Patent Publication Number: US-9422583-B2

Title: Stereospecific carbonyl reductases

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of U.S. application Ser. No. 13/379,942, filed Jul. 10, 2012, now abandoned, which is a 35 U.S.C. §371 application of International Application No. PCT/US2010/039666, filed Jun. 23, 2010, which claims the benefit of U.S. Provisional Application Ser. No. 61/219,610, filed Jun. 23, 2009, now expired. The entire content of the applications referenced above are hereby incorporated by reference herein. 
    
    
     STATEMENT OF GOVERNMENT SUPPORT 
     This invention was made with government support under U54 GM074958 awarded by the National Institutes of General Medical Science, Protein Structure Initiative program. The government has certain rights in the invention. 
    
    
     SEQUENCE LISTING 
     The instant application contains a Sequence Listing which has been submitted electronically in ASCII format and is hereby incorporated by reference in its entirety. Said ASCII copy is named 08035.030US1_SL.txt and is 15,864 bytes in size. 
     BACKGROUND 
     The NAD(P)H-dependent carbonyl reductases catalyze reduction of a variety of endogenous and xenobiotic carbonyl compounds, including biologically and pharmacologically active substrates (Forrest et al.,  Chem. Biol. Interact.,  129, 21-40 (2000)). There is considerable interest in the use of carbonyl reductases in the pharmaceutical and fine chemicals industries for the production of chiral alcohols, which are important building blocks for the synthesis of chirally-pure compounds, e.g., pharmaceutical agents (Panke et al.,  Curr. Opin. Biotechnol.,  15, 272-279 (2004); Schmid et al.,  Nature,  409, 258-268 (2001); and Schoemaker et al.,  Science,  299, 1694-1697 (2003)). For such chiral auxiliaries, production from their corresponding prochiral ketones, the use of carbonyl reductases has advantages over chemo-catalysts in terms of their highly chemo-, enantio-, and regioselectivities. These features make stereospecific carbonyl reductases very useful from both scientific and industrial perspectives (Kroutil et al.,  Curr. Opin. Chem. Biol.,  8, 120-126 (2004)). However, the range of current applications for stereospecific carbonyl reductases remains modest. This can be attributed to several limitations, including the stereospecificity and availability of enzymes. In addition, research on molecular mechanisms of oxidoreductases is still in its infancy. Further, most enzymes that can catalyze asymmetric reductions generally follow Prelog&#39;s rule in terms of stereochemical outcomes (Bradshaw et al.,  J. Org. Chem.,  57, 1526-1532 (1992); Ernst et al.,  Appl. Microbiol. Biotechnol.,  66, 629-634 (2005); Niefind et al.,  J. Mol. Biol.,  327, 317-328 (2003); Prelog,  Pure Appl. Chem.,  9, 119-130 (1964)). Enzymes with anti-Prelog stereospecificity are quite rare, and only few have been isolated and characterized in purified forms (De Wildeman et al.,  Acc. Chem. Res.  40, 1260-1266, (2007)). Accordingly, stereospecific carbonyl reductases are needed. In particular, stereospecific carbonyl reductases with anti-Prelog stereospecificity are needed. 
     SUMMARY OF CERTAIN EMBODIMENTS OF THE INVENTION 
     Accordingly, as described herein, three stereospecific carbonyl reductase genes (scr1, scr2, and scr3) from  C. parapsilosis  have been discovered. These genes have been cloned and expressed, and the encoded proteins purified to homogeneity and confirmed to function as stereospecific carbonyl reductases (SCR1, SCR2, and SCR3). These stereospecific carbonyl reductases have anti-Prelog selectivity and convert 2-hydroxyacetophenone to (S)-1-phenyl-1,2-ethanediol (PED). These oxidoreductases have useful specificities that are useful for fine biochemical synthesis. 
     Application of biocatalysis in the synthesis of chiral molecules is one of the greenest technologies for the replacement of chemical routes. This is due to environmentally benign reaction conditions for biocatalysis and unparalleled chemo-, regio- and stereoselectivities. The newly identified stereospecific carbonyl reductases (SCRs) showed high catalytic activities for producing (S)-1-phenyl-1,2-ethanediol (PED) from 2-hydroxyacetophenone with NADPH as the coenzyme. The enzymes from this cluster are carbonyl reductases with novel anti-Prelog stereoselectivity. Of the enzymes encoded in the gene cluster, SCR1 and SCR3 exhibited distinct specificities to acetophenone derivatives and chloro-substituted 2-hydroxyacetophenones, and especially very high activities to ethyl 4-chloro-3-oxobutyrate, which affords ethyl 4-chloro-3-hydroxybutyrate, a precursor of the chiral side chain in the synthesis of atorvastatin (Lipitor®) and rosuvastatin, e.g., rosuvastatin calcium (Crestor®). 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1 . Map of contig005802 of  Candida parapsilosis  genome including the four open reading frames, scr1, scr2, scr3, and cpadh. 
         FIG. 2 . Amino acid sequence alignment of CPADH (GenBank accession number DQ675534; SEQ ID NO:1), SCR1 (GenBank accession number FJ939565; SEQ ID NO:4), SCR2 (GenBank accession number FJ939563; SEQ ID NO:3), and SCR3 (GenBank accession number FJ939564; SEQ ID NO:2) from  C. parapsilosis . Gaps in the aligned sequences are indicated by dashes. Identical amino acid residues are enclosed in boxes. The conserved sequences of the cofactor-binding motif Gly-X-X-X-Gly-X-Gly (SEQ ID NO:9) and the catalytic tetrad of Asn-Ser-Tyr-Lys (SEQ ID NO:10) in the majority of SDRs are marked with arrows. 
         FIG. 3 . Analysis of the overexpression of SCR1, SCR2, and SCR3. The proteins were separated on a 12% SDS-polyacrylamide gel and stained with Coomassie Brilliant Blue G-250. Lane 1, total protein for SCR1; Lane 2, soluble fraction for SCR1; Lane 3, total protein for SCR2; Lane 4, soluble fraction for SCR2; Lane 5, total protein for SCR3; Lane 6, soluble fraction for SCR3; Lane 7, molecular mass standard. 
         FIG. 4 . SDS-PAGE analysis of purified enzymes. The purified proteins were resolved by SDS-PAGE on a 12% polyacrylamide gel and stained with Coomassie Brilliant Blue G-250. Lane 1, molecular mass standard; Lane 2, purified SCR1; Lane 3, purified SCR2; Lane 4, purified SCR3. 
         FIG. 5 . pH dependence of SCR1, SCR2, and SCR3 catalyzing 2-hydroxyacetophenone reduction. The enzyme activities of SCR1 (squares), SCR2 (triangles), and SCR3 (circles) were measured in 0.1 M acetate buffer (pH 4.0 to 6.0) or 0.1 M sodium phosphate buffer (pH 6.0 to 8.0) or 0.1 M Tris-HCl buffer (pH 8.0 to 8.5) with 2-hydroxyacetophenone as the substrate and NADPH as the cofactor. Maximal enzyme activity observed was set as 100% relative activity for each enzyme. 
         FIG. 6A-6E . Asymmetric reduction of 2-hydroxyacetophenone (2-HAP) to 1-phenyl-1,2-ethanediol (PED) enantiomer by SCR1, SCR2, and SCR3, respectively. ( 6 A) Standard sample of (R)-PED. ( 6 B) Standard sample of (S)-PED. ( 6 C) SCR1 catalyzed asymmetric reduction of 2-HAP. ( 6 D) SCR2 catalyzed asymmetric reduction of 2-HAP. ( 6 E) SCR3 catalyzed asymmetric reduction of 2-HAP. 
         FIG. 7A-7D . Substrate specificity of SCR1 and SCR3. The enzyme activities of SCR1 (open bars) and SCR3 (shaded bars) ( 7 A) to various substrates ( 7 B- 7 D) were measured as described herein. Maximal enzyme activity observed was set as 100% relative activity for the enzymes to various substrates. 
     
    
    
     DETAILED DESCRIPTION 
     Certain embodiments of the present invention provide a purified polypeptide, the sequence of which comprises an amino acid sequence that has at least 70% identity to a  Candida parapsilosis  stereospecific carbonyl reductase, wherein the polypeptide has carbonyl reductase activity and does not comprise SEQ ID NO:1. 
     In certain embodiments, the amino acid sequence has at least 70% identity to at least one of the  Candida parapsilosis  stereospecific carbonyl reductases represented by SEQ ID NO:2, SEQ ID NO:3 or SEQ ID NO:4. 
     In certain embodiments, the amino acid sequence has at least 70% identity to SEQ ID NO:2. 
     In certain embodiments, the amino acid sequence has at least 70% identity to SEQ ID NO:3. 
     In certain embodiments, the amino acid sequence has at least 70% identity to SEQ ID NO:4. 
     In certain embodiments, the amino acid sequence has at least 75% identity to the  Candida parapsilosis  stereospecific carbonyl reductase (e.g., to at least one of SEQ ID NO:2, SEQ ID NO:3 or SEQ ID NO:4). 
     In certain embodiments, the amino acid sequence has at least 80% identity to the  Candida parapsilosis  stereospecific carbonyl reductase (e.g., to at least one of SEQ ID NO:2, SEQ ID NO:3 or SEQ ID NO:4). 
     In certain embodiments, the amino acid sequence has at least 85% identity to the  Candida parapsilosis  stereospecific carbonyl reductase (e.g., to at least one of SEQ ID NO:2, SEQ ID NO:3 or SEQ ID NO:4). 
     In certain embodiments, the amino acid sequence has at least 90% identity to the  Candida parapsilosis  stereospecific carbonyl reductase (e.g., to at least one of SEQ ID NO:2, SEQ ID NO:3 or SEQ ID NO:4). 
     In certain embodiments, the amino acid sequence has at least 95% identity to the  Candida parapsilosis  stereospecific carbonyl reductase (e.g., to at least one of SEQ ID NO:2, SEQ ID NO:3 or SEQ ID NO:4). 
     In certain embodiments, the amino acid sequence has at least 99% identity to the  Candida parapsilosis  stereospecific carbonyl reductase (e.g., to at least one of SEQ ID NO:2, SEQ ID NO:3 or SEQ ID NO:4). 
     In certain embodiments, the amino acid sequence comprises SEQ ID NO:9, SEQ ID NO:10 or SEQ ID NO:11. 
     In certain embodiments, the amino acid sequence comprises SEQ ID NO:9, SEQ ID NO:10 and SEQ ID NO:11. 
     In certain embodiments, the amino acid sequence comprises SEQ ID NO:2. 
     In certain embodiments, the amino acid sequence comprises SEQ ID NO:3. 
     In certain embodiments, the amino acid sequence comprises SEQ ID NO:4. 
     In certain embodiments, the sequence of the polypeptide consists essentially of, or consists of, SEQ ID NO:2, SEQ ID NO:3 or SEQ ID NO:4. 
     In certain embodiments, the carbonyl reductase activity of the polypeptide is NADPH-dependent. 
     In certain embodiments, the polypeptide is an anti-Prelog-type stereospecific carbonyl reductase. 
     Certain embodiments of the present invention provide composition comprising the polypeptide as described herein. 
     Certain embodiments of the present invention provide an isolated nucleic acid sequence comprising a sequence that encodes a polypeptide described herein. 
     In certain embodiments, the sequence comprises SEQ ID NO:6 or of a degenerate variant of SEQ ID NO:6. 
     In certain embodiments, the sequence comprises SEQ ID NO:7 or of a degenerate variant of SEQ ID NO:7. 
     In certain embodiments, the sequence comprises SEQ ID NO:8 or of a degenerate variant of SEQ ID NO:8. 
     In certain embodiments, the sequence encodes SEQ ID NO:2. 
     In certain embodiments, the sequence encodes SEQ ID NO:3. 
     In certain embodiments, the sequence encodes SEQ ID NO:4. 
     Certain embodiments of the present invention provide an expression vector comprising an expression cassette operably linked to a nucleic acid molecule as described herein. 
     Certain embodiments of the present invention provide a host cell comprising a vector as described herein. 
     Certain embodiments of the present invention provide a method of reducing a carbonyl substrate, comprising contacting the substrate with a polypeptide described herein, or a composition described herein, in conditions suitable to catalyze the reduction of the carbonyl substrate. As used herein, a “carbonyl substrate” is a substrate that comprises at least one carbonyl group, such as a compound that comprises an α-ketoester, a β-ketoester, an aryl ketone or an aliphatic ketone (see, e.g.,  FIG. 7 ). The polypeptide having carbonyl reductase activity reduces a carbonyl group of the carbonyl substrate. 
     In certain embodiments, the reduction takes place in the presence of a coenzyme. 
     In certain embodiments, the coenzyme is NADPH. 
     In certain embodiments, the carbonyl substrate comprises an α-ketoester, a β-ketoester, an aryl ketone or an aliphatic ketone. 
     In certain embodiments, the carbonyl substrate comprises an α-ketoester. 
     In certain embodiments, the α-ketoester is methyl pyruvate, methyl phenylglyoxylate, ethyl pyruvate or ethyl benzoylformate. 
     In certain embodiments, the carbonyl substrate comprises a β-ketoester. 
     In certain embodiments, the β-ketoester is ethyl trifluoroacetoacetate, methyl acetoacetate, methyl 3-oxovalerate, methyl 4-fluorobenzoylacetate, ethyl acetoacetate, ethyl 3-oxovalerate, ethyl 4-chloroacetoacetate, ethyl benzoylacetate, or ethyl 3,4-dimethoxybenzoylacetate. 
     In certain embodiments, the carbonyl substrate comprises an aryl ketone. 
     In certain embodiments, the aryl ketone is 2-hydroxyacetophenone, or a derivative thereof. 
     In certain embodiments, the aryl ketone is 2′-chloro-2-hydroxyacetophenone, 3′-chloro-2-hydroxyacetophenone, 4′-chloro-2-hydroxyacetophenone or 4′-methoxy-2-hydroxyacetophenone. 
     In certain embodiments, the carbonyl substrate comprises an aliphatic ketone. 
     In certain embodiments, the aliphatic ketone is 2-butanone, 2-pentanone, 2-hexanone, 2-heptanone, 2-octanone or 2-methyl-3-pentanone 
     In certain embodiments, the carbonyl substrate is ethyl 4-chloro-3-oxobutyrate. 
     In certain embodiments, the reduction takes place at pH ranging from 5.0 to 6.0 (e.g., at about 5.0, 5.5 or 6.0). 
     Described herein is a new gene cluster of enantioselective oxidoreductases with unusual stereospecificity in  C. parapsilosis . It was confirmed that these genes code for three unique stereospecific carbonyl reductases through cloning, expression, and purification of the corresponding gene products, and verification of enantiomer configuration of the enzymatic products of asymmetric reduction of prochiral carbonyl groups of multiple substrates. SCR1, SCR2, and SCR3 all exhibit a novel anti-Prelog stereospecificity in reducing prochiral carbonyl groups; e.g., forming (S)-1-phenyl-1,2-ethanediol from the corresponding ketone substrate, 2-hydroxyacetophenone. The enzymes are, however, distinct in their catalytic properties, including their pH dependency and substrate specificity spectrum. 
     According to catalytic properties and primary structure information, stereospecific oxidoreductases, including alcohol dehydrogenases and carbonyl reductases, are mainly classified into three different groups, the zinc-dependent alcohol dehydrogenase, the short-chain dehydrogenase/reductase (SDR), and the aldo-keto reductase (AKR) (Kamitori et al.,  J. Mol. Biol.,  352, 551-558 (2005); Reid and Fewson,  Crit. Rev. Microbiol.,  20, 13-56 (1994)). These proteins share sequence motifs characteristic of the SDR superfamily, including the cofactor-binding motif Gly-X-X-X-Gly-X-Gly (X denotes any amino acid; SEQ ID NO:9), the catalytic triad of Ser-Tyr-Lys (SEQ ID NO:11), and also the extended tetrad of Asn-Ser-Tyr-Lys (SEQ ID NO:10) observed in the majority of SDRs (Filling et al.,  J. Biol. Chem.  277, 25677-25684 (2002)). In addition, the SCRs also have the conserved sequence motifs of secondary structural elements and key positions for assignment of coenzyme specificity of the cP2 subfamily in classic SDRs, except that the conserved basic residue K/R responsible for binding phosphate group in NADPH is replaced by weak basic residue H (Kallberg et al.,  Eur. J. Biochem.,  269, 4409-4417 (2002)). These highly-conserved, characteristic sequence motifs indicate that the SCRs belong to the cP2 subfamily of the classical SDR superfamily, one of the three NADPH-dependent subfamilies (Kallberg et al.,  Eur. J. Biochem.,  269, 4409-4417 (2002)). 
     Oxidoreductases perform a wide variety of asymmetric reductions, differing in stereospecificity and substrate specificity, and have been used for producing optically active alcohols from various prochiral ketones, ketoacids, and ketoesters. The SCRs catalyze (S)-specific reduction of 2-hydroxyacetophenone, an anti-Prelog type reaction (Manzocchi et al.,  J. Org. Chem.,  53, 4405-4407, (1988); Prelog,  Pure Appl. Chem.,  9, 119-130 (1964)). Therefore, these new enzymes complement the stereospecific oxidoreductases described to date for catalysis of the reduction of prochiral carbonyl compounds to the corresponding optically pure alcohols with anti-Prelog stereopreference. Additionally, the finding of stereospecific carbonyl reductases from the same host provides profound knowledge on the reaction mechanism of  C. parapsilosis  whole-cell mediated stereoinversion, involving the oxidation step of (R)-PED to the intermediate (2-hydroxyacetophenone) and the reduction step of the intermediate to (S)-PED (Gruber et al.,  Adv. Synth. Catal.,  348, 1789-1805 (2006); Nie et al.,  Org. Process Res. Dev.,  8, 246-251 (2004); Nie et al.,  Appl. Environ. Microbiol.,  73, 3759-3764 (2007); Voss et al.,  Angew. Chem. Int. Ed.,  47, 741-745 (2008); Voss et al.,  J. Am. Chem. Soc.,  130, 13969-13972 (2008)). It is worthy to note that SCR1 catalyzes the reduction of a broad spectrum of ketones including aryl, aliphatic ketones, α- and β-ketoesters, and shows a particular highest substrate specificity towards ethyl 4-chloro-3-oxobutyrate, a precursor for the synthesis of an important pharmaceutical intermediate. Therefore, the new discovered stereospecific carbonyl reductases will be useful enzymes with application potential. 
     The discovery of novel stereospecific carbonyl reductases of anti-Prelog selectivity further demonstrates the diversity of stereospecific oxidoreductases in microorganisms. Such enzymes provide a basis for elucidating the molecular mechanisms of enzyme-mediated asymmetric reactions involving stereo-recognition between proteins and chiral molecules, and mechanisms of electron transfer between functional groups of chiral molecules and key amino acid residues in enzymes. Apart from their unique value in studies of mechanisms of stereospecific oxidoreduction reactions, these novel carbonyl reductases of anti-Prelog stereopreference, have multiple potential uses in industrial applications to produce chiral alcohols useful as intermediates in fine chemical synthesis. 
     In some embodiments of the invention, the carbonyl reductase can catalyze asymmetric reduction of 2-hydroxyacetophenone into (S)-1-phenyl-1,2-ethanediol (PED) (Nie et al.,  Appl. Environ. Microbiol.,  73, 3759-3764 (2007)), a versatile chiral building block for the synthesis of pharmaceuticals, agrochemicals, and liquid crystals. PED is also a precursor for the production of chiral biphosphines and a chiral initiator for stereoselective polymerization (Iwasaki et al.,  Org. Lett.,  1, 969-972 (1999); Liese et al.,  Biotechnol. Bioeng.,  51, 544-550 (1996)). 
     In some embodiments, the carbonyl reductase can catalyze the reduction of a compound that comprises an aryl ketone, an aliphatic ketone, an α-ketoester, or β-ketoester. In some embodiments, the carbonyl reductase catalyzes the reduction of an aryl ketone. In some embodiments, the carbonyl reductase catalyzes the reduction of an aliphatic ketone. In some embodiments, the carbonyl reductase catalyzes the reduction of an α-ketoester. In some embodiments, the carbonyl reductase catalyzes the reduction of β-ketoester. 
     The term “nucleic acid” refers to deoxyribonucleotides or ribonucleotides and polymers thereof in either single or double stranded form, made of monomers (nucleotides) containing a sugar, phosphate and a base that is either a purine or pyrimidine. Unless specifically limited, the term encompasses known analogs of natural nucleotides that have similar binding properties as the reference nucleic acid and are metabolized in a manner similar to naturally occurring nucleotides. Unless otherwise indicated, a particular nucleic acid sequence also encompasses conservatively modified variants thereof (e.g., degenerate codon substitutions) and complementary sequences, as well as the sequence explicitly indicated. Specifically, degenerate codon substitutions may be achieved by generating sequences in which the third position of one or more selected (or all) codons is substituted with mixed-base and/or deoxyinosine residues. 
     The term “nucleotide sequence” refers to a polymer of DNA or RNA which can be single-stranded or double-stranded, optionally containing synthetic, non-natural or altered nucleotide bases capable of incorporation into DNA or RNA polymers. The terms “nucleic acid,” “nucleic acid molecule,” and “polynucleotide” are used interchangeably. 
     Certain embodiments of the invention encompass compositions that comprise isolated or substantially purified nucleic acid. In the context of the present invention, an “isolated” or “purified” DNA molecule or RNA molecule is a DNA molecule or RNA molecule that exists apart from its native environment and is therefore not a product of nature. An isolated DNA molecule or RNA molecule may exist in a purified form or may exist in a non-native environment such as, for example, a transgenic host cell. For example, an “isolated” or “purified” nucleic acid molecule is substantially free of other cellular material or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized. In one embodiment, an “isolated” nucleic acid is free of sequences that naturally flank the nucleic acid (i.e., sequences located at the 5′ and 3′ ends of the nucleic acid) in the genomic DNA of the organism from which the nucleic acid is derived. 
     The following terms are used to describe the sequence relationships between two or more nucleic acids or polynucleotides: (a) “reference sequence,” (b) “comparison window,” (c) “sequence identity,” (d) “percentage of sequence identity,” and (e) “substantial identity.” 
     (a) As used herein, “reference sequence” is a defined sequence used as a basis for sequence comparison. A reference sequence may be a subset or the entirety of a specified sequence; for example, as a segment of a full-length cDNA or gene sequence, or the complete cDNA or gene sequence. 
     (b) As used herein, “comparison window” makes reference to a contiguous and specified segment of a polynucleotide sequence, wherein the polynucleotide sequence in the comparison window may comprise additions or deletions (i.e., gaps) compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. Generally, the comparison window is at least 20 contiguous nucleotides in length, and optionally can be 30, 40, 50, 100, or longer. Those of skill in the art understand that to avoid a high similarity to a reference sequence due to inclusion of gaps in the polynucleotide sequence a gap penalty is typically introduced and is subtracted from the number of matches. 
     Methods of alignment of sequences for comparison are well-known in the art. Thus, the determination of percent identity between any two sequences can be accomplished using a mathematical algorithm. Non-limiting examples of such mathematical algorithms are the algorithm of Myers and Miller (Myers and Miller,  CABIOS,  4, 11 (1988)); the local homology algorithm of Smith et al. (Smith et al.,  Adv. Appl. Math.,  2, 482 (1981)); the homology alignment algorithm of Needleman and Wunsch (Needleman and Wunsch,  JMB,  48, 443 (1970)); the search-for-similarity-method of Pearson and Lipman (Pearson and Lipman,  Proc. Natl. Acad. Sci. USA,  85, 2444 (1988)); the algorithm of Karlin and Altschul (Karlin and Altschul,  Proc. Natl. Acad. Sci. USA,  87, 2264 (1990)), modified as in Karlin and Altschul (Karlin and Altschul,  Proc. Natl. Acad. Sci. USA,  90, 5873 (1993)). 
     Computer implementations of these mathematical algorithms can be utilized for comparison of sequences to determine sequence identity. Such implementations include, but are not limited to: CLUSTAL in the PC/Gene program (available from Intelligenetics, Mountain View, Calif.); the ALIGN program and GAP, BESTFIT, BLAST, FASTA, and TFASTA in the Wisconsin Genetics Software Package (available from Genetics Computer Group (GCG), 575 Science Drive, Madison, Wis., USA). Alignments using these programs can be performed using the default parameters. The CLUSTAL program is well described by Higgins et al. (Higgins et al.,  CABIOS,  5, 151 (1989)); Corpet et al. (Corpet et al.,  Nucl. Acids Res.,  16, 10881 (1988)); Huang et al. (Huang et al.,  CABIOS,  8, 155 (1992)); and Pearson et al. (Pearson et al.,  Meth. Mol. Biol.,  24, 307 (1994)). The ALIGN program is based on the algorithm of Myers and Miller, supra. The BLAST programs of Altschul et al. (Altschul et al.,  JMB,  215, 403 (1990)) are based on the algorithm of Karlin and Altschul supra. 
     Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information. In addition to calculating percent sequence identity, the BLAST algorithm also performs a statistical analysis of the similarity between two sequences. To obtain gapped alignments for comparison purposes, Gapped BLAST can be utilized. Alternatively, PSI-BLAST can be used to perform an iterated search that detects distant relationships between molecules. When utilizing BLAST, Gapped BLAST, PSI-BLAST, the default parameters of the respective programs (e.g., BLASTN for nucleotide sequences, BLASTX for proteins) can be used. 
     For purposes of the present invention, comparison of nucleotide sequences for determination of percent sequence identity to another sequence may be made using the BlastN program (version 1.4.7 or later) with its default parameters or any equivalent program. By “equivalent program” is intended any sequence comparison program that, for any two sequences in question, generates an alignment having identical nucleotide or amino acid residue matches and an identical percent sequence identity when compared to the corresponding alignment generated by the program. 
     (c) As used herein, “sequence identity” or “identity” in the context of two nucleic acid or polypeptide sequences makes reference to a specified percentage of residues in the two sequences that are the same when aligned for maximum correspondence over a specified comparison window, as measured by sequence comparison algorithms. When percentage of sequence identity is used in reference to proteins it is recognized that residue positions which are not identical often differ by conservative amino acid substitutions, where amino acid residues are substituted for other amino acid residues with similar chemical properties (e.g., charge or hydrophobicity) and therefore may not change the functional properties of the molecule. When sequences differ in conservative substitutions, the percent sequence identity may be adjusted upwards to correct for the conservative nature of the substitution. Sequences that differ by such conservative substitutions are said to have “sequence similarity” or “similarity.” Means for making this adjustment are well known to those of skill in the art. Typically this involves scoring a conservative substitution as a partial rather than a full mismatch, thereby increasing the percentage sequence identity. Thus, for example, where an identical amino acid is given a score of 1 and a non-conservative substitution is given a score of zero, a conservative substitution is given a score between zero and 1. The scoring of conservative substitutions is calculated, e.g., as implemented in the program PC/GENE (Intelligenetics, Mountain View, Calif.). 
     (d) As used herein, “percentage of sequence identity” means the value determined by comparing two optimally aligned sequences over a comparison window, wherein the portion of the polynucleotide sequence in the comparison window may comprise additions or deletions (i.e., gaps) as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. The percentage is calculated by determining the number of positions at which the identical nucleic acid base or amino acid residue occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison, and multiplying the result by 100 to yield the percentage of sequence identity. 
     (e) (i) The term “substantial identity” of polynucleotide sequences means that a polynucleotide comprises a sequence that has at least 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, or 94%, or even at least 95%, 96%, 97%, 98%, or 99% sequence identity, compared to a reference sequence using one of the alignment programs described using standard parameters. One of skill in the art will recognize that these values can be appropriately adjusted to determine corresponding identity of proteins encoded by two nucleotide sequences by taking into account codon degeneracy, amino acid similarity, reading frame positioning, and the like. Substantial identity of amino acid sequences for these purposes normally means sequence identity of at least 70%, 80%, 90%, or even at least 95%. 
     Another indication that nucleotide sequences are substantially identical is if two molecules hybridize to each other under stringent conditions. Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point (T m ) for the specific sequence at a defined ionic strength and pH. However, stringent conditions encompass temperatures in the range of about 1° C. to about 20° C., depending upon the desired degree of stringency as otherwise qualified herein. Nucleic acids that do not hybridize to each other under stringent conditions are still substantially identical if the polypeptides they encode are substantially identical. This may occur, e.g., when a copy of a nucleic acid is created using the maximum codon degeneracy permitted by the genetic code. One indication that two nucleic acid sequences are substantially identical is when the polypeptide encoded by the first nucleic acid is immunologically cross reactive with the polypeptide encoded by the second nucleic acid. 
     (e) (ii) The term “substantial identity” in the context of a peptide indicates that a peptide comprises an amino acid sequence with at least 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, or 94%, or even 95%, 96%, 97%, 98% or 99%, sequence identity to a reference sequence over a specified comparison window. In certain embodiments, optimal alignment is conducted using the homology alignment algorithm of Needleman and Wunsch (Needleman and Wunsch,  JMB,  48, 443 (1970)). An indication that two peptide sequences are substantially identical is that one peptide is immunologically reactive with antibodies raised against the second peptide. Thus, a peptide is substantially identical to a second peptide, for example, where the two peptides differ only by a conservative substitution. Thus, certain embodiments of the invention provide amino acid sequences that are substantially identical to the amino acid sequences described herein. 
     For sequence comparison, typically one sequence acts as a reference sequence to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are input into a computer, subsequence coordinates are designated if necessary, and sequence algorithm program parameters are designated. The sequence comparison algorithm then calculates the percent sequence identity for the test sequence(s) relative to the reference sequence, based on the designated program parameters. 
     As noted above, another indication that two nucleic acid sequences are substantially identical is that the two molecules hybridize to each other under stringent conditions. The phrase “hybridizing specifically to” refers to the binding, duplexing, or hybridizing of a molecule only to a particular nucleotide sequence under stringent conditions when that sequence is present in a complex mixture (e.g., total cellular) DNA or RNA. “Bind(s) substantially” refers to complementary hybridization between a probe nucleic acid and a target nucleic acid and embraces minor mismatches that can be accommodated by reducing the stringency of the hybridization media to achieve the desired detection of the target nucleic acid sequence. 
     “Stringent hybridization conditions” and “stringent hybridization wash conditions” in the context of nucleic acid hybridization experiments such as Southern and Northern hybridizations are sequence dependent, and are different under different environmental parameters. Longer sequences hybridize specifically at higher temperatures. The thermal melting point (Tm) is the temperature (under defined ionic strength and pH) at which 50% of the target sequence hybridizes to a perfectly matched probe. Specificity is typically the function of post-hybridization washes, the critical factors being the ionic strength and temperature of the final wash solution. For DNA-DNA hybrids, the T m  can be approximated from the equation of Meinkoth and Wahl (1984); T m  81.5° C.+16.6 (log M)+0.41 (% GC)−0.61 (% form)−500/L; where M is the molarity of monovalent cations, % GC is the percentage of guanosine and cytosine nucleotides in the DNA, % form is the percentage of formamide in the hybridization solution, and L is the length of the hybrid in base pairs. T m  is reduced by about 1° C. for each 1% of mismatching; thus, T m , hybridization, and/or wash conditions can be adjusted to hybridize to sequences of the desired identity. For example, if sequences with &gt;90% identity are sought, the T m  can be decreased 10° C. Generally, stringent conditions are selected to be about 5° C. lower than the T m  for the specific sequence and its complement at a defined ionic strength and pH. However, severely stringent conditions can utilize a hybridization and/or wash at 1, 2, 3, or 4° C. lower than the T m ; moderately stringent conditions can utilize a hybridization and/or wash at 6, 7, 8, 9, or 10° C. lower than the T m ; low stringency conditions can utilize a hybridization and/or wash at 11, 12, 13, 14, 15, or 20° C. lower than the T m . Using the equation, hybridization and wash compositions, and desired temperature, those of ordinary skill will understand that variations in the stringency of hybridization and/or wash solutions are inherently described. If the desired degree of mismatching results in a temperature of less than 45° C. (aqueous solution) or 32° C. (formamide solution), the SSC concentration is increased so that a higher temperature can be used. Generally, highly stringent hybridization and wash conditions are selected to be about 5° C. lower than the T m  for the specific sequence at a defined ionic strength and pH. 
     An example of highly stringent wash conditions is 0.15 M NaCl at 72° C. for about 15 minutes. An example of stringent wash conditions is a 0.2×SSC wash at 65° C. for 15 minutes. Often, a high stringency wash is preceded by a low stringency wash to remove background probe signal. An example medium stringency wash for a duplex of, e.g., more than 100 nucleotides, is 1×SSC at 45° C. for 15 minutes. For short nucleotide sequences (e.g., about 10 to 50 nucleotides), stringent conditions typically involve salt concentrations of less than about 1.5 M, less than about 0.01 to 1.0 M, Na ion concentration (or other salts) at pH 7.0 to 8.3, and the temperature is typically at least about 30° C. and at least about 60° C. for long probes (e.g., &gt;50 nucleotides). Stringent conditions may also be achieved with the addition of destabilizing agents such as formamide. In general, a signal to noise ratio of 2×(or higher) than that observed for an unrelated probe in the particular hybridization assay indicates detection of a specific hybridization. Nucleic acids that do not hybridize to each other under stringent conditions are still substantially identical if the proteins that they encode are substantially identical. This occurs, e.g., when a copy of a nucleic acid is created using the maximum codon degeneracy permitted by the genetic code. 
     Very stringent conditions are selected to be equal to the T m  for a particular probe. An example of stringent conditions for hybridization of complementary nucleic acids that have more than 100 complementary residues on a filter in a Southern or Northern blot is 50% formamide, e.g., hybridization in 50% formamide, 1 M NaCl, 1% SDS at 37° C., and a wash in 0.1×SSC at 60 to 65° C. Exemplary low stringency conditions include hybridization with a buffer solution of 30 to 35% formamide, 1 M NaCl, 1% SDS (sodium dodecyl sulphate) at 37° C., and a wash in 1× to 2×SSC (20×SSC=3.0 M NaCl/0.3 M trisodium citrate) at 50 to 55° C. Exemplary moderate stringency conditions include hybridization in 40 to 45% formamide, 1.0 M NaCl, 1% SDS at 37° C., and a wash in 0.5× to 1×SSC at 55 to 60° C. 
     In addition to the chemical optimization of stringency conditions, analytical models and algorithms can be applied to hybridization data-sets (e.g. microarray data) to improve stringency. 
     The invention will now be illustrated by the following non-limiting Example. 
     EXAMPLE 1 
     Anti-Prelog Stereospecific Carbonyl Reductases 
     Three open reading frames (ORFs) in the 960-kb contig005802 of  C. parapsilosis  have been discovered. These ORFs encode the stereospecific carbonyl reductase genes (scr1, scr2, and scr3). These ORFs have been cloned and expressed, and the encoded proteins purified to homogeneity and confirmed to function as stereospecific carbonyl reductases (SCR1, SCR2, and SCR3). 
     Identification of Putative Stereospecific Carbonyl Reductases-Encoding Genes. Three ORFs, named here as scr1, scr2, and scr3 coding for putative stereospecific carbonyl reductases (SCRs) were identified. As shown in  FIG. 1 , these three ORFs, as well as the cpadh gene, locate in the 960-kb contig005802 of the  C. parapsilosis  genome. The scr1, scr2, and scr3 genes comprise 846, 840, and 840 bp, encoding polypeptides of 281, 279, and 279 amino acid residues with the calculated molecular masses of 30,061, 29,993, and 30,097 Da, respectively. Multiple sequence alignment of these four ORFs ( FIG. 2 ) revealed high sequence identity between CPADH and SCR1 (68%), SCR2 (88%), and SCR3 (84%). Key active-site residues in the catalytic domain of the short-chain dehydrogenase/reductase (SDR) superfamily, including the cofactor-binding motif of Gly-x-x-x-Gly-x-Gly (x denotes any amino acid; SEQ ID NO:9) and the catalytic triad of Ser-Tyr-Lys (SEQ ID NO:11) are also found in SCR1, SCR2, and SCR3. 
     Cloning, Expression, and Purification of SCRs. From the nucleotide sequence of the ORFs, scr1, scr2, and scr3 were amplified by PCR from genomic DNA of  C. parapsilosis  CCTCC M203011, and the PCR products were inserted into pET21c vector by ligation-independent cloning to construct the recombinant plasmids. These three plasmids, pET21-SCR1, pET21-SCR2, and pET21-SCR3, were then transformed into expression host  E. coli  BL21(DE3) pMgK cells, and recombinant SCR1, SCR2, and SCR3 were produced in  E. coli  as fusion proteins containing a C-terminal His 6  tag (SEQ ID NO:12). All three recombinant enzymes were expressed at very high levels. Of them, SCR1 and SCR3 were expressed as soluble form at yields of 50 mg/l broth and 46 mg/l broth, respectively, while SCR2 has relatively low solubility with a yield of 5 mg/l broth ( FIG. 3 ). 
     The three recombinant enzymes were purified to homogeneity as judged by Coomassie Brilliant Blue staining of SDS-PAGE ( FIG. 4 ) by Ni affinity purification followed by gel filtration chromatography. The relative molecular mass of the SCR1 and SCR3 were estimated to be 124.6 kDa, and 123.4 kDa by analytic gel filtration and static light scattering using the same low salt buffer (Acton et al.,  Methods Enzymol.,  394, 210-243 (2005)), but SCR2 was detected as aggregated form. Since the relative molecular mass of the monomer of the recombinant enzymes should be around 30 kDa based on their amino acid composition, these results suggested that both SCR1 and SCR3 have tetrameric structures. 
     Catalytic Properties of Recombinant SCRs. The enzymatic activities of SCR1, SCR2, and SCR3 were investigated for reduction of 2-hydroxyacetophenone. Under the assay conditions, SCR1 gave the highest specific activity of 5.16 μmol/min mg protein, and SCR3 had the catalytic activity of 4.23 μmol/min mg protein, while SCR2 has a lower specific activity of 1.55 mol/min mg protein. In addition, the SCRs all displayed catalytic activity with NADPH as the coenzyme, but very low activities with NADH, indicating that these three enzymes are NADPH-dependent oxidoreductases. 
     Since environmental pH value can have an influence on the stereochemistry of enzymatic reactions (Secundo and Phillips,  Enzyme Microb. Technol.,  19, 487-492 (1996)), the effect of the reaction pH on the activities of SCRs catalyzing 2-hydroxyacetophenone reduction was also investigated. All three enzymes exhibited the highest activity at the pH ranging from 5.0 to 6.0 ( FIG. 5 ). Subsequently, the enzymes were evaluated under their individual optimal pH, and apparent kinetic parameters were further measured by double reciprocal Lineweaver-Burk plots at various 2-hydroxyacetophenone concentrations with fixed NADPH concentrations. As shown in Table 1, for reduction of 2-hydroxyacetophenone, these three enzymes exhibited different kinetic parameters. The values are the average of three independent experiments. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Activities and kinetic parameters for reduction of 2- 
               
               
                 hydroxyacetophenone (2-HAP) by SCR1, SCR2, SCR3, and CPADH, using 
               
               
                 NADPH as a cofactor 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Specific activity  
                   
                 V m   
               
               
                   
                   
                 (μmol/min mg 
                   
                 (μmol/min mg 
               
               
                 Enzyme 
                 pH  a   
                 protein) 
                 K m  (mM) 
                 protein) 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 SCR1 
                 5.0 
                 5.16 
                 9.83 
                 42.0 
               
               
                 SCR2 
                 5.5 
                 1.55 
                 4.81 
                 8.83 
               
               
                 SCR3 
                 6.0 
                 4.23 
                 4.68 
                 32.3 
               
               
                 CPADH 
                 4.5 
                 2.70 
                 5.83 
                 18.3 
               
               
                   
               
               
                   a  Activity assay was carried out at the optimum pH for each enzyme. 
               
            
           
         
       
     
     Stereoselectivity to Prochiral Carbonyl Group. Using 2-hydroxyacetophenone as the substrate, optically pure 1-phenyl-1,2-ethanediol (PED) of (S)-enantiomer (&gt;99% e.e.) was produced by each of SCR1, SCR2, and SCR3, respectively ( FIG. 6 ). These three enzymes catalyze asymmetric reduction of prochiral carbonyl compounds and are all (S)-specific carbonyl reductase toward 2-hydroxyacetophenone. Of them, however, SCR2 is not as efficient as the other two enzymes, corresponding to its lower activity. These data demonstrate that the SCR1, SCR2, and SCR3 enzymes are anti-Prelog-type stereospecific carbonyl reductases (Manzocchi et al.,  J. Org. Chem.,  53, 4405-4407 (1988); Nie et al.,  Appl. Environ. Microbiol.,  73, 3759-3764 (2007); Prelog,  Pure Appl. Chem.,  9, 119-130 (1964)). 
     Substrate Specificity. Since SCR1 and SCR3 showed distinct (S)-specific carbonyl-reducing activity, the substrate specificity of these two enzymes was further examined to various carbonyl compounds including aryl ketones, aliphatic ketones, α- and β-ketoesters. As shown in  FIG. 7 , on the one hand, both the enzymes exhibited higher catalytic activity to ketoesters than to alkyl and aromatic ketones; on the other hand, the enzymes showed diversity on specificity to aryl ketones and ketoesters, respectively. For both substituted acetophenone and 2-hydroxyacetophenones derivatives, bearing chloro or methyl at various positions of the phenyl ring, the ortho substituents were poor substrates for these enzymes, indicating that the substitution at ortho position might have steric influence on the hydrogen attack from electron donator NADPH to the carbonyl group and significant influence on the reactivity of the enzymes. However, SCR3 was more specific to p-Cl-2-hydroxyacetophenone, while SCR1 had higher activity to m-Cl-2-hydroxyacetophenone. For ketoesters, the enzymes both exhibited high activity to those with small groups, but compared with SCR3, SCR1 was more active to bulky ketoesters with phenyl ring and generally showed higher activities to β-ketoesters. Worth to note, among the tested ≢2-ketoesters, SCR1 preformed the highest activity for the reduction of ethyl 4-chloro-3-oxobutyrate, which affords ethyl 4-chloro-3-hydroxybutyrate, an important pharmaceutical intermediate (Thayer,  Chem. Eng. News,  84, 26-27 (2006). 
     Materials and Methods 
     Materials.  C. parapsilosis  strain CCTCC M203011 was obtained from the China Center for Type Culture Collection (CCTCC, Wuhan, China).  Escherichia coli  XL-10 gold cells were used for gene cloning and plasmid preparation, and  E. coli  BL21 (DE3) pMgK competent cells, a rare codon-enhanced strain, were used for gene expression. High-fidelity PCR kit including DNA polymerase was purchased from FINNZYMES (Finland). The plasmid pET21c was obtained from Novagen (USA). (R)- and (S)-1-phenyl-1,2-ethanediol, all the aliphatic ketones and ketone esters, aryl ketones including acetophenone and its derivatives, 2-hydroxyacetophenone, propiophenone, butyrophenone, valerophenone, hexanophenone, and coenzymes including NAD(P)H and NAD(P) +  were purchased from Sigma-Aldrich (USA). All other 2-hydroxyacetophenone derivatives including o-Cl-2-hydroxyacetophenone, m-Cl-2-hydroxyacetophenone, p-Cl-2-hydroxyacetophenone, and p-CH 3 O-2-hydroxyacetophenone were prepared using the method described by Itsuno (Itsuno et al.,  J. Chem. Soc. Perkin Trans. I,  2039-2044, (1985)). All other chemicals used in this work were of analytical grade and commercially available. 
     Cloning and Expression of Genes Encoding Stereospecific Reductases. The genes encoding SCR1, SCR2, and SCR3 were amplified by polymerase chain reaction from  C. parapsilosis  genomic DNA. PCR-amplified DNA products were purified by QIAquick PCR Purification Kit (QIAGEN, USA) and inserted into pET21c expression vector (Novagen, USA) by ligation-independent cloning (LIC) using In-Fusion PCR Cloning Kit (Clontech, USA) for construction of recombinant plasmids. The infusion reaction mixtures were used to transform  E. coli  XL-10 gold cells. The plasmids isolated from these transformants were verified by DNA sequence analysis using BigDye Terminator cycle sequencing kit and an ABI PRISM 310 Genetic Analyzer (Applied Biosystems, USA). The plasmids with the correct inserts, pET21-SCR1, pET21-SCR2, and pET21-SCR3, were transformed into  E. coli  BL21(DE3) pMgK competent cells for the production of SCRs. These plasmids provide SCRs with a six-His tag (SEQ ID NO:12) fused at the C-terminus. 
       E. coli  BL21 (DE3) pMgK transformants were cultivated at 37° C. in Luria broth (LB) medium in the presence of ampicillin (100 μg/ml) and kanamycin (50 μg/ml). When the optical density of the culture at 600 nm reached 0.6, the temperature was changed to 17° C. and isopropyl-β-D-thiogalactopyranoside (IPTG) was added to the culture to give a final concentration of 1 mM for induction of gene expression. After an additional incubation of 20 h at 17° C., cells harvested by centrifugation were disrupted by sonication, and expressions of the recombinant proteins were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). 
     Purification of Recombinant Enzymes. The cells were suspended in binding buffer (20 mM Tris-HCl, pH7.5, 0.3 M NaCl, 40 mM imidazole, 1× protease inhibitors, 1 mM Tris(2-carboxyethyl)phosphine (TCEP)) and disrupted on ice by sonication. The supernatant of the cell lysate was collected by centrifugation at 26,000×g for 40 min at 4° C. and purified by an AKTAxpress system using HisTrap HP affinity column followed by Superdex 75 gel filtration column (GE Healthcare, USA), and the purified fractions were exchanged into low salt buffer (10 mM Tris-HCl, pH 7.5, 0.1 M NaCl, 0.02% NaN 3 , 5 mM D,L-dithiothreitol) (Acton et al.,  Methods Enzymol.,  394, 210-243 (2005)). The final recombinant enzymes were purified with an apparent homogeneity on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with 12% polyacrylamide gels. Their molecular masses were measured by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (Applied Biosystems, USA), and their oligomerization states were determined by analytic gel filtration using Agilent 1200 seriers HPLC system followed by static light scattering (Wyatt Technology, USA). These final preparations of purified SCRs were used in all of the experiments in this study. 
     Enzyme Assays. Carbonyl reductase activity was measured by a continuous spectrophotometric assay using 2-hydroxyacetophenone as a substrate. One unit of enzyme activity was defined as the amount of enzyme catalyzing the oxidation of 1 μmol NAD(P)H per min under the assay conditions. The standard assay mixture for the enzyme activity comprised of 0.1 M potassium phosphate buffer (pH 6.5), 0.3 mM NAD(P)H, 0.7 mM 2-hydroxyacetophenone and appropriate enzyme in a total volume of 100 μl. The decrease in the amount of the coenzyme was measured spectrophotometrically at 340 nm (extinction coefficient [ε]=6.22 mM −1  cm −1 ). Protein concentration was determined using Bradford reagents (Bio-Rad) with bovine serum albumin as a standard. The pH dependence of enzyme activity was determined over a pH range of 4.0 to 8.5 using the following buffers: 0.1 M acetate (pH 4.0 to 6.0), 0.1 M sodium phosphate (pH 6.0 to 8.0) and 0.1 M Tris-HCl (pH 8.0 to 8.5). 
     The substrate specificity of SCRs was investigated under the same conditions as described above. Various carbonyl compounds including aryl ketones, aliphatic ketones, α- and β-ketone esters were used as the substrates with the cofactor of NADPH. 
     Asymmetric Reduction and Stereoselectivity Assay. Asymmetric reduction of 2-hydroxyacetophenone by the purified enzymes were carried out at 30° C. for 6 h with shaking in a reaction mixture comprising 0.1 M potassium phosphate buffer (pH 6.5), 1 g/12-hydroxyacetophenone, NADPH (7 mM) and 0.5 mg of the purified enzyme in a total volume of 0.5 ml. The reaction products were extracted with ethyl acetate and the organic layer was used for analysis. The optical purity of reaction products were analyzed by HPLC using a Chiralcel OB-H column (4.6×250 mm, Daicel Chemical Ind., Ltd., Japan). Enantiomers were eluted with hexane and 2-propanol (9:1) at a flow rate of 0.5 ml/min. The effluent was monitored at 215 nm, and the areas under each peak were integrated (Nie et al.,  Org. Process Res. Dev.,  8, 246-251 (2004)). 
     Nucleotide Sequence Accession Number. The nucleotide sequence for the stereospecific carbonyl reductase genes scr1, scr2, and scr3 have been deposited in the GenBank database under accession numbers FJ939565 (SEQ ID NO:8), FJ939563 (SEQ ID NO:7), and FJ939564 (SEQ ID NO:6), respectively. 
     All publications, patents and patent applications cited herein are incorporated herein by reference. While in the foregoing specification this invention has been described in relation to certain embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein may be varied considerably without departing from the basic principles of the invention. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.