# Patent

## Title
Polypeptide having esterase activity and recombinant esterase and use thereof

## Abstract

Method of resolving stereoselectively racemic 2-alkyl-5-halopent-4-ene-carboxylic esters of the formula (I)




    
    
        in which R is a C1-C6-alkyl radical, R1 is C1-C4-alkyl and X is chlorine, bromine or iodine, by contacting a racemic ester of formula (I) with a polypeptide or recombinant protein having esterase activity and exhibiting an amino acid sequence that is at least 98% identical to the amino acid sequence of SEQ. ID. No. 1.

## Background
This application is the U.S. national phase under 35 U.S.C. 371 of International Application No. PCT/EP2006/011832, filed 8 Dec. 2006, which designated the U.S. and claims priority to Austrian Application No. A2081/2005, filed 27 Dec. 2005; the entire contents of each of which are hereby incorporated by reference.
The invention relates to a novel polypeptide having esterase activity, especially having 2-alkyl-5-halopent-4-enecarboxylesterase activity, and to an enzymatically active recombinant protein having esterase activity and to the use thereof for resolving racemates of 2-alkyl-5-halopent-4-enecarboxylic ester enantiomer mixtures.
BACKGROUND OF THE INVENTION
Enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic acids and their esters are valuable intermediates for preparing pharmaceuticals, such as, for instance, for delta-amino-gamma-hydroxy-omega-arylalkanecarboxamides, which have renin-inhibiting properties and can be used as antihypertensive agents in pharmaceutical preparations.
Esterases are generally employed in the resolution of racemates and asymmetrization.
However, only very few esterases suitable for preparing chiral compounds are commercially available.
The use of esterase extracts from pig liver is known on the preparative scale. Pig liver esterase (PLE) was isolated long ago from natural sources, and its activity has also been known for a long time (Simonds, J. P. (1919) Amer. J. Physiol. 48, 141; Bamann, E. et al. (1934) Hoppe-Seyler Z. 229, 15; Falconer J. S. and Taylor, D. B. (1946) Biochem. J. 40, 831-834).
Various studies have also already been carried out in order to characterize PLE (Heymann, E. and Junge, W. (1979) Eur. J. Biochem. 95, 509-518; Lehner, R. and Verger, T. (1997) Biochemistry 36, 1861-1868). It has further been possible to show, for example in WO 01/09079, that esterase extracts from pig liver can selectively hydrolyze the (R) enantiomer of methyl 5-chloro-2-(1-methylethyl)-4-pentenoate.
However, the use of such esterase extracts from natural sources, such as pig liver, is associated with disadvantages.
In the first place, the qualities of the different batches vary and thus make it difficult to optimize industrial processes. Secondly, the use of animal resources in the manufacture of pharmaceutical products is undesired because the presence of viruses and prions cannot always be precluded.
For these reasons there is a need to produce recombinant pig liver esterases of standardized quality in microorganisms.
The cloning of putative esterase genes is described for example in FEBS Lett. (1991), 293, 37-41. The first functional expression of an active pig liver esterase enzyme was described for the first time in WO 02/48322. WO 2004/055177 describes the preparation of further recombinant esterases by site directed mutagenesis of the recombinant pig liver esterase of seq. ID No. 1 (rPLE) from WO 02/48322. As is evident from the description of WO 2004/055177 and from the article authored by the same inventors in Protein Engineering, 16, 1139-1145, 2003, the modifications of the rPLE sequence from WO 02/48322 were chosen so that a recombinant intestinal pig esterase (PICE) disclosed in David et al., (1998) Eur. J. Biochem. 257, 142-148, is obtained.
The resolution of racemic 2-alkyl-5-halopent-4-enecarboxylic esters is not described in any of these articles.
However, since the need for esterases which have the desired stereoselective activity for 2-alkyl-5-halopent-4-enecarboxylic esters and which can easily be prepared biotechnologically is not met, it was an object of the present invention to provide a corresponding novel recombinant esterase.
SUMMARY OF THE INVENTION
In an attempt to isolate and to clone the gene described in FEBS Lett. (1991), 293, 37-41 and WO 02/48322 for known pig liver esterase (PLE) as cDNA starting from mRNA from pig liver, a second, novel esterase sequence was found in addition to the known PLE sequence. Following expression of the two sequences, in which the corresponding proteins or esterases, namely the known rPLE and a novel, recombinant “alternative” esterase (rAPLE), was prepared, it unexpectedly emerged that only the rAPLE is capable of selective resolution of racemic 2-alkyl-5-halopent-4-enecarboxylic esters.
It was thus possible to achieve the object of the present invention by a novel polypeptide having esterase activity and a novel recombinant esterase (rAPLE), whose amino acid sequence differs in 21 of a total of 548 amino acids from the known PLE sequence. The novel rAPLE differs in the amino acid sequence also from the known pig intestinal carboxylesterase (PICE) in 12 of a total of 548 amino acids. However, PICE is found in the pig intestinal tract.
The present invention accordingly relates to a polypeptide having esterase activity, which comprises the amino acid sequence SEQ. ID. No. 1.
The present invention further relates to a novel recombinant protein having esterase activity, which comprises the amino acid sequence SEQ. ID. No. 1.
The polypeptide and the recombinant rAPLE of the invention have the ability to resolve stereoselectively racemic 2-alkyl-5-halopent-4-enecarboxylic esters of the formula (I)




in which R is a C1-C6-alkyl radical, R1 is C1-C4-alkyl and X is chlorine, bromine or iodine.

The polypeptide of the invention having esterase activity, and the novel recombinant esterase rAPLE differ, as stated above, in 21 of a total of 548 amino acids of the known sequence disclosed in FEBS Lett. (1991), 293, 37-41 and in 12 of a total of 548 amino acids from the known PICE protein disclosed in David et al., (1998) Eur. J. Biochem. 257, 142-148.
The sequence of the protein of the novel rAPLE of the invention differs in the following amino acid positions from the known sequence of the PLE protein:















APLE
Position
PLE




















Glu
73
Asp



Ile
75
Val



Gly
76
Val



Gly
77
Glu



Leu
80
Thr



Arg
87
Gly



Ile
92
Thr



Pro
93
Leu



Val
129
Leu



Ser
133
Pro



Thr
134
Met



Leu
138
Val



Ala
139
Val



Phe
234
Leu



Ala
236
Val



Gly
237
Ala



Phe
286
Leu



Ala
287
Thr



Leu
290
Phe



Pro
294
Gln



Thr
302
Pro










The protein of the invention and the novel recombinant rAPLE may moreover be in the form of a modified sequence as shown in SEQ ID No 1, which can be obtained for example by usual modifications such as, for instance, exchange, deletion or attachment of amino acid(s) in the sequence at the N or C terminus, such as, for instance, GluAlaGluAla from the α factor signal sequence, or by fusion to other proteins.
The invention also further includes muteins having modifications within the protein sequence of the enzyme of the invention having the appropriate activity, in particular on 2-alkyl-5-halopent-4-enecarboxylic esters. Muteins can be obtained for example by modifications of the DNA which codes for the enzyme of the invention, by known mutagenesis techniques (random mutagenesis, site-directed mutagenesis, directed evolution, gene shuffling etc.) so that the DNA codes for an enzyme which differs at least by one amino acid from the enzyme of the invention, and subsequent expression of the modified DNA in a suitable host cell. The invention thus also includes modified DNA sequences as shown in SEQ ID. No 1, obtained by the mutations, deletions, extensions, fusions described above, and which code for enzymes having the desired esterase activity.
Esterase activity, especially 2-alkyl-5-halopent-4-ene-carboxylesterase activity, is defined in this connection as the ability to resolve racemates of 2-alkyl-5-halopent-4-enecarboxylic esters of the formula (I).
The polypeptide of the invention and the recombinant rAPLE can be prepared as described below:
Firstly, mRNA is isolated from pig liver using a suitable kit, and then the cDNA is generated by reverse transcription based on the mRNA extract.
Subsequently, specific PCR primers based on the sequence of the known pig liver esterase gene of GenBank accession No. X63323 (Matsushima et al., 1991) is prepared, followed by amplification and cloning.
These specific primers are:









Primer 1:



5′-CAGAATTCATGGCTATCGGGCAGCCAGCCTCGC-3′



 



Primer 2:



5′-CCGGAATTCAGCCTCCCCTTCACAGCTCAG-3′






This part of the primers which comprises the appropriate nucleotide sequences coding for the PLE protein and which is obligatorily present in the primers is in bold script.
The other sequence part of the primers comprises for example information for cleavage sites for restriction endonucleases (in italics) or sequence elements which are important for expression. This part may vary in the preparation of the rAPLE of the invention.
Amplification then takes place with primers 1 and 2 by prior art PCR methods.
The PCR product is subsequently used to prepare by prior art methods expression constructs for heterologous expression of the encoded rAPLE protein in suitable host organisms. This preferably entails the PCR product being initially cloned into suitable plasmid vectors.
The recombinant plasmids obtained in this way are then transformed into a suitable host, for example Escherichia coli. Inserts of several resulting clones are then sequenced.
Unexpectedly, 2 groups of recombinant clones with different sequences were identified therein, one being 100% identical to the expected sequence for PLE according to Matsushima et al., (1991) FEBS Lett. 293, 37-41, and a novel nucleotide sequence as shown in SEQ. ID. No. 2 (APLE sequence) which leads after expression to the amino acid sequence SEQ. ID. No. 1 of the invention.
The present invention further relates to a nucleic acid or nucleotide sequence which codes for the polypeptide of the invention and the recombinant esterase rAPLE.
For example, such a nucleic acid has the nucleotide sequence shown in SEQ. ID. No. 2.
The invention also relates further to nucleotide sequences which include a nucleotide sequence which codes for the polypeptide of the invention and the recombinant esterase rAPLE, or comprises the nucleotide sequence shown in SEQ. ID. No. 2.
A further possibility is to prepare appropriate oligonucleotides corresponding to nucleic acid sequences according to the present invention which code for the esterase of the invention by standardized synthetic techniques, for example with use of automated DNA synthesizers.
The purely synthetic preparation of the nucleic acid sequences which code for the esterase of the invention is particularly advantageous for use in the production of pharmaceuticals or their intermediates, because enzymes are thus not obtained from animal sources.
Expression of the two sequences found (PLE and APLE sequences) then takes place.
The known pig liver esterase (PLE, Swiss-Prot ID Q29550) comprises an N-terminal signal sequence and a C-terminal ER retention signal, the last 4 amino acids HAEL.
In order to express the known PLE and the novel APLE, vectors in which the sequences are introduced into suitable expression systems constructed. These expression constructs are then transformed into suitable host cells.
Suitable host cells in this connection are for example microorganisms, animal cell lines and plants.
Both prokaryotic and eukaryotic microorganisms can be employed. Preferred prokarytic hosts (bacteria) are Escherichia coli, and strains from the genera Bacillus (e.g. B. subtilis, B. licheniformis, B. amyloliquefaciens), Pseudomonas (e.g. P. fluorescens, P. putida), or Streptomyces (e.g. S. lividans, S. tendae) Eukaryotic microorganisms are preferred, and fungi are particularly preferred. Examples thereof are Saccharomyces cerevisiae, Pichia pastoris, Kluyveromyces lactis or Aspergillus sp.
Expression may be secretory or intracellular and both inducible and constitutive.
For bacterial expression a choice of species-specific signals can be obtained, a.o. as commercially available strains and vectors for protein expression (e.g provided by companies like Invitrogen, Novagen, New England Biolabs), that allow inducible or constitutive expression, intracellular and secretory localization of the target protein; in addition, technology to enable or promote the correct folding of proteins in order to result in soluble and active protein may be applied.
The proteins are preferably expressed in a secretory manner, in which case vectors in which the sequences of PLE and APLE are linked N-terminally to the α factor signal sequence of S. cerevisiae are preferably constructed.
It is further possible to prepare constructs in which the C-terminal tetrapeptide HAEL, which serves as ER retention signal as described for example in Hardwick et al., (1990) EMBO J. 9, 623-630, is additionally deleted.
A further preferred expression is inducible expression of constructs with or without ER retention signal.
Unexpectedly, constructs having the ER retention signal can also be expressed and lead to an rAPLE which is capable of selective resolution of racemic 2-alkyl-5-halopent-4-enecarboxylic esters.
The amino acid sequence of the novel esterase rAPLE which is derived from the nucleotide sequence of the APLE gene is depicted in SEQ ID No. 1.
It has unexpectedly been possible to find that the novel polypeptide or the rAPLE protein is able, in contrast to the known rPLE, in each case obtained by expression of the DNA segments coding for APLE and PLE, respectively, for example in P. pastoris cells, to resolve racemic 2-alkyl-5-halopent-4-enecarboxylic esters stereoselectively.
The invention accordingly further relates to the use of the polypeptide having esterase activity and of the recombinant esterase (rAPLE) of the invention, which have at least 80% identity to the sequence shown in SEQ ID No. 1, for resolving racemates of 2-alkyl-5-halopent-4-enecarboxylic esters of the formula (I)




in which R is a C1-C6-alkyl radical, R1 is C1-C4-alkyl and X is chlorine, bromine or iodine.

The polypeptide having esterase activity and the recombinant esterase (rAPLE) of the invention preferably have at least 90%, particularly preferably at least 98%, identity to the sequence of the protein shown in SEQ ID No. 1. It is also possible to employ polypeptide having an esterase activity or the recombinant esterase (rAPLE) of the invention with modified DNA sequences shown in SEQ ID. No. 1, obtained by usual modifications such as, for instance, mutations, deletions, extensions, fusions, which code for enzymes having the desired esterase activity.
In this connection, enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic acids or their esters of the formula (II)




in which R is a C1-C6-alkyl radical, A is equal to H, R1, where R1 may be C1-C4-alkyl, or R2, where R2 is an alkyl group, but is not equal to R1, and X is chlorine, bromine or iodine, are obtained by an enantiomeric mixture of a 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (I)





in which R, R1 and X are as defined above,

being converted by means of the polypeptide of the invention or of the rAPLE of the invention in the presence of water or an alcohol of the formula R2OH, where R2 is an alkyl group which is not equal to R1, as nucleophile, and

    a) either the remaining enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (II) with A equal to R1 being isolated or
    b) if an alcohol is employed as nucleophile, the resulting enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (II) with A equal to R2 being isolated, or
    c) if water is employed as nucleophile, the resulting 2-alkyl-5-halopent-4-enecarboxylic acid of the formula (II) with A equal to H being isolated.


R in the formula (II) is a C1-C6-alkyl radical such as, for instance, methyl, ethyl, n- and i-propyl, n-, and t-butyl, pentyl and hexyl.
C1-C4-Alkyl radicals are preferred, and the i-propyl radical is particularly preferred.
A is H, R1, where R1 is a C1-C4-alkyl radical, preferably a C1-C2-alkyl radical and particularly preferably a methyl radical, or R2, where R2 is an alkyl radical which is not equal to R1. R2 is particularly preferably a C1-C6-alkyl radical.
X is chlorine, bromine or iodine, preferably chlorine.
Enantiomerically enriched compounds mean in this connection those which exhibit an enantiomeric excess (ee) of >80%, preferably of >90% and particularly preferably of >97%.
The enzyme of the invention can moreover be used in any form. For example as dispersion, as solution, immobilized, as crude enzyme, as enzyme which has been obtained from its source by a combination of known purification methods, as whole cells (where appropriate immobilized and/or permeabilized) which have the required enzymatic activity (naturally or through genetic modifications) or in a lysate of such cells.
The reaction temperature for the conversion of the invention is normally between 0 and 90° C., preferably between 10 and 60° C. The pH of the reaction solution is between 4 and 11, preferably between 6 and 9.
The choice of the solvent depends on the nucleophile employed.
If, for example, water is the nucleophile, solvents which can be employed are water, a mixture of water with a water-miscible solvent, for example with an alcohol such as, for instance, methanol, ethanol, isopropanol, t-butanol, etc., dioxane, tetrahydrofuran, acetone or dimethyl sulfoxide or a two-phase system of water and of a water-immiscible solvent, for example an aromatic compound such as, for instance, toluene, xylene, etc., an alkane such as, for instance, hexane, heptane, cyclohexane, etc., ether such as, for instance, diisopropyl ether, methyl t-butyl ether, etc. If the nucleophile is an alcohol, the solvent preferably employed is the alcohol R2OH where R2 is an alkyl group which is not, however, equal to R1. However, it is also possible to use mixtures of the alcohol with an organic solvent such as, for instance, tetrahydrofuran, heptane, toluene, hexane, CH3CN, methyl t-butyl ether etc.
After the enzymatically catalyzed racemate resolution has taken place, the desired final product is isolated. This may be either the remaining enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (II) with A equal to R1, or if water is the nucleophile the enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic acid of the formula (II) with A equal to H which has formed, or if the alcohol is the nucleophile the enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (II) with A equal to R2.
The isolation can take place for example by conventional methods such as, for instance, extraction, crystallization, column chromatography, distillation, etc.
The method of the invention results in the corresponding acids or esters of the formula (I) in theoretical yields of up to 98% yield and with an e.e. of up to >99%.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further described with reference to the accompanying drawings, in which:
FIG. 1 shows stereoselective esterase activity on methyl-5-chloro-2-(1-methylethyl)-4-pentenoate;
FIG. 2 shows DSD-PAGE of recombinant APLE;
FIG. 3 shows induced expression of pig liver esterases with the AOX1 promoter;
FIG. 4 shows SEQ ID No 1 Protein sequence of rAPLE SEQ ID No 2: Nucleotide sequence of rAPLE
FIG. 5 shows SEQ ID No 3: Primer 1
SEQ ID No. 4: Primer 2
SEQ ID No. 5: EcoRlalpha1
SEQ ID No. 6: alphaPLE2
SEQ ID No. 7: PLEalpha1
SEQ ID No. 8: EcoRIPLE+ER2
SEQ ID No. 9: EcoRIPLE2.



EXAMPLES
Example 1
mRNA Isolation and Generation of cDNA
0.7 g of liver from a freshly slaughtered pig, obtained from a local abattoir, was frozen in liquid nitrogen and homogenized with a mortar, and the liberated mRNA was isolated or extracted using the Fast Track mRNA extraction kit 2.0 (Invitrogen, Carlsbad, Calif., USA) in accordance with the statements made by the manufacturer (Fast Track 2.0 kit manual; version J; 082301; 25-0099). The extraction afforded a total amount of 12.9 μg of mRNA.
0.26 μg of this mRNA was then used as template for generating cDNA using the SuperScript III First-Strand synthesis system for RT-PCR according to the manufacturer's statements.
Example 2
Amplification and Cloning of cDNA Fragments from Pig Liver
Specific primers based on the sequence of the pig liver esterase gene of GenBank accession No. X63323 (Matsushima et al., 1991) were prepared:









Primer 1:







(SEQ ID. No. 3)









5′-CAGAATTCATGGCTATCGGGCAGCCAGCCTCGC-3′



 



Primer 2:







(SEQ ID No. 4)









5′-CCGGAATTCAGCCTCCCCTTCACAGCTCAG-3′






Bases homologous to the known PLE sequence are in bold script. Recognition sequences for restriction endonucleases are italicized for emphasis.
The amplification took place in a 50 μl mixture with 1 U of Phusion DNA polymerase (Finnzymes, Espoo, Finland), with 500 ng of cDNA as template, 20 μmol each of primer 1 and 2, 5 μl of a dNTP mix (2 mM each), all in 1× Phusion HF buffer in accordance with ‘Phusion High-Fidelity DNA Polymerase’ manual (Finnzymes), starting with a 30 second denaturation step at 98° C., followed by 30 cycles (10 sec 98° C., 20 sec 68° C., 1 min 72° C.) for amplification and a final incubation at 70° C. for 8 min to prepare complete products.
This PCR resulted in a DNA fragment with a size of 1.8 kb (found by agarose gel electrophoresis).
This PCR product was then purified using the Qiaquick kit (Qiagen, Hilden, Germany) in accordance with the manual included.
About 0.1 μg of the purified PCR product was cut with the restriction endonuclease EcoRI and cloned into the plasmid vectors pHILZ and pHIL-D2 via the EcoRI cleavage sites.
The vectors were then transformed into TOP10 electrocompetent cells prepared in accordance with ‘Current Protocols in Molecular Biology’.
Inserts of several resulting clones were sequenced using the ‘Dye Deoxy Terminator Cycle Sequencing’ kit (Applied Biosystems Inc., Forster City, Calif., USA).
Two sequences were identified thereby, one corresponding 100% to the expected sequence published by Matsushima et al., (1991) FEBS Lett. 293, 37-41, and the other sequence corresponding to SEQ ID No. 2.
Example 3
Introduction of the α Factor Signal Sequence and Variations of the C-Terminal End
In order to enable secretory expression of the known protein PLE and the protein rAPLE of the invention, vectors in which the sequence of PLE and APLE was connected N-terminally to the α factor start sequence of the cloning vector pPICZ α (Invitrogen) were constructed. In addition, constructs in which the C-terminal tetrapeptide HAEL was deleted were prepared.
PCR I: The EcoRIalpha1/alphaPLE2 primer pair was used to amplify the α factor signal sequence of the cloning vector pPICZ α (Invitrogen). The PCR was carried out in a 50 μl mixture (2 ng of template, 0.5 μM of each primer, 0.2 mM dNTPs, 1 U of the Phusion DNA polymerase (Finnzymes) all in 1× Phusion HF buffer in accordance with the ‘Phusion High-Fidelity DNA Polymerase’ manual (Finnzymes)).
Denaturation at 95° C. for 3 minutes was followed by amplification in 30 cycles (30 sec 95° C., 30 sec 57° C., 15 sec 72° C.) and a final step at 72° C. for 7 min.
PCR II: The PLE and APLE sequences were amplified from pHILZ plasmids using either the PLEalpha1/EcoRIPLE+ER2 primer pair or the PLEalpha1/EcoRIPLE2 (deletion of the C-terminal HAEL tetrapeptide) primer pair.
These PCRs were again carried out in 50 μl mixtures (2 ng of template, 0.5 μM of each primer, 0.2 mM dNTPs, 1 U of the Phusion DNA polymerase (Finnzymes) all in 1× Phusion HF buffer in accordance with the ‘Phusion High-Fidelity DNA Polymerase’ Manual (Finnzymes)).
A denaturation at 95° C. for 3 minutes was followed by amplification in 30 cycles (30 sec 95° C., 30 sec 57° C., 15 sec 72° C.) and a final step at 72° C. for 7 min.
PCR III: 3 μl of the products from PCR 1 and PCR II were used to combine these two products by primerless PCR.
The extension was carried out in a 45 μl mixture with 0.2 mM dNTPs, 1 U of the Phusion DNA polymerase (Finnzymes) all in 1× Phusion HF buffer.
The reaction mixture was heated at 95° C. for 3 minutes and then 10 cycles with 30 sec at 95° C. and 45 sec at 72° C. were carried out. To amplify these overlapping extension products, 5 μl of primer mix (3 μl of water, 1 μl of 5 μM EcoRIalpha1 primer and 1 μl of 5 μM EcoRIPLE+ER2 or EcoRIPLE2 primer) were added. The products were amplified with 20 PCR cycles (30 sec 95° C., 30 sec 57° C., 1 min 72° C.) and a single temperature stop at 72° C. for 7 min.







Primer sequences:


EcoRlalpha1:







(SEQ ID No. 5)







5′-TCTTCGAAGAATTCACGATGAGATTTCCTTCAATTTTTACTGC-3′


 


alphaPLE2:







(SEQ ID No. 6)







5′-GAGGCTGGCTGCCCAGCTTCAGCCTCTCTTTTCTCG-3′


 


PLEalpha1:







(SEQ ID No. 7)







5′-AGAGAGGCTGAAGCTGGGCAGCCAGCCTCGCCG-3′


 


EcoRlPLE + ER2:







(SEQ ID No. 8)







5′-ATGGTACCGAATTCTCACAGCTCAGCATGCTTTATCTTG-3′


 


EcoRlPLE2:







(SEQ ID No. 9)







5′-ATGGTACCGAATTCTCACTTTATCTTGGGTGGCTTCTTTG-3′






Regions with homology to the templates in bold, recognition sequences for restriction endonucleases in italics.
Example 4
Construction of Expression Constructs for the Heterologous Expression of Pig Liver Esterases in Pichia pastoris 
The overlapping extension PCR products from example 3 were purified using the Qiaquick kit (Qiagen, Hilden, Germany) in accordance with the manual included. About 0.1 μg of the purified PCR products was cut using the EcoRI restriction endonuclease and cloned via the EcoRI cleavage site into the plasmid vector pGAPZ A (Invitrogen).
Correct orientation of the insert in relation to the promoters was checked with the aid of control cleavages, for example with NcoI.
In each case, a clone with correctly oriented insert was selected, sequenced and preserved.
The corresponding plasmids were named as follows:
Plasmids which contained the known PLE sequence as disclosed in Matsushima et al., (1991) FEES Lett. 293, 37-41, were called pGAPZ A PLE−ER (the HAEL tetrapeptide was deleted) and pGAPZ A PLE+ER (HAEL tetrapeptide still present).
Plasmids derived from the novel APLE sequence were called pGAPZ A APLE−ER (the HAEL tetrapeptide was deleted) and pGAPZ A APLE−ER (HAEL tetrapeptide still present).
Example 5
Constitutive Expression of Pig Liver Esterases in Pichia pastoris 
The plasmids pGAPZ A PLE−ER, pGAPZ A PLE+ER, pGAPZ A APLE−ER and pGAPZ A APLE+ER were transformed into P. pastoris X-33. The transformation took place in accordance with the instructions of the protocol for the Pichia Expressions kit from Invitrogen. The transformants were selected on YPD plates (1% yeast extract, 2% peptone, 2% D-glucose, 2% agar) which contained 100 mg/l zeocin. 52 zeocin-resistant clones were streaked onto YPD plates with 100 mg/l zeocin and preserved in 15% glycerol.
Example 6
Qualitative Analysis of the Esterase Activity
P. pastoris transformants were cultured on YPD plates with 100 mg/l zeocin at 30° C. for 48 h. The cells were lifted onto Whatman 541 hardened ashless 70 mmØ filters and air-dried. The filters were incubated with a solution of 6 mg of α-naphthyl acetate (Sigma, dissolved in 500 μl of acetone), 2.5 mg of tetrazotized o-dianisidine (Fast Blue Salt BN, Sigma, dissolved in 125 μl of water) and 5 ml of 0.1 M potassium phosphate buffer, pH 7, in order to visualize the esterase activity by a color reaction.
Activities were detected in all transformants which had integrated one of the 4 plasmids pGAPZ A PLE−ER, pGAPZ A PLE+ER, pGAPZ A APLE−ER and pGAPZ A APLE+ER. This proves the expression of functional proteins having esterase activity. As a check, a clone with integrated empty vector was also tested in the same way. In this case, no significant esterase activity was visible in the comparable reaction period.
Example 7
Stereoselective Esterase Activity in Relation to methyl 5-chloro-2-(1-methylethyl)-4-pentenoate
P. pastoris transformants as described in example 6 were cultured on YPD plates with 100 mg/l zeocin at 30° C. for 48 h. The cells were lifted onto Whatman 541 hardened ashless 70 mmØ filters and air-dried. The filters were incubated either with substrate solution A (100 μl of racemic methyl 5-chloro-2-(1-methylethyl)-4-pentenoate, 200 μl of 0.1 M potassium phosphate buffer, pH 8; 150 μl of 10 mg/ml phenol red; 450 μl of DMSO; 650 μl of H2O) or with substrate solution B (identical to solution A but employing methyl (2S,4E)-5-chloro-2-(1-methylethyl)-4-pentenoate instead of the racemate). Owing to the specific esterase activity on the substrates, the liberation of acid on hydrolysis of the ester substrate results in a pH decrease which in turn leads to a change in the color of the phenol red indicator to yellow.
It emerged from this that transformants containing the plasmid pGAPZ A APLE−ER gave signals (yellow coloration around the colony) after incubation for 3 to 4 hours if they were tested on substrate solution A, whereas no significant conversion could be found with substrate solution B (FIG. 1).
Transformants obtained with the plasmids pGAPZ A PLE−ER or pGAPZ A PLE+ER showed no reaction with substrate solutions A or B under the same conditions.
This showed that the recombinant rAPLE has a substrate specificity different from recombinant rPLE and that hydrolysis of methyl 5-chloro-2-(1-methylethyl)-4-pentenoate using rAPLE takes place stereoselectively for the (R) enantiomer.
Example 8
SDS Polyacrylamide Gel Electrophoresis
10 μl of a 2×SDS sample buffer (125 mM Tris-HCl, pH 6.8; 4% SDS, 20% glycerol, 5% β-mercaptoethanol, 0.05% bromophenol blue) were added to 10 μl of commercially available pig liver esterase or 10 μl of the 60-fold concentrated (Centricon Ultrafiltrations-Spin Columns, from Sartorius) supernatants of the P. pastoris cultures (72 h at 100 rpm and 28° C. in 250 ml of YPD medium in 2 l Erlenmeyer flasks with baffles) which contained either the plasmid pGAPZ A APLE−ER or an empty pGAPZ A plasmid (control strain).
After the samples had been heated at 95° C. for 5 minutes, the proteins were separated on a 12.5% polyacrylamide gel (4% stacking gel) and stained with Coomassie Brilliant Blue R250 for detection. The SDS-PAGE shows a protein band with the expected size for rAPLE ( ̃60 kDa) in the yeast strain having the pGAPZ A APLE plasmid, but not in the control strain (FIG. 2).
The commercially available pig liver esterase which was also analyzed for comparison showed two protein bands in the same size range (arrow in FIG. 2).
Example 9
Induced Expression of Pig Liver Esterases with the AOX1 Promoter
The plasmids pGAPZ A PLE−ER, pGAPZ A PLE+ER, pGAPZ A APLE−ER and pGAPZ A APLE+ER were cut with the restriction endonuclease XhoI, and the respective fragments coding for APLE and PLE proteins, with or without ER retention signal, were cloned via the XhoI cleavage site into the pPIC9 vector (Invitrogen). Correct orientation of the fragments in relation to the AOX1 promoter was checked by means of control cuts with the restriction endonuclease NcoI. The vectors having the AOX1 promoter were named, in analogy to the plasmids named in example 4, pPIC9 PLE−ER, pPIC9 PLE+ER, pPIC9 APLE−ER and pPIC9 APLE+ER, linearized with SalI and transformed into P. pastoris KM71. Transformation and selection for His prototrophy took place in accordance with the instructions of the Pichia expression kit from Invitrogen. Selected transformants and the KM71 strain were cultured on complete medium in accordance with the Pichia expression kit from Invitrogen overnight and induced with 1% methanol for 48 h. The resulting cultures were analyzed by means of the qualitative pH-shift assay described in example 7, testing 2 μl of the cultures in the mixtures in each case. Expression under the control of the inducible AOX1 promoter led to very much higher, by comparison with the situation described for constitutive expression in example 7, rAPLE enzymic activities in relation to racemic methyl 5-chloro-2-(1-methylethyl)-4-pentenoate. The phenyl red color change (red to yellow) was detectable after only a few minutes (FIG. 3). Unexpectedly, the rAPLE activity was independent of the presence of the ER retention signal HAEL at the C terminus, i.e. even cells which expressed rAPLE with ER retention signal showed activity. By contrast, yeast strains for producing rPLE had no activity in relation to racemic methyl 5-chloro-2-(1-methylethyl)-4-pentenoate.

## Claims

1. A method of resolving stereoselectively racemic 2-alkyl-5-halopent-4-ene-carboxylic esters of the formula (I)



in which R is a C1-C6-alkyl radical, R1 is C1-C4-alkyl and X is chlorine, bromine or iodine, comprising contacting a racemic ester of formula (I) with a polypeptide or recombinant protein having esterase activity and exhibiting an amino acid sequence that is at least 98% identical to the amino acid sequence of SEQ. ID. No. 1.

2. A method for preparing enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic acids or esters thereof of the formula (II)



in which R is a C1-C6-alkyl radical, A is equal to H, R1, where R1 may be C1-C4-alkyl, or R2, where R2 is an alkyl group, but is not equal to R1, and X is chlorine, bromine or iodine, the method comprising converting a mixture of enantiomers of a 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (I)




in which R, R1 and X are as defined above,
using a polypeptide or a recombinant esterase having esterase activity and exhibiting an amino acid sequence that is at least 98% identical to the amino acid sequence of SEQ. ID. No. 1 in the presence of water or an alcohol of the formula R2OH, where R2 is an alkyl group which is not equal to R1, as nucleophile, and isolating
a) either the remaining enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (II) with A equal to R1 or
b) if an alcohol is employed as nucleophile, the resulting enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (II) with A equal to R2 or
c) if water is employed as nucleophile, the resulting 2-alkyl-5-halopent-4-enecarboxylic acid of the formula (II) with A equal to H.