductase from Lactobacillus kefir is described in Genbank accession no. AAP94029 GI:33112056. Wild type ketoreductase from Lactobacillus brevis is described in CAD66648 GI:28400789.
In some embodiments, the ketoreductase is an engineered ketoreductase derived from a wild type ketoreductase of Lactobacillus. Engineered ketoreductases of Lactobacillus, for example, L. kefir, L. brevis, and L. minor, with improved enzyme properties are described in US patent publications US20080318295, US20090093031, US20090191605, US20090155863, US20090162909, U.S. Ser. No. 12/545,034, filed Aug. 20, 2009, U.S. Ser. No. 12/545,761, filed Aug. 21, 2009, U.S. Ser. No. 12/549,154, filed Aug. 27, 2009, and U.S. Ser. No. 12/549,293, filed Aug. 27, 2009, of which each of the ketoreductase polypeptides disclosed therein are hereby incorporated by reference herein.
In some embodiments, the ketoreductase of Candida is from Candida magnoliae. Wild type ketoreductase of Candida magnoliae is described in e.g., Wada et al., Biosci. Biotechnol. Biochem. 62(2): 280-285 (1998). Engineered ketoreductases with improved enzyme properties derived from Candida magnoliae ketoreductase is described in US20060195947, of which each of the ketoreductase polypeptides disclosed therein is hereby incorporated by reference herein.
In some embodiments, the ketoreductase of Saccharomyces is from Saccharomyces cerevisiae. Wild type ketoreductase from Saccharomyces cerevisiae is described in US20080248539. Engineered ketoreductases with improved enzyme properties derived from Saccharomyces cerevisiae ketoreductase are described in US20080248539. Each of the ketoreductase polypeptides disclosed therein are incorporated by reference herein.
In some embodiments, the ketoreductase of Novosphingobium is from Novosphingobium aromaticivorans. A wild type ketoreductase gene from Novosphingobium aromaticivorans is provided as GenBank accession CP000677.1, and the encoded polypeptide sequence is accession no. gi|145322460|gb|ABP64403.1|[145322460]. Engineered ketoreductases derived from Novosphingobium aromaticivorans wild type are described in U.S. provisional application 61/219,162, filed Jun. 22, 2009, which is hereby incorporated by reference herein. Exemplary engineered polynucleotides and the corresponding ketoreductase polypeptides derived from the Novosphingobium aromaticivorans wild type and having the improved property of increased activity in converting isopropanol to acetone are presented in the sequence listing incorporated herein as SEQ ID NO: 3-24. In some embodiments, the engineered ketoreductase comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24.
In some embodiments, the ketoreductase used in the process is capable of recycling cofactor by converting isopropanol (IPA) to acetone in a reaction medium of 3 to 20% IPA at a pH of about 9.0 to 10.5 with an activity at least 1.5-fold greater than the reference ketoreductase of SEQ ID NO: 2.
In some embodiments, the ketoreductase can be present in the form of whole cells, including whole cells transformed with polynucleotide constructs expressing wild type or engineered ketoreductases. In some embodiments, the ketoreductase can present as cell extracts and/or lysates thereof, and may be employed in a variety of different forms, including solid (e.g., lyophilized, spray-dried, and the like) or semisolid (e.g., a crude paste). In some embodiments of the process, the ketoreductase is isolated, and can be in substantially purified form.
In the cofactor regenerating process carried out by the ketoreductase, an alcohol is used as the substrate reductant for the generation of reduced cofactor NADH or NADPH. As noted above, while primary alcohols substrates recognized by the ketoreductase, such as ethanol, can be used, preferable are secondary alcohols, particularly lower secondary alcohols. Suitable secondary alcohols include lower secondary alkanols and aryl-alkyl carbinols. Examples of lower secondary alcohols include isopropanol, 2-butanol, 3-methyl-2-butanol, 2-pentanol, 3-pentanol, 3,3-dimethyl-2-butanol, and the like. Suitable aryl-alkyl carbinols include unsubstituted and substituted 1-arylethanols. In some embodiments, the secondary alcohol is isopropanol.
The alcohol, particularly a secondary alcohol (e.g., isopropanol), can be present at about 1% to 60% v/v, about 1% to 50% v/v, about 1% to 40% v/v, about 1% to 30% v/v, about 1% to 20% v/v, or about 1% to 10% v/v of the reaction medium. In some embodiments, the alcohol can be present at about 10% to 60% v/v, about 10% to 50% v/v, about 10% to 40% v/v, about 10% to 30% v/v, or about 10% to 20% v/v of the reaction medium. In some embodiments, the alcohol can be present at about 20% to 60% v/v, about 20% to 50% v/v, about 20% to 40% v/v, or about 20% to 30% v/v of the reaction medium. The amount of alcohol useful in the process can be determined based on the activities of the amino acid dehydrogenase and the ketoreductase in the presence of a defined amount of alcohol.
In some embodiments of the process, the product formed from the ketoreductase can be removed from the reaction medium to improve conversion of the alcohol to the corresponding carbonyl product, and thereby pushing the equilibrium of the process to the reduction of NAD+ or NADP+ to NADH or NADPH. For instance, where the carbonyl product is volatile, the product can be removed by sparging the reaction medium with an non-reactive gas or by lowering the vapor pressure of the reaction medium and removing the volatile carbonyl product. In some embodiments of the process, the alcohol substrate for the ketoreductase is a lower secondary alcohol, and the corresponding lower alkyl ketone formed from the lower secondary alcohol is removed from the reaction medium. In some embodiments, where the alcohol is isopropanol, the product acetone can be removed by sparging the reaction medium with a non-reactive gas, such as nitrogen, or by applying a vacuum to the reaction medium and removing the acetone by condensation.
Where appropriate for use in the processes, the amino acid dehydrogenases and ketoreductase enzymes present in cells, such as engineered enzymes expressed in host cells, can be recovered from the cells and or the culture medium using any one or more of the well known techniques for protein purification, including, among others, lysozyme treatment, sonication, filtration, salting-out, ultra-centrifugation, and chromatography. Suitable solutions for lysing and the high efficiency extraction of proteins from bacteria, such as E. coli, are commercially available under the trade name CelLytic BTM from Sigma-Aldrich of St. Louis Mo. The cell extracts or cell lysates may be partially purified by precipitation (ammonium sulfate, polyethyleneimine, heat treatment or the like, followed by a desalting procedure prior to lyophilization (e.g., ultrafiltration, dialysis, and the like). Any of the cell preparations may be stabilized by crosslinking using known crosslinking agents, such as, for example, glutaraldehyde or immobilization to a solid phase (e.g., Eupergit C, and the like).
The reactions described herein are generally carried out in a solvent. Suitable solvents include water, organic solvents (e.g., ethyl acetate, butyl acetate, 1-octanol, heptane, octane, methyl t-butyl ether (MTBE), toluene, and the like), and ionic liquids (e.g., 1-ethyl 4-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate, and the like). In some embodiments, aqueous solvents, including water and aqueous co-solvent systems, can be used.
Exemplary aqueous co-solvent systems have water and one or more organic solvent. In general, an organic solvent component of an aqueous co-solvent system is selected such that it does not significantly inactivate the enzymes (i.e., amino acid dehydrogenase and ketoreductase). Appropriate co-solvent systems can be readily identified by measuring the enzymatic activity of the specified enzyme with a defined substrate of interest in the candidate solvent system, utilizing an enzyme activity assay, such as those described herein.
The organic solvent component of an aqueous co-solvent system may be miscible with the aqueous component, providing a single liquid phase, or may be partly miscible or immiscible with the aqueous component, providing two liquid phases. Generally, when an aqueous co-solvent system is employed, it is selected to be biphasic, with water dispersed in an organic solvent, or vice-versa. Generally, when an aqueous co-solvent system is utilized, it is desirable to select an organic solvent that can be readily separated from the aqueous phase. In general, the ratio of water to organic solvent in the co-solvent system is typically in the range of from about 90:10 to about 10:90 (v/v) organic solvent to water, and between 80:20 and 20:80 (v/v) organic solvent to water. The co-solvent system may be pre-formed prior to addition to the reaction mixture, or it may be formed in situ in the reaction vessel.
During the course of the amino acid dehydrogenase and ketoreductase mediated process, the pH of the reaction medium may change. The pH of the reaction medium may be maintained at a desired pH or within a desired pH range by the addition of an acid or a base during the course of the reaction. Alternatively, the pH may be controlled by using an aqueous solvent that comprises a buffer. Suitable buffers to maintain desired pH ranges are known in the art and include, for example, phosphate buffer, triethanolamine buffer, and the like. Combinations of buffering and acid or base addition may also be used. Typically, bases added to unbuffered or partially buffered reaction mixtures over the course of the reduction are added in aqueous solutions.
In carrying out embodiments of the process described herein, either the oxidized or reduced form of the cofactor may be provided initially. As described above, the cofactor regenerating system converts oxidized cofactor to its reduced form (or vice-versa), which is then utilized in the reduction (or oxidation) of the substrate.
The solid reactants (e.g., enzyme, salts, etc.) may be provided to the reaction in a variety of different forms, including powder (e.g., lyophilized, spray dried, and the like), solution, emulsion, suspension, and the like. The reactants can be readily lyophilized or spray dried using methods and equipment that are known to those having ordinary skill in the art. For example, the protein solution can be frozen at −80° C. in small aliquots, then added to a pre-chilled lyophilization chamber, followed by the application of a vacuum. After the removal of water from the samples, the temperature is typically raised to about 4° C. before release of the vacuum and retrieval of the lyophilized samples.
The quantities of reactants used in the reduction reaction will generally vary depending on the quantities of product desired, and concomitantly, the amount of substrate employed. The following guidelines can be used to determine the amounts of amino acid dehydrogenase, ketoreductase, and cofactor. Those having ordinary skill in the art will readily understand how to vary these quantities to tailor them to the desired level of productivity and scale of production. Appropriate quantities of cofactor regenerating system may be readily determined by routine experimentation based on the amount of amino acid dehydrogenase and ketoreductase utilized.
The order of addition of reactants is not critical. The reactants may be added together at the same time to a solvent (e.g., monophasic solvent, biphasic aqueous co-solvent system, and the like), or alternatively, some of the reactants may be added separately, and some together at different time points.
For improved mixing efficiency when an aqueous co-solvent system is used, the amino acid dehydrogenase and the ketoreductase may be added and mixed into the aqueous phase first. The organic phase may then be added and mixed in, followed by addition of the enzyme substrates. Alternatively, the enzymes may be premixed in the organic phase, prior to addition to the aqueous phase.
Suitable conditions for carrying out the amino acid dehydrogenase/ketoreductase mediated process described herein include a wide variety of conditions which can be optimized by routine experimentation that includes, but is not limited to, contacting the enzymes and substrates at an experimental pH and temperature and detecting product, for example, using the methods described in the Examples provided herein.
Generally, the process can be carried out at a pH of about 11 or below, usually in the range of from about 8.0 to about 11. In some embodiments, the process may be carried out a neutral pH, i.e., about 7.0. The optimal pH of the reaction medium can be determined based on the pH sensitivities of the amino acid dehydrogenase and ketoreductase enzymes. In some embodiments, the process is carried out at a pH of about 9.5 or below, usually in the range of from about 8.5 to about 9.5, and in some embodiments at a pH of about 8.75 to about 9.25, and in some embodiments the process is carried out at about pH 9. In some embodiments, the process may can be carried out at a pH of about 9 to about 11, particularly at about pH 10 to about 11.
In some embodiments, the process described herein can be carried out at a temperature in the range of from about 15° C. to about 75° C. In some embodiments, the reaction is carried out at a temperature in the range of from about 20° C. to about 55° C. In some embodiments, it is carried out at a temperature in the range of from about 20° C. to about 45° C. In some embodiments, the process is carried out at a temperature of about 35° C. to about 40° C. The reaction may also be carried out under ambient conditions.
The process is generally allowed to proceed until there is no further conversion or substantial conversion of substrate (e.g., oxo acid) to the product (e.g., amino acid) or until there is essentially complete, or nearly complete, conversion of substrate to product. Conversion of substrate to product can be monitored using known methods by detecting substrate and/or product. Suitable methods include gas chromatography, HPLC, and the like. Conversion yields of the product amino acid generated in the reaction mixture can be greater than about 50%, greater than about 60%, greater than about 70%, greater than about 80%, greater than 90%, and are often greater than about 97%.
In some embodiments, the process described herein can be used in the conversion of a compound of formula I which is 3,3-dimethyl-2-oxobutanoic acid (also referred to herein as “trimethylpyruvic acid” or “TMP”) to the chiral amino acid (S)-2-amino-3,3-dimethylbutanoic acid (also referred to herein as “L-tert-leucine”), where the amino acid dehydrogenase comprises a L-leucine dehydrogenase (“LeuDH”), as illustrated by Scheme 2.




The chiral amino acid L-tert-leucine is useful in the synthesis of intermediates, particularly advanced pharmaceutical intermediate used in the preparation of drug compounds, e.g., atazanavir, boceprevir, and telaprevir.
In some embodiments, the process for producing L-tert-leucine can comprise contacting the substrate 3,3-dimethyl-2-oxobutanoic acid with a reaction medium comprising a L-leucine dehydrogenase, an ammonium ion donor, NAD+/NADH or NADP+/NADPH, and a cofactor regenerating system comprising a ketoreductase and an alcohol, under suitable reaction conditions to convert the 3,3-dimethyl-2-oxobutanoic acid to product (S)-2-amino-3,3-dimethylbutanoic acid, and the alcohol to the corresponding carbonyl compound. In particular, the alcohol is a secondary alcohol, as described herein.
In some embodiments, the L-leucine dehydrogenase used in the process can be a wild type leucine dehydrogenase or an engineered leucine dehydrogenase. L-leucine dehydrogenases can be from genus Bacillus, Clostridium, Corynebacterium, Geobacillus, Natronobacterium, Thermoactinomyces, Thermos, Thermomicrobium, or Carderia. 
In some embodiments, the leucine dehydrogenase is from Bacillus acidokaludarius, Bacillus brevis, Bacillus caldolyticus, Bacillus cereus, Bacillus megaterium, Bacillus mesentericus, Bacillus mycoides, Bacillus natto, Bacillus pumilus, Bacillus sphaericus, Bacillus stearothermophilus, Bacillus subtilis, Clostridium thermoaceticum, Corynebacterium pseudodiphtheriticum, Geobacillus stearothermophilus, Natronobacterium magadii, or Thermoactinomyces intermedius. 
The process disclosed herein could be used with any wild-type L-leucine dehydrogenase. Sequences of such wild-type enzymes are publicly available, for example from the GenBank database available at the NCBI web-site.
A few exemplary wild-type leucine dehydrogenase enzyme sequences listed by GenBank accession include: leucine dehydrogenase [Geobacillus stearothermophilus] gi|34014423|dbj|BAC81829.1|; leucine dehydrogenase [Geobacillus stearothermophilus] gi|143145|gb|M22977.1|; leucine dehydrogenase [Geobacillus stearothermophilus] gi|34014421|dbj|AB103384.1|; leucine dehydrogenase [Bacillus licheniformis] gi|1477946|gb|AAB36205.1∥bbm1385403|bbs|177171[1477946]; LEUCINE-DEHYDROGENASE [Bacillus cereus] gi|6741939|emb|CAB69610.1|[6741939]; leucine dehydrogenase [Streptosporangium roseum DSM 43021] gi|271970173|gnl|REF_jgi|Sros_8995|ref|YP_003344369.1|[271970173]; leucine dehydrogenase [Streptosporangium roseum DSM 43021] gi|270513348|gnl|jgi|Sros_8995|gb|ACZ91626.1|[270513348]; leucine dehydrogenase [Natranaerobius thermophilus JW/NM-WN-LF] gi|179351985|gnl|jgi|Nther_2701|gb|ACB86255.1|[179351985]; leucine dehydrogenase [Natranaerobius thermophilus JW/NM-WN-LF] gi|179351644|gnl|jgi|Nther_2349|gb|ACB85914.1|[179351644]; leucine dehydrogenase [Natranaerobius thermophilus JW/NM-WN-LF] gi|179350985|gnl|jgi|Nther_1681|gb|ACB85255.1|[179350985]; leucine dehydrogenase [Shewanella amazonensis SB2B] gi|119767702|gnl|jgi|Sama_2067|gb|ABM00273.1|[119767702]; leucine dehydrogenase [Shewanella sp. ANA-3] gi|117612521|gnl|jgi|Shewana3_1742|gb|ABK47975.1|[117612521]; leucine dehydrogenase [Shewanella frigidimarina NCIMB 400] gi|114334729|gnl|jgi|Sfri_2265|gb|ABI72111.1|[114334729]; leucine dehydrogenase [Shewanella sp. MR-7] gi|113888616|gnl|jgi|Shewmr7_1673|gb|ABI42667.1|[113888616]; leucine dehydrogenase [Shewanella sp. MR-4] gi|113884623|gnl|jgi|Shewmr4_1598|gb|ABI38675.1|[113884623]; leucine dehydrogenase [Chelativorans sp. BNC1] gi|110286554|gnl|jgi|Meso_3241|gb|ABG64613.1|[110286554]; leucine dehydrogenase [Pseudoalteromonas atlantica T6c] gi|109701583|gnl|jgi|Patl_2995|gb|ABG41503.1|[109701583]; leucine dehydrogenase [Rubrobacter xylanophilus DSM 9941] gi|108766512|gnl|jgi|Rxyl_2467|gb|ABG05394.1|[108766512]; leucine dehydrogenase [Chlamydophila pneumoniae TW-183] gi|33236793|gb|AAP98880.1∥gnl|byk|CpB0951[33236793]; Leucine dehydrogenase [Bacillus cereus m1293] gi|229198293|ref|ZP_04325000.1∥gnl|WGS:NZ_ACLS01|bcere0001_38230[229198293]; Leucine dehydrogenase [Bacillus cereus ATCC 10876] gi|229192377|ref|ZP_04319341.1∥gnl|WGS:NZ_ACLT01|bcere0002_40300[229192377]; Leucine dehydrogenase [Bacillus cereus BGSC 6E1] gi|229186408|ref|ZP_04313572.1∥gnl|WGS:NZ_ACLU01|bcere0004_39530[229186408]; Leucine dehydrogenase [Bacillus cereus 172560W] gi|229180445|ref|ZP_04307788.1∥gnl|WGS:NZ_ACLV01|bcere0005_37900[229180445]; Leucine dehydrogenase [Bacillus cereus MM3] gi|229174841|ref|ZP_04302361.1∥gnl|WGS:NZ_ACLW01|bcere0006_39250[229174841]; Leucine dehydrogenase [Bacillus cereus AH621] gi|229168909|ref|ZP_04296626.1∥gnl|WGS:NZ_ACLX01|bcere0007_38620[229168909]; Leucine dehydrogenase [Bacillus cereus R309803] gi|229163100|ref|ZP_04291056.1∥gnl|WGS:NZ_ACLY01|bcere0009_38690[229163100]; Leucine dehydrogenase [Bacillus cereus ATCC 4342] gi|229157746|ref|ZP_04285821.1∥gnl|WGS:NZ_ACLZ01|bcere0010_39270[229157746]; Leucine dehydrogenase [Bacillus cereus m1550] gi|229152367|ref|ZP_04280559.1∥gnl|WGS:NZ_ACMA01|bcere0011_39050[229152367]; Leucine dehydrogenase [Bacillus cereus BDRD-ST24] gi|229146739|ref|ZP_04275105.1∥gnl|WGS:NZ_ACMB01|bcere0012_38800[229146739]; Leucine dehydrogenase [Bacillus cereus BDRD-ST26] gi|229140899|ref|ZP_04269444.1∥gnl|WGS:NZ_ACMC01|bcere0013_39930[229140899]; Leucine dehydrogenase [Bacillus cereus BDRD-ST196] gi|229134979|ref|ZP_04263785.1∥gnl|WGS:NZ_ACMD01|bcere0014_38860[229134979]; and Leucine dehydrogenase [Bacillus cereus BDRD-Cer4] gi|229129445|ref|ZP_04258416.1∥gnl|WGS:NZ_ACME01|bcere0015_38880[229129445].
Leucine dehydrogenases useful with the process of the present disclosure include an L-leucine dehydrogenase from Bacillus cereus such as the enzyme disclosed in e.g., U.S. Pat. No. 5,854,035, which is hereby incorporated by reference herein.
Exemplary leucine dehydrogenases useful with process of the disclosure can also be obtained from Geobacillus stearothermophilus (e.g., gi|34014423|dbj|BAC81829.1|; gi|143145|gb|M22977.1|; gi|34014421|dbj|AB103384.11). In one embodiment, an L-leucine dehydrogenase from G. stearothermophilus useful with the present process comprises an amino acid sequence of SEQ ID NO: 26. Further suitable LeuDH enzymes useful with the present process can be engineered by site-directed mutagenesis and/or directed evolution methods using the polynucleotide of SEQ ID NO: 25, which encodes the wild-type LeuDH of SEQ ID NO: 26.
As noted above, the ammonium ion donor in the process mediated by the leucine dehydrogenase can be any suitable ammonium ion donor, which provides the NH3 for formation of the amino acid. Exemplary ammonium source includes, among others, various ammonium salts, such as ammonium halide (e.g., ammonium chloride), ammonium formate, ammonium sulfate, ammonium phosphate, ammonium nitrate, ammonium tartrate, and ammonium acetate. In particular, the process for forming L-tert-leucine with leucine dehydrogenase can use ammonium chloride as the ammonium donor.
In the process for formation of L-tert-leucine, the ketoreductase of the cofactor regenerating system can be any suitable ketoreductase capable of forming reduced cofactor NADH and/or NADPH utilizing the oxidization of the alcohol to the corresponding carbonyl compound to drive the reaction towards reduced cofactor formation. As described herein, the ketoreductase can be a wild type ketoreductase, or an engineered ketoreductase. The engineered ketoreductase can have an improved enzyme property, such as improvements in enzymatic activity, thermostability, solvent stability, pH stability, inhibitor resistance, or cofactor preference. Ketoreductases found in or derived from any number of organisms, as discussed above, can be used in conjunction with the leucine dehydrogenase.
In some embodiments, the ketoreductase is an engineered enzyme derived from Candida magnoliae, Lactobacillus kefir, Lactobacillus brevis, Lactobacillus minor, Saccharomyces cerevisiae or Novosphingobium aromaticivorans, as discussed above. Exemplary engineered ketoreductases useful with the leucine dehydrogenase in forming L-tert-leucine can comprise an amino acid sequence selected from the group consisting of SEQ ID NO: 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24.
In the leucine dehydrogenase mediated process, the substrate alcohol for the ketoreductase can comprise any alcohol that is recognized and converted by the ketoreductase to the corresponding carbonyl compound. The alcohol can be a primary alcohol or a secondary alcohol. An exemplary primary alcohol recognized by ketoreductases is ethanol. In some embodiments, the alcohol is a suitable secondary alcohol, including lower secondary alkanols and aryl-alkyl carbinols, as noted above. Exemplary secondary alcohol for use in the cofactor regenerating system with the leucine dehydrogenase includes, by way of example and not limitation, include isopropanol, 2-butanol, 3-methyl-2-butanol, 2-pentanol, 3-pentanol, 3,3-dimethyl-2-butanol, and the like, particularly isopropanol.
The alcohol, particularly a secondary alcohol, such as isopropanol, can be present at about 1% to 60% v/v, about 1% to 50% v/v, about 1% to 40% v/v, about 1% to 30% v/v, about 1% to 20% v/v, or about 1% to 10% v/v of the reaction medium. In some embodiments, the alcohol can be present at about 10% to 60% v/v, about 10% to 50% v/v, about 10% to 40% v/v, about 10% to 30% v/v, or about 10% to 20% v/v of the reaction medium. In some embodiments, the alcohol can be present at about 20% to 60% v/v, about 20% to 50% v/v, about 20% to 40% v/v, or about 20% to 30% v/v of the reaction medium. In some embodiments, the process with the leucine dehydrogenase has about 5% to 20% isopropanol, particularly 5% to 15% isopropanol. An exemplary amount of isopropanol is about 8% to 12% of the reaction medium v/v. In some embodiments, the secondary alcohol is present in at least 1.5 fold stoichiometric excess of substrate.
Generally, the process with the leucine dehydrogenase can be carried out at a pH of about 11 or below, usually in the range of from about 8.0 to about 11. In some embodiments, the process is carried out at a pH of about 9.0 or below, usually in the range of from about 8.5 to about 9.0. In some embodiments, the process for forming L-tert-leucine with leucine dehydrogenase, can be carried out at a pH of about 9 to about 11, particularly at about pH 9 to 10, more particularly at about pH 9.5. In some embodiments, the process may be carried out a neutral pH, i.e., about 7.0. It is to be understood that the pH of the reaction medium for the formation of L-tert-leucine can be determined based on the activities of the leucine dehydrogenase and ketoreductase at different pHs.
In some embodiments, the process described herein can be carried out at a temperature in the range of from about 15° C. to about 75° C. In some embodiments, the reaction is carried out at a temperature in the range of from about 20° C. to about 55° C. In some embodiments, it is carried out at a temperature in the range of from about 20° C. to about 45° C. In some embodiments, the process is carried out at a temperature of about 35° C. to about 45° C. The reaction may also be carried out under ambient conditions.
In view of the foregoing, a process for producing (S)-2-amino-3,3-dimethylbutanoic acid (also referred to herein as “L-tert-leucine”), can comprise: contacting 3,3-dimethyl-2-oxobutanoic acid (also referred to herein as “trimethyl pyruvic acid”, “TMP”, or “trimethyl pyruvate”) with a reaction medium comprising a leucine dehydrogenase, an ammonium ion donor, NAD+/NADH or NADP+/NADPH, and a cofactor recycling system comprising a ketoreductase and a lower secondary alcohol, under conditions where the 3,3-dimethyl-2-oxobutanoic acid is converted to (S)-2-amino-3,3-dimethylbutanoic acid, wherein the 3,3-dimethyl-2-oxobutanoic acid is at about 75 g/L to 125 g/L, the cofactor is at about 0.30 g/L to 0.70 g/L, and the leucine dehydrogenase and ketoreductase are each independently at about 0.5 to about 1.0 g/L.
The process for forming L-tert-leucine with a leucine dehydrogenase can be carried out with whole cells or in a cell free system. In some embodiments, the leucine dehydrogenase can be present in whole cells while the ketoreductase is present in a cell free system. Conversely, in some embodiments, the leucine dehydrogenase can be present in cell free system while the ketoreductase is present in whole cells. In some embodiments, the whole process is carried out in a cell free system, where the leucine dehydrogenase and the ketoreductase is present a crude extract or in isolated form. In some embodiments, the leucine dehydrogenase and ketoreductase is provided in substantially purified form.
In some embodiments, the process for forming L-tert-leucine of the present disclosure can be carried out wherein the leucine dehydrogenase polypeptide and/or the ketoreductase polypeptide is immobilized on a surface, for example wherein the enzyme is linked to the surface of a solid-phase particle (e.g., beads) or resin that is present in the solution or otherwise is contacted by the solution (e.g., a flow-through system in a column) Methods for linking (covalently or non-covalently) enzymes to solid-phase supports or particles (e.g., porous or non-porous beads, resins, or solid supports) such that they retain activity for use in industrial bioreactors are known in the art (see e.g., Hermanson, G. T., Bioconjugate Techniques, Second Edition, Academic Press; (2008), and “Bioconjugation Protocols: Strategies and Methods,” in Methods in Molecular Biology, C. M. Niemeyer ed., Humana Press (2004); Mateo et al., “Epoxy sepabeads: a novel epoxy support for stabilization of industrial enzymes via very intense multipoint covalent attachment,” Biotechnol Prog. 18(3):629-34 (2002); the disclosures of which are incorporated herein by reference). Useful solid supports can be in the form of beads, spheres, particles, granules, a gel, a membrane or a surface, can be porous or non-porous, can have swelling or non-swelling characteristics, can be composed of organic polymers, such as polystyrene, polyethylene, polypropylene, polyfluoroethylene, polyethyleneoxy, and polyacrylamide, as well as co-polymers and grafts thereof, or can be composed of inorganic solids, such as glass, silica, controlled pore glass (CPG), reverse phase silica or metal, such as gold or platinum. A variety of useful solid supports for the immobilization of enzymes are commercially available (e.g., SEPABEADS resins; available from Sigma-Aldrich, USA).
In some embodiments of the process using immobilized enzymes, the immobilized LeuDH and KRED polypeptides can be on different particles, which can be mixed in the reaction chamber, or in some embodiments, both enzymes can be immobilized on the same particles. In some embodiments, the methods using immobilized polypeptides can be carried out wherein the method further comprises a step of isolating or separating the immobilized enzymes from the reaction solution containing the product so that the enzymes can be recycled. In continuous flow-through embodiments, the particles comprising the immobilized LeuDH and/or KRED are maintained in a reaction chamber or column and the reacting solution flows through at a rate (and under suitable conditions) to allow for complete conversion of the substrate the reaction solution to product whereupon it exits the reaction chamber or column.
As noted above, to facilitate the reaction toward formation of product, the process of can further comprise removing from the reaction medium the carbonyl product formed from the alcohol used as the substrate to the ketoreductase. In the exemplary embodiments using isopropanol as the alcohol, the corresponding product acetone can be removed by sparging the reaction medium with an inert gas, such as nitrogen gas, or by lowering the pressure of the reaction medium and trapping the volatile acetone in a condenser.
As will be apparent to the skilled artisan, the process for forming L-tert-leucine can be varied with respect to amounts of L-leucine dehydrogenase, 3,3-dimethyl-2-oxobutanoic acid, cofactor, ketoreductase, and alcohol for the efficient conversion of the substrate to the product (S)-2-amino-3,3-dimethylbutanoic acid. In some embodiments, at a substrate loading of at about 75 g/L to 125 g/L, the process is capable of converting at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or about 100% of the substrate to the corresponding product.
In some embodiments, the present disclosure provides a process for preparing an N-protected chiral amino acid compound, where the process comprises: (i) a biocatalytic step for converting a compound of formula I to a chiral amino acid compound of formula IIa; and (ii) a chemical step where the amino acid compound of formula IIa is converted to an N-protected chiral amino acid compound.
The present disclosure contemplates that the biocatalytic step for converting a compound of formula I to a chiral amino acid compound of formula IIa may be carried out using any of the processes disclosed herein wherein the biocatalytic step comprises contacting a compound of formula I with a reaction medium comprising an amino acid dehydrogenase, an ammonium ion donor, NAD+/NADH or NADP+/NADPH, and a cofactor regenerating system comprising a ketoreductase and a lower secondary alcohol under suitable conditions where the compound of formula I is converted to the chiral amino acid compound of formula IIa and the lower secondary alcohol is converted to a ketone.
The present disclosure contemplates that the chemical step where the amino acid compound of formula IIa is converted to an N-protected chiral amino acid compound is carried out by contacting the amino acid compound of formula IIa (produced in the biocatalytic step) with a N-protecting group reagent under conditions where the N-protecting group reacts with the amine group nitrogen of the compound of formula IIa.
Examples of such N-protecting groups useful in the processes of the disclosure include the formyl group, the trityl group, the methoxytrityl group, the tosyl group, the phthalimido group, the acetyl group, the trichloroacetyl group, the chloroacetyl, bromoacetyl, and iodoacetyl groups, benzyloxycarbonyl (Cbz), methoxycarbonyl (MOC), 9-fluorenylmethoxycarbonyl (FMOC), 2-trimethylsilylethoxycarbonyl (Teoc), 1-methyl-1-(4-biphenylyl)ethoxycarbonyl (Bpoc), t-butoxycarbonyl (BOC), allyloxycarbonyl (Alloc), trihaloacetyl, benzyl, benzoyl, and nitrophenylacetyl and the like. Further examples of protecting groups useful with the embodiments of the present disclosure can be found in P. G. M. Wuts and T. W. Greene, “Greene's Protective Groups in Organic Synthesis—Fourth Edition,” John Wiley and Sons, New York, N.Y., 2007, Chapter 7 (“Greene”).
In one embodiment, the compound of formula I is trimethylpyruvic acid (TMP) and the chiral amino acid compound of formula IIa is L-tert-leucine. According to the process of the present disclosure an N-protected L-tert-leucine (e.g., an L-tert-leucine with its amine group nitrogen protected) can be prepared with e.g., a t-butoxycarbonyl (BOC), benzyloxycarbonyl (Cbz), 9-fluorenylmethoxycarbonyl (FMOC), or methoxycarbonyl (MOC) protecting group. Processes for preparing the BOC and MOC protected L-tert-Leucine are described in Examples 1-4.
Various features and embodiments of the disclosure are illustrated in the following representative examples, which are intended to be illustrative, and not limiting.
6. EXAMPLES
Example 1
Biocatalytic Process for Preparation of Enantiomerically Pure L-tert-Leucine from Trimethylpyruvic Acid (TMP)
This Example illustrates a biocatalytic process for production of enantiomerically pure L-tert-leucine from trimethylpyruvic acid at a 20 g scale using leucine dehydrogenase (LeuDH) in the presence of a ketoreductase (KRED) recycling system.
Materials
The materials used and their sources are provided in Table 5. Specific vendors and grades are given for commercially available inputs, although it is expected that the source of such reagents would not have an impact upon the reaction.







TABLE 5







Materials List










Material
Amount















Trimethylpyruvic acid
20
g



(60% aqueous solution)
(33.4
mL)



Water
118.6
mL



Ammonium hydroxide
19
mL



(28-30% aqueous solution)



Ammonium chloride
9.0
g



Isopropanol (min 99.7%)
20
mL



β - NAD+
100
mg



Leucine dehydrogenase
150
mg



(LeuDH) of SEQ ID NO: 26



Ketoreductase (KRED) of
150
mg



SEQ ID NO: 18










Summary of Reaction Conditions

A summary of the reaction conditions used in the biocatalytic process are shown below in Table 6.







TABLE 6





Reactant parameters and conditions for biocatalytic process



















Trimethylpyruvic acid (TMP)
100
g/L



Ammonium chloride
1.1
equiv










pH
9.0



IPA
10%











β - NAD+
0.50
g/L



LeuDH of SEQ ID NO: 26
0.75
g/L



KRED of SEQ ID NO: 18
0.75
g/L



Temperature
40°
C.










Reaction Protocol

A 250 mL 2-neck round bottom flask was equipped with pH probe and a magnetic stirrer. The pH probe also served as an internal thermometer. The flask was charged sequentially with 100 mL water and 20.0 g (33.4 mL) of trimethylpyruvic acid. The resulting aqueous solution was pH 1.1 (measured at 23° C.) and was stirred for 10 minutes at 400 rpm at room temperature to obtain homogeneity. The solution was cooled to 15° C. using ice water bath Ammonium hydroxide was added in portions to neutralize the acid and attain a pH between 7 and 7.5, maintaining the temperature below 20° C. during addition. Approximately 13 mL of ammonium hydroxide (28-30% aqueous solution) adjusted the pH to 7.34 at 21° C. Ammonium chloride (1.1 equiv, 9.0 g) was added and stirred until a clear solution was obtained. After addition of ammonium chloride, reaction mixture was allowed to reach room temperature and the pH at room temperature approximately 7.0. The reaction mixture was readjusted to pH 9.0 by adding ammonium hydroxide in portions, maintaining the temperature below 25° C. during addition. Approximately 6 mL of ammonium hydroxide (28-30% aqueous solution) adjusted the pH to 9.03 at 22° C. The pH was found to have a significant impact on the rate of reaction Reactions did not proceed to complete conversion in 24 h when NaOH was used to adjust pH to pH 9.5 or above. 20 mL isopropanol (IPA) was added to the reaction mixture. The pH dropped to 8.95 upon addition of IPA, but no further re-adjustment to pH 9.0 was carried out. NAD (100 mg) was charged to the stirred mixture as a powder and 18.6 mL of water as added to the reaction mixture to adjust the final volume to 200 mL. The leucine dehydrogenase (LeuDH) of SEQ ID NO: 26 (150 mg) and the engineered ketoreductase (KRED) of SEQ ID NO: 18 (150 mg) biocatalysts were charged to the stirred mixture as powders. The reaction mixture was heated to 40° C. (internal temperature) stirring at 600 rpm. The final temperature was reached within 30 min. The reaction course was followed by taking samples periodically out of the reaction mixture and analyzing as described below. For the purpose of monitoring the reaction, t=0 was the time at which the KRED was added into the reaction mixture. After in-process analyses indicated maximum conversion, the reaction was cooled to room temperature (in this Example, the reaction was stirred for a total time of 24 h). As shown by the reaction profile based on the in-process analyses (described further below) provided in Table 7 below, the biocatalytic reaction achieved nearly 88% conversion of TMP substrate to L-tert-leucine product in only 7.5 hours, and complete (i.e., 99.9%) conversion of substrate to L-tert-leucine product in 24 hours.







TABLE 7







Reaction profile










Time (h)
% Conversion














0
0



1.5
53.7



3
64.0



4.5
75.6



6
83.7



7.5
87.9



22
99.5



24
99.9










White turbidity was observed as reaction progressed and the pH of the reaction mixture after complete conversion was 8.83 at room temperature. A sample (10 mL) of the reaction mixture was taken and heated at 40° C. for an additional 24 h. No increase in impurities was observed by GC analysis.
L-tert Leucine Product Isolation and Work-Up
The reaction mixture was adjusted to pH 4.5 using 6.0 M HCl solution under controlled exothermic conditions. Celite (3.0 g) was added at room temperature and stirred to obtain homogeneity. The mixture was filtered through a sintered glass funnel and the filter cake washed with 10 mL of water. The combined aqueous solution was concentrated to about one-third the original volume. Precipitation of a white solid was observed during concentration. The suspension was cooled in an ice bath for a few hours and the solid was filtered, washed with 20 mL of a 1:1 mixture of isopropanol:water followed by 20 mL of acetonitrile. A repeat washing with 1:1 isopropanol:water (50 mL) and acetonitrile (50 mL) afforded 3.0 g (approx. yield=50%) of white solid. The solid was identified as the desired product, L-tert-leucine, with about 90% purity when compared with a standard sample of L-tert-leucine (peak area by HPLC-ELSD).
Analytical Methods
Analytical methods used in the biocatalytic process and described below include: Method 1, a GC analysis method used to monitor % conversion of trimethylpyruvic acid substrate to L-tert-leucine product; Method 2, an HPLC-UV-ELSD method to monitor trimethylpyruvic acid substrate to and L-tert-leucine product in the reaction; and Method 3, an HPLC method to determine the enantiomeric excess of L-tert-leucine formed during the course of the reaction. Results obtained by the Method 1 were verified using Method 2.
Method 1: GC Analysis Method for Monitoring % Conversion
Sample Preparation: In a 1.5 mL glass vial, samples from the reaction mixture (5 μL) were diluted in 1.0 mL of acetonitrile containing 10 μL of pyridine. Methyl chloroformate (10 μL) was added and the mixture was agitated for a few seconds. The exothermic reaction accompanied by release of CO2 was left standing until the precipitate settled. A sample (300 μL) from the supernatant of the derivatized reaction sample was taken and analyzed by gas chromatography (GC). A summary of the analytical parameters and conditions are provided in Table 8 below.










TABLE 8









Instrument
Agilent 6890N series



Column:
HP-5 (30.0 μm × 320 μm × 0.25 μm




nominal)



Temperature profile:
90° C. (hold for 2.8 min) then




ramp to 210° C. @ 20° C./min



Pressure:
12 psi (constant)











Inlet:
250°
C.










Split ratio
20:1



Detector
FID, 250° C.











Run time
8.8
min










Retention times
Trimethylpyruvic acid methyl ester




(Peak 1): 2.50 min




Derivatized L-tert-leucine




(Peak 2): 6.6 min










The % conversion based on consumption of trimethylpyruvic acid was calculated as follows:




  
    %
    ⁢
    
      
    
    ⁢
    Conversion
  
  =
  
    
      
        
          
            (
            
              Area
              ⁢
              
                
              
              ⁢
              of
              ⁢
              
                
              
              ⁢
              Peak
              ⁢
              
                
              
              ⁢
              1
            
            )
          
          
            t
            =
            0
          
        
        -
        
          
            (
            
              Area
              ⁢
              
                
              
              ⁢
              of
              ⁢
              
                
              
              ⁢
              Peak
              ⁢
              
                
              
              ⁢
              1
            
            )
          
          
            t
            =
            x
          
        
      
      
        
          (
          
            Area
            ⁢
            
              
            
            ⁢
            of
            ⁢
            
              
            
            ⁢
            Peak
            ⁢
            
              
            
            ⁢
            1
          
          )
        
        
          t
          =
          0
        
      
    
    ×
    100
  




Method 2: HPLC-UV-ELSD Analysis Method for Monitoring % Conversion

HPLC Sample Preparation:
20 μL of the reaction mixture was taken and added to 980 μL of mobile phase A. Injection is neat into the HPLC equilibrated and set up according to the analytical parameters and conditions are provided in Table 9 below.








TABLE 9







Instrument
Agilent HPLC 1200 series


Column
Mightysil RP18 GP, 250 × 4.6 mm, 5 μm



(1 x Aqua R18 guard column before &



after analytical column).


Mobile Phase
A: 76% 20 mM NH4OAc, pH 4.87 + 0.3



mM dodecyltriethylammonium phosphate



[Regis Cat No: 404021]



B: 24% MeCN









Flow rate
1.0
mL/min


Run Time
15
min


ELSD detector temperature
35°
C.


Column temperature
29°
C.


Injection volume
10
μL








Gain
3









UV Wavelength
230
nm








Retention Times
Peak 1 (L-tert-leucine): 2.725 min



Peak 2 (TMP): 13.587 min









The % conversion is calculated as follows:




  
    %
    ⁢
    
      
    
    ⁢
    Conversion
  
  =
  
    
      
        (
        
          Area
          ⁢
          
            
          
          ⁢
          of
          ⁢
          
            
          
          ⁢
          Peak
          ⁢
          
            
          
          ⁢
          1
        
        )
      
      
        [
        
          
            (
            
              Area
              ⁢
              
                
              
              ⁢
              of
              ⁢
              
                
              
              ⁢
              Peak
              ⁢
              
                
              
              ⁢
              1
            
            )
          
          +
          
            (
            
              Area
              ⁢
              
                
              
              ⁢
              of
              ⁢
              
                
              
              ⁢
              Peak
              ⁢
              
                
              
              ⁢
              2
              ×
              Response
              ⁢
              
                
              
              ⁢
              factor
            
            )
          
        
        ]
      
    
    ×
    100
  




where the Response Factor is calculated as follows:





  
    Response
    ⁢
    
      
    
    ⁢
    Factor
  
  =
  
    
      (
      
        
          Area
          ⁢
          
            
          
          ⁢
          of
          ⁢
          
            
          
          ⁢
          Peak
          ⁢
          
            
          
          ⁢
          1
          ⁢
          
            
          
          ⁢
          at
          ⁢
          
            
          
          ⁢
          time
        
        =
        x
      
      )
    
    
      [
      
        
          (
          
            
              Area
              ⁢
              
                
              
              ⁢
              of
              ⁢
              
                
              
              ⁢
              Peak
              ⁢
              
                
              
              ⁢
              2
              ⁢
              
                
              
              ⁢
              at
              ⁢
              
                
              
              ⁢
              time
            
            =
            0
          
          )
        
        -
        
          (
          
            
              Area
              ⁢
              
                
              
              ⁢
              of
              ⁢
              
                
              
              ⁢
              Peak
              ⁢
              
                
              
              ⁢
              2
              ⁢
              
                
              
              ⁢
              at
              ⁢
              
                
              
              ⁢
              time
            
            =
            x
          
          )
        
      
      ]
    
  




Method 3: HPLC Analysis of Enantiomeric Purity

HPLC Sample Preparation:
10 μL from the reaction mixture was taken and added 990 μL of mobile phase in a 2 mL HPLC glass vial. Injection is neat into the HPLC equilibrated and set up according to the analytical parameters and conditions are provided in Table 10 below.








TABLE 10







Instrument
Agilent HPLC 1200 series


Column
Phenomenex Chirex 4.6 × 150 mm (5 μm)


Mobile Phase
85% 3 mM CuSO4, 15% MeOH









Flow Rate
1.00
mL/min


Detection Wavelength
254.0
nm


Detector Temperature
25°
C.


Injection Volume
10
μL


Run time
20.0
min








Retention Times:
Peak 1 (L-tert-leucine): 11.319 min



Peak 2 (D-tert-leucine): 19.560









Example 2
800 Gram-Scale Biocatalytic Process for Preparation of Enantiomerically Pure L-tert-Leucine from Trimethylpyruvic Acid (TMP)
This Example illustrates a scaled up version of the biocatalytic process for production of enantiomerically pure L-tert-leucine from trimethylpyruvic acid as described in Example 1 to near-kilogram scale. Generally, the reaction protocol followed the same general scheme described in Example 1, only at a larger scale.
The materials used and their sources are provided in Table 11.










TABLE 11







Material
Quantity




















Trimethyl pyruvate
800
g



(TMP)



Ammonium formate
128
g



Ammonia (25% w/w)
750
mL



Isopropanol (IPA)
800
mL



NAD
4
g



LeuDH
12
g



(SEQ ID NO: 26)



KRED
12
g



(SEQ ID NO: 18)



Deionized water
5.5
L



(DIW)



Acetone
11
L



Celite 545
250
g










Reaction Protocol

A 10 L two neck round bottom flask in a water bath was charged 800 g TMP substrate in 5 L water. Overhead stirrer (equipped with flat two-blade paddle; 8 cm diameter) was started and water bath heated to 40.0° C. The ammonia was added in portions until all TMP was dissolved (approx. temperature increase of 4° C.). The flask was then charged ammonium formate and IPA. The pH was adjusted with 170 mL ammonia to pH 9.0±0.1 at 40.0° C.±0.5° C. The solution was charged with 12 g of KRED of SEQ ID NO: 18, 12 g of LeuDH of SEQ ID NO: 26, and 4 g of NAD in 500 mL water. The reaction volume was adjusted to 8 L with water as necessary. After 18 h the reaction was complete (as determined by Method 1 analysis of Example 1).
Workup Procedure
Charged 250 g Celite 545 and stir for about 15 min then filtered off Celite with a 5 L P4 fritted funnel. Distilled off ammonia, acetone, IPA and water at 50° C. under reduced pressure up to 60 mbar, until a vessel volume of 2.5 L is achieved. Charged residue in a 15 L bucket with 10 L cold acetone (4 vols) and mixed for 5 min. Closed bucket was stored overnight at 4° C. Precipitate was filtered off and washed two times with 500 mL cold acetone. Filter cake was dried at 50° C. under reduced pressure yielding 624 g L-tert-Leucine as a beige powder having >95% e.e.
The crude beige powder from this workup procedure could be immediately used in the reaction with the N-protecting groups MOC or BOC as described in Example 3, to produce the N-protected version of L-tert-leucine.
Example 3
Preparation of MOC-Protected L-tert-Leucine
This Example illustrates a process for production of a MOC-protected L-tert-leucine compound made using L-tert-leucine prepared biocatalytically using LeuDH and an engineered KRED recycling system as described in Examples 1 or 2.
Materials used in the process are shown in Table 12.










TABLE 12







Material
Quantity




















L-tert-Leucine
10.0
g



Deionized water (DIW)
80
mL



Methyl chloroformate (MOC-Cl)
14.4
g



NaOH, 50% wt/wt aqueous
21.4
g



Hydrochloric acid, 12N
12
mL



Isopropyl acetate (iPrOAc)
8.0
mL



Heptane (Hept)
125
mL



Ethyl acetate (EtOAc)
120
mL



NaCl, saturated aqueous
10
mL










Equipment and Reaction Protocol

A 300 mL round-bottomed flask fitted with a mechanical stirrer, thermocouple, and graduated addition funnel was used for the reaction, extractive workup. Temperature was controlled with a water/ice bath for below ambient temperature and a calibrated pH probe and meter used at certain stages. For isolation the vessel was fitted with a short-path distillation head.
The reaction vessel was charged with 10.0 g crude L-tert-Leucine (e.g., made as in Example 2) and 70 mL water, and the stirrer set at 150 rpm. 21.4 g of NaOH added to the addition funnel and fed dropwise into the reaction over 5 min (temperature remains <35 C) resulting in a straw-colored solution. The addition funnel was rinsed with 10 mL water and then charged with 14.4 g MOC-Cl, 14.4 g. The MOC-Cl was fed into the reaction dropwise over 30-40 min (temperature does not exceed 50° C.). HPLC was used to monitor the reaction for completion of conversion.
Workup Procedure
Upon complete conversion a pH probe was inserted and the flask allowed to cool <25 C. Approximately 12 mL of 12 N HCl was fed drop wise using the addition funnel until the pH was 2 while keeping the temperature <30° C. The reaction was charged with 60 mL ethyl acetatate which was mixed for 2 min and then layers allowed to separate (emulsions were filtered through an “M” sintered frit). The upper organic layer was removed and stored. The pH was re-adjusted to  ̃pH 3 and the extraction was repeated two more time with 40 mL then 20 mL of ethyl acetate. The extracted upper organic layers were combined and charged the 10 mL saturated NaCl with mixing for 2 min followed by layer separation. The organic layers ( ̃130 mL) were charged to a reaction vessel fitted with a distillation kit. Heat was applied to distill solvent (with 200 rpm stirring) until a vessel volume of 80 mL is achieved. Continue distillation with heptane fed dropwise to maintain 80 mL vessel volume. Continue distillation and feed until the pot temperature is  ̃98.2 C. Cool to 80° C. and feed iPrOAc, 8 mL. Cool to 65° C. and charge product seed crystals, 0.1 g. Slowly cool to ambient and then to 5 C. Collect product by filtration on an “M” sintered glass funnel; wash through with recycled liquors until all solid is collected. Wash solid with heptane, 15 mL, and suction dry. Vacuum dry to constant weight at 40° C.
Analytical Methods
HPLC was utilized for in-process analysis of % conversion and final product quality assay. H-NMR was utilized only for final product quality assay for general purity and as a check for residual solvents.
Sample Preparation:
During the reaction, with mixer on, a 200 μL aliquot is withdrawn via pipette and diluted into 3000 μL of acetonitrile/water (50:50). This sample is shaken by hand to ensure homogeneity. The sample is further diluted 1:2 prior to injection.
The analytical instrumentation and relevant parameters are shown in Table 13.








TABLE 13







Instrument
Agilent 1100


Column
TOSOH Bioscience, #19533, TSKGel Amide80



(4.6 mm × 10 cm, 5 μm)


Column temp
30° C. 


Detection
205 nm


Flow rate
   1.2 mL/min


Mobile phase
C = water;



D = acetonitrile













Timetable:
Time (min)
% C
% D






0
0.2
99.8



2.00
1
99.0



10.00
60
40



12.00
0.2
99.8








Injection volume
 10 μL


Sample Concentration
  ~5 mg/mL


Retention Times
tert-leucine: 7.8 min



MOC-tert-leucine: 1.7 min









Example 4
Preparation of BOC-Protected L-tert-Leucine
This Example illustrates a process for production of a BOC-protected L-tert-leucine compound made using L-tert-leucine prepared biocatalytically using a ketoreductase (KRED) recycling system as described in Examples 1 or 2.
Materials used in the process are shown in Table 14.










TABLE 14







Material
Quantity




















L-tert-Leucine
10
g



Deionized water (DIW)
70
mL



Triethylamine (TEA)
7.71
g



di-tert-butyl dicarbonate (BOC2O)
20.8
g



NaOH, 50% wt/wt aqueous
9.15
g



Tetrahydrofuran (THF)
30
mL



Heptane
285
mL



Ethyl acetate
145
mL



NaCl, saturated aqueous
10
mL










Equipment and Reaction Protocol

Reaction Vessel:
A 300 mL round-bottomed flask fitted with a mechanical stirrer, thermocouple, and graduated addition funnel was used for the reaction and extractive workup. Temperature was controlled with a water/ice bath for below ambient temperature. A calibrated pH probe and meter was used at certain stages.
Isolation Vessel:
A 500 mL round-bottomed flask fitted with a mechanical stirrer, thermocouple with temperature controller and heating mantle, graduated addition funnel, and distillation kit was used for the solvent exchange and crystallization.
Reaction vessel was charged 10.0 g crude L-tert-Leucine product (as made in Example 2), and 70 mL to the reaction vessel and set stirrer at 150 rpm. Solution was a suspension. Charged TEA, 7.71 g, over 5 min so that T remains <25° C., which resulted in a straw-colored solution. Charged reaction vessel with NaOH, 4.57 g, over 2 min, resulting in an opaque solution. Prepared a solution of BOC2O, 20.8 g, in THF, 30 mL, and fed dropwise to reaction vessel over 30-40 min, without allowing temperature to exceed 32° C. The reaction was monitored for complete conversion by HPLC as described in Example 3, except that retention time for BOC-tert-leucine was 1.5 min.
Workup Procedure
Checked pH and adjusted with 50% NaOH to pH>9.2. Charged vessel with heptanes (30 mL) with mixing. Upper organic layer was removed and a pH probe and flask cooled to <25° C. before charging with 32 mL 3 N HCl dropwise (pH ̃3.5) and keeping temperature <30° C. Remove cooling and charged with ethyl acetate (50 mL) followed by mixing for 2 min and removal of upper organic layer. pH was readjusted to  ̃3.5, if necessary, and the vessel charged with ethyl acetate (70 mL) and heptane (30 mL). Upper organic layer removed and stored. Charged again with ethyl acetate (25 mL) and heptane (25 mL). Upper organic layer removed and stored. All organic phases were combined and filtered through an “M” sintered frit if an emulsion were present.
The combined organic phases were charged with sat'd NaCl, 10 mL, mixed for 2 min and allowed to separate. Lower layer was removed ( ̃235 mL) and charged to a crystallizer fitted with a distillation kit. Heat was applied to distill solvent until a vessel volume of 100 mL is achieved. Continue distillation and begin feeding heptanes dropwise so as to maintain 100 mL vessel volume. Continue distillation and feed until the pot temperature 98° C. Cool to 5° C. and collect product by filtration on an “M” sintered glass funnel. Wash through with recycled liquors until all solid is collected. Wash solid with pre-cooled (5° C.) heptane, 15 mL. Vacuum dry to constant weight at 40° C.
All publications, patents, patent applications and other documents cited in this application are hereby incorporated by reference in their entireties for all purposes to the same extent as if each individual publication, patent, patent application or other document were individually indicated to be incorporated by reference for all purposes.
While various specific embodiments have been illustrated and described, it will be appreciated that various changes can be made without departing from the spirit and scope of the invention(s).

## Claims

1. A process for converting a compound of formula I which is a substrate for an amino acid dehydrogenase to a chiral amino acid of formula IIa:




comprising contacting the compound of formula I with a reaction medium comprising an amino acid dehydrogenase, an ammonium ion donor selected from ammonia or an ammonium salt, a cofactor selected from NAD+/NADH or NADP+/NADPH, and a cofactor regenerating system comprising an engineered ketoreductase having at least 90% sequence identity to SEQ ID NO: 18, which has less than 5% of the activity of the amino acid dehydrogenase, with compound of formula I and a lower secondary alcohol, under conditions where the compound of formula I is converted to the chiral amino acid of formula IIa and the lower secondary alcohol is converted to a ketone, wherein R is a substituted or unsubstituted —(C1-C10)alkyl, —(C2-C6)alkenyl, —(C2-C6)alkynyl, —(C3-C8)cycloalkyl, heterocycloalkyl, aryl, or heteroaryl and wherein the said ketoreductase is capable of recycling cofactor by converting isopropanol (IPA) to acetone with an activity at least 2.0-fold greater than a reference ketoreductase of SEQ ID NO: 2.

2. The process of claim 1, wherein the amino acid dehydrogenase is an L-amino acid dehydrogenase is selected from L-alanine dehydrogenase, L-aspartate dehydrogenase, L-erythro-3,5-diaminohexanoate dehydrogenase, L-leucine dehydrogenase, L-glutamate dehydrogenase, lysine dehydrogenase, L-phenylalanine dehydrogenase, L-serine dehydrogenase, L-valine dehydrogenase, L-2,4-diaminopentanoate dehydrogenase, L-glutamate synthase, L-diaminopimelate dehydrogenase, L-N-methylalanine dehydrogenase, L-lysine 6-dehydrogenase, and L-tryptophan dehydrogenase.

3. The process of claim 1, wherein the lower secondary alcohol is isopropanol and the ketone is acetone.

4. The process of claim 1, wherein the reaction medium is at a pH of about 8.5 to about 10.

5. The process of claim 1, wherein the reaction medium is at a temperature of about 35° C. to about 40° C.

6. The process of claim 1, wherein the secondary alcohol is present in at least 1.5 fold stoichiometric excess of substrate.

7. The process of claim 1, wherein the ketoreductase is capable of recycling cofactor by converting isopropanol (IPA) to acetone in a reaction medium of 3 to 20% IPA at a pH of about 9.0 to 10.5 with an activity at least 5.0-fold greater than a reference ketoreductase of SEQ ID NO:2.

8. The process of claim 1, wherein the activity of the ketoreductase with compound of formula I is less than 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the activity of the amino acid dehydrogenase with the compound of formula I, or optionally no activity with the compound of formula I.

9. The process of claim 1, wherein the compound of formula IIa is 3,3-dimethyl-2-oxobutanoic acid and the product of formula I is (S)-2-amino-3,3-dimethylbutanoic acid and the amino acid dehydrogenase comprises a L-leucine dehydrogenase.

10. The process of claim 9, wherein the leucine dehydrogenase is engineered leucine dehydrogenase or is derived from a wild-type leucine dehydrogenase from Bacillus, Clostridium, Corynebacterium, Geobacillus, Natronobacterium, Thermoactinomyces, Thermos, Thermomicrobium, or Carderia.

11. The process of claim 10, wherein the leucine dehydrogenase comprises the amino acid sequence of SEQ ID NO: 26.

12. A process for producing (S)-2-amino-3,3-dimethylbutanoic acid, comprising: contacting 3,3-dimethyl-2-oxobutanoic acid with an enantioselective leucine dehydrogenase in a reaction medium comprising an ammonium ion donor, cofactor NAD+/NADH or NADP+/NADPH, and a cofactor recycling system comprising a ketoreductase and a lower secondary alcohol, under conditions where the 3,3-dimethyl-2-oxobutanoic acid is converted to (S)-2-amino-3,3-dimethylbutanoic acid,
wherein the 3,3-dimethyl-2-oxobutanoic acid is at about 75 g/L to 125 g/L, the secondary alcohol is about 1.5 fold stoichiometric excess of the 3,3-dimethyl-2-oxobutanoic acid, the cofactor is at about 0.30 g/L to 0.70 g/L, and the leucine dehydrogenase and ketoreductase are each independently at about 0.5 to about wherein the said ketoreductase having at least 90% sequence identity to SEQ ID NO: 18 and capable of recycling cofactor by converting isopropanol (IPA) to acetone with an activity at least 2.0-fold greater than a reference ketoreductase of SEQ ID NO: 2.

13. The process of claim 12, wherein the secondary alcohol is isopropanol, wherein the isopropanol is at about 7% to 12% volume of the reaction medium by (weight/volume).

14. A process for converting a compound mixture of formula IId which comprises a substrate for an amino acid dehydrogenase to a composition of formula I and a chiral amino acid of formula IIa:



comprising contacting the compound mixture of formula IId with an enantioselective amino acid leucine dehydrogenase in a reaction medium comprising NAD+/NADH or NADP+/NADPH and a cofactor recycling system comprising a ketoreductase and a lower alkyl ketone, under conditions where the compound mixture of formula IId is converted to the composition of formula I and a chiral amino acid of formula IIa, and the lower alkyl ketone is converted to a lower secondary alcohol; wherein R is a substituted or unsubstituted —(C1-C10)alkyl, —(C2-C6)alkenyl, —(C2-C6)alkynyl, —(C3-C8)cycloalkyl, heterocycloalkyl, aryl, or heteroaryl, wherein the said ketoreductase having at least 90% sequence identity to SEQ ID NO: 18 and capable of recycling cofactor by converting isopropanol (IPA) to acetone with an activity at least 2.0-fold greater than a reference ketoreductase of SEQ ID NO: 2.

15. The process of claim 1, wherein the process further comprises: (i) a biocatalytic step comprising contacting a compound of formula I with a reaction medium comprising an amino acid dehydrogenase, an ammonium ion donor, NAD+/NADH or NADP+/NADPH, and a cofactor regenerating system comprising the ketoreductase of the claim 1 and a lower secondary alcohol under suitable conditions where the compound of formula I is converted to a chiral amino acid compound of formula IIa and the lower secondary alcohol is converted to a ketone; and (ii) a chemical step comprising contacting the chiral amino acid compound of formula IIa with a compound comprising an N-protecting group under conditions where the N-protecting group reacts with the chiral amino acid compound of formula IIa to form an N-protected chiral amino acid compound.

16. The process of claim 15, wherein the N-protecting group is selected from Cbz, FMOC, BOC and MOC.