WO2007073846A1

WO2007073846A1 - Method for preparing enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic acids and -carboxylic esters

Info

Publication number: WO2007073846A1
Application number: PCT/EP2006/011833
Authority: WO
Inventors: Gerhard Steinbauer; Michael Stanek; Peter Pojarliev; Wolfgang Skranc; Helmut Schwab; Marcel Gerhardus Wubbolts; Joannes Kierkels; Harald Pichler; Manuela Hermann; Christoph Zenzmaier
Original assignee: Dsm Fine Chemicals Austria Nfg Gmbh & Co Kg
Priority date: 2005-12-27
Filing date: 2006-12-08
Publication date: 2007-07-05
Also published as: AT502992A1; AT502992B1

Abstract

A method for preparing 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, by racemate resolution of 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, by means of a polypeptide having esterase activity or of a recombinant protein having esterase activity, which have an amino acid sequence as shown in SEQ ID No. 1 or is at least 80 % identical thereto, and polypeptide and recombinant esterase which has the amino acid sequence SEQ. ID. No. 1, and the use thereof .

Description

Method for preparing enantiomerically enriched 2-alkyl- 5-halopent-4-enecarboxylic acids and -carboxylic esters

The present invention relates to a method for preparing enantiomerically enriched 2-alkyl-5-halopent-4- enecarboxylic acids and their esters in an optical purity of up to e.e. > 99% and up to 98% yield of theory . 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.

One variant for preparing alkyl-5-halopent-4- enecarboxylic esters is described for example in WO 01/09079, according to which the desired esters are obtained by reaction isovaleric ester with 1,3-dihalo- 1-propene in the presence of a strong base such as, for instance, alkali metal amides (LDA) , in a yield of 84% as racemate. The desired enantiomer is obtained from the racemate by treatment with esterases, for example with pig liver esterase (PLE) , in yields of about 32 to 46%.

A substantial disadvantage of this method is the use of the enzyme pig liver esterase (PLE) which is of animal origin.

J. Agric. Food Chem. , 32(1), pp. 85-92, describes for example the preparation of various haloalkenecarboxylic acids such as, for instance, racemic 2-isopropyl-5- chloropent-4 -enecarboxylic acid starting from the corresponding dialkyl isopropylmalonate . The malonate is in this case first alkylated with 1 , 3-dichloro-l- propene and then a decarboxylation takes place, converting the ester into the racemic 2-isopropyl-5- chloropent-4-enecarboxγlic acid. A racemate separation is not described.

According to WO 2004/052828, the method of J^". Agric. Food Chem. , 32(1), I₁ pp. 85-92, is slightly modified in relation to some parameters of the method. This method again has the disadvantage of the racemate separation described in the WO publication through the use of the enzyme pig liver esterase (PLE) . The use of such esterase extracts from natural sources such as pig liver is associated with disadvantages, however. Firstly, the qualities of 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 in principle 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.l (rPLE) from WO 02/48322. As is evident from the description of WO 02/48322 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 . Attempts to resolve racemic 2-alkyl-5-halopent-4- enecarboxylic esters with the recombinant pig liver esterase as shown in Seq. ID No. 1 (rPLE) from WO 02/48322 have shown, however, that only slight, or no, selective resolution takes place.

It was an object of the present invention to find a method for preparing enantiomerically enriched 2-alkyl- 5-halopent-4-enecarboxylic acids and their esters which makes it possible to prepare the desired compounds in optical purities which are higher than in the prior art and are up to e.e. > 99% and in higher yields of up to 98% of theory in a simple manner avoiding the natural pig liver esterase (PLE) , and to provide a corresponding novel recombinant esterase.

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

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

(I)

in which R, R₁ and X are as defined above, by means of a polypeptide having esterase activity or of a recombinant protein having esterase activity, which have an amino acid sequence as shown in SEQ ID NO 1 or is at least 80% identical thereto, in the presence of water or of an alcohol of the formula R₂OH, where R₂ is an alkyl group which is not equal to R_x, 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.

Enantiopure 2-alkyl-5-halopent-4-enecarboxylic acids or their esters of the formula (I) are prepared by the method of the invention.

R in the formula (I) is a Ci-C₆-alkyl radical such as, for instance, methyl, ethyl, n- and i-propyl, n-, i- and t-butyl, pentyl and hexyl . Ci-C₄-Alkyl radicals are preferred, and the i-propyl radical is particularly preferred.

Ri is H in the case of carboxylic acids and is a Ci-C₄- alkyl radical, preferably a Ci-C₂-alkyl radical and particularly preferably a methyl radical in the case of the esters . X is chlorine, bromine or iodine, preferably chlorine.

Suitable starting compounds of the formula (II) can be prepared for example as described in the prior art such as, for instance, in J. Agric. Food Chem. , 32 (1), 1, pp. 85-92, WO 2004/052828 or WO 01/09079.

Step a) is carried out in a suitable solvent.

The reaction temperature for the reaction of the invention is normally between 0 and 90⁰C, preferably between 10 and 6O⁰C. 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, 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 Ri. 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%.

The polypeptide having esterase activity and the recombinant protein (rAPLE) preferably exhibit an at least 90%, particularly preferably an at least 98%, identity with the sequence of the protein shown in SEQ ID No 1. It is also possible to employ a polypeptide having esterase activity or the recombinant esterase (rAPLE) with modified DNA sequences as shown in SEQ ID No 1, obtained by usual modifications such as, for instance, mutations, deletions, extensions and/or fusions, and which code for enzymes having the desired esterase activity.

The present invention accordingly relates further to a polypeptide which has esterase activity and which has the amino acid sequence SEQ. ID. No. 1.

The present invention likewise relates to a novel recombinant protein which has esterase activity and which has 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)

(I) in which R is a Ci-C₃-alkyl radical, Ri is C₁-C₄-alkyl and X is chlorine, bromine or iodine.

The polypeptide of the invention having esterase activity, and the novel recombinant pig liver esterase 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.

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 lie 75 VaI

GIy 76 VaI

GIy 77 GIu

Leu 80 Thr

Arg 87 GIy

He 92 Thr

Pro 93 Leu

VaI 129 Leu

Ser 133 Pro

Thr 134 Met

Leu 138 VaI

Ala 139 VaI

Phe 234 Leu

Ala 236 VaI

GIy 237 Ala

Phe 286 Leu

Ala 287 Thr

Leu 290 Phe

Pro 294 GIn

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, and which code for the desired esterase activity.

The invention 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, insertions, 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-B ' Primer 2: 5'-CCGGAATTCAGCCTCCCCTTCACAGCTCAG-B'

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 turned 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 are constructed. These expression constructs are then transformed into suitable host cells.

Suitable host cells in this connection are for example microorganisms and plants. 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.

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 selectively racemic 2-alkyl-5-halopent-4- enecarboxylic esters.

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: δ'-CAG/AΛTTCATGGCTATCGGGCAGCCAGCCTCGC-S¹ (SEQ ID. No.3)

Primer 2: 5'-CCGG>AA7TCAGCCTCCCCTTCACAGCTCAG -3¹ (SEQ ID No.4)

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 I 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 lxPhusion HF buffer in accordance with 'Phusion High-Fidelity DNA Polymerase' manual (Finnzymes), starting with a 30 second denaturation step at 98⁰C, followed by 30 cycles (10 sec 98°C, 20 sec 68°C, 1 min 72°C) for amplification and a final incubation at 72°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 TOPlO electrocompetent cells prepared in accordance with ^v 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 EcoRIalphal/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 Ix Phusion HF buffer in accordance with the ^v Phusion High-Fidelity DNA Polymerase' manual (Finnzymes) ) . Denaturation at 95⁰C 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 72⁰C for 7 min.

PCR II: The PLE and APLE sequences were amplified from pHILZ plasmids using either the PLEalphal/EcoRIPLE+ER2 primer pair or the PLEalphal/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 Ix Phusion HF buffer in accordance with the ^λ 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 III: 3 μl of the products from PCR I 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, I U of the Phusion DNA polymerase (Finnzymes) all in Ix Phusion HF buffer. The reaction mixture was heated at 95⁰C for 3 minutes and then 10 cycles with 30 sec at 95⁰C and 45 sec at 72⁰C were carried out. To amplify these overlapping extension products, 5 μl of primer mix (3 μl of water, 1 μl of 5 μM EcoRIalphal primer and 1 μl of 5 μM EcoRIPLE+ER2 or EcoRIPLE2 primer) were added. The products were amplified with 20 PCR cycles (30 sec 95⁰C, 30 sec 57°C, 1 min 72°C) and a single temperature stop at 72⁰C for 7 min. Primer sequences : EcoRlalphai : δ'-TCTTCGAAGAATTCACGATGAGATTTCCTTCAATTTTTACTGC-S'

(SEQ ID No. 5) alphaPLE2: 5'-GAGGCTGGCTGCCCAGCTTCAGCCTCTCTTTTCTCG-3'

(SEQ ID No. 6)

PLEalphal : 5'-AGAGAGGCTGAAGCTGGGCAGCCAGCCTCGCCG-3^I

(SEQ ID No. 7)

EcoRIPLE+ER2: 5'-ATGGT/ACCGΛΛ 7TCTCACAGCTCAGCATGCTTTATCTTG-S'

(SEQ ID No. 8)

EcoRIPLE2: 5"-ATGGLACCG>AyA 7TCTCACTTTATCTTGGGTGGCTTCTTTG-3'

(SEQ ID No. 9)

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 Pich±a 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 Ncol .

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) FEBS 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 expression kit from Invitrogen. The transformants were selected on YPD plates (1% yeast extract, 2% peptone, 2% D-glucose, 2% agar) which contained 100 mg/1 zeocin. 52 zeocin-resistant clones were streaked onto YPD plates with 100 mg/1 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/1 zeocin at 30⁰C for 48 h. The cells were lifted onto Whatman 541 hardened ashless 70 mm0 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/1 zeocin at 30⁰C for 48 h. The cells were lifted onto Whatman 541 hardened ashless 70 mm0 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 (figure 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 μ of a 2x 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 1 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 (figure 2) . The commercially available pig liver esterase which was also analyzed for comparison showed two protein bands in the same size range (arrow in figure 2) .

Example 9: Induced expression of pig liver esterases with the AOXl 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 Xhol, and the respective fragments coding for APLE and PLE proteins, with or without ER retention signal, were cloned via the Xhol cleavage site into the pPIC9 vector (Invitrogen) . Correct orientation of the fragments in relation to the AOXl promoter was checked by means of control cleavages with the restriction endonuclease Ncol . The vectors having the AOXl 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 Sail and transformed into P. pastoris YMIl. 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 AOXl 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

(figure 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

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

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

(I)

in which R, Ri and X are as defined above, by means of a polypeptide having esterase activity or of a recombinant protein having esterase activity, which have an amino acid sequence as shown in SEQ ID NO. 1 or is at least 80% identical thereto, in the presence of water or of an alcohol of the formula R₂OH, where R₂ is an alkyl group which is not equal to Ri, as nucleophile, and a) either the remaining enantiomerically enriched 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (II) with A equal to Ri 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.

2. A polypeptide having esterase activity and exhibiting the amino acid sequence SEQ. ID. No. 1.

3. A recombinant protein having esterase activity and exhibiting the amino acid sequence SEQ. ID. No. 1.

4. A polypeptide or recombinant protein which exhibits at least 80% identity to the amino acid sequence shown in SEQ ID No. 1 and has activity for racemate resolution of 2-alkyl-5-halopent-4- enecarboxylic esters of the formula (I)

(I)

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

5. A polypeptide or recombinant protein having esterase activity as claimed in any of claims 2-4, which exhibits an amino acid sequence modified by usual modifications from the group of mutation, deletion, insertion, extension and/or fusion.

6. A nucleotide sequence which codes for a polypeptide as claimed in claim 2 or a recombinant protein as claimed in claim 3.

7. A nucleotide sequence which has the sequence shown in SEQ ID No. 2.

8. A nucleotide sequence which comprises a nucleotide sequence as claimed in claim 6 or 7.

9. The use of a polypeptide or of a recombinant esterase as claimed in claim 2, 3 or 4 for racemate resolution of 2-alkyl-5-halopent-4-ene- carboxylic esters of the formula (I)

(I)

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