Epoxide hydrolases: new tools for the synthesis of fine organic chemicals

Deciphering the stereo-specific catalytic mechanisms of cis-epoxysuccinate hydrolases producing L(+)-tartaric acid

Microbial epoxide hydrolases, cis-epoxysuccinate hydrolases (CESHs), have been utilized for commercial production of enantiomerically pure L(+)- and D(-)-tartaric acids for decades. However, the stereo-catalytic mechanism of CESH producing L(+)-tartaric acid (CESH[L]) remains unclear. Herein, the crystal structures of two CESH[L]s in ligand-free, product-complexed, and catalytic intermediate forms were determined. These structures revealed the unique specific binding mode for the mirror-symmetric substrate, an active catalytic triad consisting of Asp-His-Glu, and an arginine providing a proton to the oxirane oxygen to facilitate the epoxide ring-opening reaction, which has been pursued for decades. These results provide the structural basis for the rational engineering of these industrial biocatalysts.

Potential of Y. lipolytica epoxide hydrolase for efficient production of enantiopure (R)-1,2-octanediol

The recombinant Yleh from a tropical marine yeast Yarrowia lipolytica NCIM 3589 exhibited a high epoxide hydrolase activity of 9.34 ± 1.80 µmol min^-1 mg^-1 protein towards 1,2-epoxyoctane (EO), at pH 8.0 and 30 °C. The reaction product was identified as 1,2-Octanediol (OD) by GC-MS using EO and H₂O¹⁸ as substrate, affirming the functionality of Yleh as an epoxide hydrolase. For EO, the K_m, V_max, and k_cat/K_m values were 0.43 ± 0.017 mM, 0.042 ± 0.003 mM min^-1, and 467.17 ± 39.43 mM^-1 min^-1, respectively. To optimize the reaction conditions for conversion of racemic EO by Yleh catalyst to enantiopure (R)-1,2-octanediol, initially, Response Surface Methodology was employed. Under optimized reaction conditions of 15 mM EO, 150 µg purified Yleh at 30 °C a maximal diol production of 7.11 mM was attained in a short span of 65 min with a yield of 47.4%. Green technology using deep eutectic solvents for the hydrophobic substrate (EO) were tested as co-solvents in Yleh catalyzed EO hydrolysis. Choline chloride-Glycerol, produced 9.08 mM OD with an increased OD yield of 60.5%. Thus, results showed that deep eutectic solvents could be a promising solvent for Yleh-catalyzed reactions making Yleh a potential biocatalyst for the biosynthesis of enantiopure synthons.

Epoxide Hydrolases: Multipotential Biocatalysts

Epoxide hydrolases are attractive and industrially important biocatalysts. They can catalyze the enantioselective hydrolysis of epoxides to the corresponding diols as chiral building blocks for bioactive compounds and drugs. In this review article, we discuss the state of the art and development potential of epoxide hydrolases as biocatalysts based on the most recent approaches and techniques. The review covers new approaches to discover epoxide hydrolases using genome mining and enzyme metagenomics, as well as improving enzyme activity, enantioselectivity, enantioconvergence, and thermostability by directed evolution and a rational design. Further improvements in operational and storage stabilization, reusability, pH stabilization, and thermal stabilization by immobilization techniques are discussed in this study. New possibilities for expanding the synthetic capabilities of epoxide hydrolases by their involvement in non-natural enzyme cascade reactions are described.

Epoxide containing molecules: A good or a bad drug design approach

Functional group modification is one of the main strategies used in drug discovery and development. Despite the controversy of being identified for many years as a biologically hazardous functional group, the introduction of an epoxide function in a structural backbone is still one of the possible modifications being implemented in drug design. In this manner, it is our intention to prove with this work that epoxides can have significant interest in medicinal chemistry, not only as anticancer agents, but also as important drugs for other pathologies. Thus, this revision paper aims to highlight the biological activity and the proposed mechanisms of action of several epoxide-containing molecules either in preclinical studies or in clinical development or even in clinical use. An overview of the chemistry of epoxides is also reported. Some of the conclusions are that effectively most of the epoxide-containing molecules referred in this work were being studied or are in the market as anticancer drugs. However, some of them in preclinical studies, were also associated with other different activities such as anti-malarial, anti-arthritic, insecticidal, antithrombotic, and selective inhibitory activity of FXIII-A (a transglutaminase). As for the epoxide-containing molecules in clinical trials, some of them are being tested for obesity and schizophrenia. Finally, drugs containing epoxide groups already in the market are mostly used for the treatment of different types of cancer, such as breast cancer and multiple myeloma. Other diseases for which the referred drugs are being used include heart failure, infections and gastrointestinal disturbs. In summary, epoxides can be a suitable option in drug design, particularly in the design of anticancer agents, and deserve to be better explored. However, and despite the promising results, it is imperative to explore the mechanisms of action of these compounds in order to have a better picture of their efficiency and safety.

Enantiomeric Tartaric Acid Production Using cis-Epoxysuccinate Hydrolase: History and Perspectives

Tartaric acid is an important chiral chemical building block with broad industrial and scientific applications. The enantioselective synthesis of l(+)- and d(−)-tartaric acids has been successfully achieved using bacteria presenting cis-epoxysuccinate hydrolase (CESH) activity, while the catalytic mechanisms of CESHs were not elucidated clearly until very recently. As biocatalysts, CESHs are unique epoxide hydrolases because their substrate is a small, mirror-symmetric, highly hydrophilic molecule, and their products show very high enantiomeric purity with nearly 100% enantiomeric excess. In this paper, we review over forty years of the history, process and mechanism studies of CESHs as well as our perspective on the future research and applications of CESH in enantiomeric tartaric acid production.

Exploring the origins of selectivity in soluble epoxide hydrolase from Bacillus megaterium

Epoxide hydrolase (EH) enzymes catalyze the hydration of racemic epoxides to yield their corresponding vicinal diols. In this work, the Bacillus megaterium epoxide hydrolase (BmEH)-mediated hydrolysis of racemic styrene oxide (rac-SO) and its para-nitro styrene oxide (rac-p-NSO) derivative are computationally investigated using density functional theory (DFT).

Epoxide hydrolase (EH) enzymes catalyze the hydration of racemic epoxides to yield their corresponding vicinal diols. These enzymes present different enantio- and regioselectivity depending upon either the substrate structure or the substitution pattern of the epoxide ring. In this study, we computationally investigate the Bacillus megaterium epoxide hydrolase (BmEH)-mediated hydrolysis of racemic styrene oxide (rac-SO) and its para-nitro styrene oxide (rac-p-NSO) derivative using density functional theory (DFT) and an active site cluster model consisting of 195 and 197 atoms, respectively. Full reaction mechanisms for epoxide ring opening were evaluated considering the attack at both oxirane carbons and considering two possible orientations of the substrate at the BmEH active site. Our results indicate that for both SO and p-NSO substrates the BmEH enantio- and regioselectivity is opposite to the inherent (R)-BmEH selectivity, the attack at the benzylic position (C1) of the (S)-enantiomer being the most favoured chemical outcome.

Enzyme-catalyzed cationic epoxide rearrangements in quinolone alkaloid biosynthesis

Epoxides are highly useful synthons and biosynthons for the construction of complex natural products during total synthesis and biosynthesis, respectively. Among enzyme-catalyzed epoxide transformations, a reaction that is notably missing, in regard to the synthetic toolbox, is cationic rearrangement that takes place under strong acid. This is a challenging transformation for enzyme catalysis, as stabilization of the carbocation intermediate upon epoxide cleavage is required. Here, we discovered two Brønsted acid enzymes that can catalyze two unprecedented epoxide transformations in biology. PenF from the penigequinolone pathway catalyzes a cationic epoxide rearrangement under physiological conditions to generate a quaternary carbon center, while AsqO from the aspoquinolone pathway catalyzes a 3-exo-tet cyclization to forge a cyclopropane-tetrahydrofuran ring system. The discovery of these new epoxide-modifying enzymes further highlights the versatility of epoxides in complexity generation during natural product biosynthesis.

Expression and production of recombinant cis-epoxysuccinate hydrolase in Escherichia coli under the control of temperature-dependent promoter

cis-Epoxysuccinate hydrolase (ESH) from Nocardia tartaricans CAS-52 could catalyze the stereospecific hydrolysis of cis-epoxysuccinate to L-(+)-tartrate. The ESH gene of 762 bp was cloned and its open reading frame (ORF) sequence predicted a protein of 253 amino acids. An expression plasmid carrying the ESH gene under the control of the P(L)P(R) promoter was introduced into Escherichia coli, and the ESH gene was successfully expressed in the recombinant strain. The expression conditions and scale-up production were also studied. Fed-batch fermentation of E. coli Trans 1-T1 transformant was carried out in a 2000 L fermentor to product recombinant ESH. The results showed that wet cell concentration reached to 62.45 g L(-1), and the specific activity of ESH was 380.17 U mg(-1), which could meet the requirements of industrial production of L-(+)-tartaric acid.

Microsomal epoxide hydrolase gene polymorphisms and susceptibility to chronic obstructive pulmonary disease in the Tunisian population

It is well known that cigarette smoking is the major risk factor for chronic obstructive pulmonary disease (COPD). However, only 10%-20% of chronic heavy cigarette smokers develop symptomatic disease, which suggests the presence of genetic susceptibility. Microsomal epoxide hydrolase (EPHX1) is an enzyme involved in the protective mechanism against oxidative stress. It has been reported that gene polymorphisms of this enzyme may be associated with variations in EPHX1 activity. In this study, we aimed at investigating the relationship between EPHX1 polymorphisms and susceptibility to COPD in the Tunisian population. EPHX1 exon 3 (rs1051740, Tyr113His) and exon 4 (rs2234922, His139Arg) polymorphisms were genotyped by polymerase chain reaction followed by restriction fragment length polymorphism analysis. These techniques were used to examine a total of 416 Tunisian individuals, including 182 blood donors and a group of 234 COPD patients. All subjects were not related. An increased risk for COPD was observed in subjects with EPHX1 His113-His113 genotype (odds ratio = 2.168; confidence interval 1.098-4.283; p = 0.02386). However, multivariate logistic regression analysis showed no significant relationship between the mutant genotype and the disease after adjustment for sex, age, body mass index, smoking status, and pack-year smoking (odds ratio = 1.524; confidence interval, 0.991-6.058; p = 0.06137). Regarding the two subtypes of COPD, our investigations demonstrated that there is no significant correlation between exon 3 polymorphism and the chronic bronchitis subgroup (p = 0.09034). The relation between exon 3 polymorphism and emphysema was significant in the univariate analysis (p = 0.02257), but no association was found after controlling for classic risk factors (p = 0.06273). In conclusion, our results showed that there is a weak relation between 113His genotype and COPD, and no apparent relation between 139Arg and COPD in the studied Tunisian population.

Optimization of culture conditions for production of cis-epoxysuccinic acid hydrolase using response surface methodology

Response surface methodology, which allows for rapid identification of important factors and optimization of them to enhance enzyme production, was employed here to optimize culture conditions for the production of cis-epoxysuccinic acid hydrolase from Bordetella sp. strain 1-3. In the first step, a Plackett-Burman design was used to evaluate the effects of nine variables (yeast extract, cis-epoxysuccinic acid, KH(2)PO(4), K(2)HPO(4).3H(2)O, MgSO(4).7H(2)O, trace minerals solution, culture volume, initial pH and incubation time) on the enzyme production. Yeast extract, cis-epoxysuccinic acid and KH(2)PO(4) had significant influences on cis-epoxysuccinic acid hydrolase production and their concentrations were further optimized using central composite design and response surface analysis. A combination of adjusting the concentration of yeast extract to 7.8 g/l, cis-epoxysuccinic acid to 9.8 g/l, and KH(2)PO(4) to 1.12 g/l would favor maximum cis-epoxysuccinic acid hydrolase production. An enhancement of cis-epoxysuccinic acid hydrolase production from 5.6 U/ml to 9.27 U/ml was gained after optimization.

Cloning of an epoxide hydrolase-encoding gene from Aspergillus niger M200, overexpression in E. coli, and modification of activity and enantioselectivity of the enzyme by protein engineering

The gene encoding an epoxide hydrolase from Aspergillus niger M200 has been cloned and its sequence determined. The gene is interrupted by seven introns, one exon being only nine nucleotides long. The non-coding 5'- and 3'-regions of the mRNA are composed of 47 and 76 nucleotides, respectively. Overexpression of the fungal epoxide hydrolase in E. coli TOP10 has led to a 15-fold increase in specific activity (compared to the wild-type strain). Saturation mutagenesis at codon 217 resulted in the discovery of nine enzyme variants showing in several cases profound differences in activity and enantioselectivity towards various epoxides when compared to the data of the wild-type enzyme. The site 217 is located at the entrance of the tunnel that provides the substrate with access to the active site. The exchange of Ala at this position for Cys has led to a doubled enantioselectivity (E-value of 5.0) towards benzyl glycidyl ether. The same substitution resulted in a threefold-enhanced activity of the enzyme towards allyl glycidyl ether and styrene oxide without affecting enantioselectivity. The variant A217L showed an enhanced enantioselectivity towards tert-butyl glycidyl ether reaching an E-value of 100 (from 60 for the wild-type enzyme). Replacement of A217 by Val has led to higher activity towards allyl glycidyl ether by a factor of six. The substitutions Ala-->Glu and Ala-->Gln increased the enantioselectivity towards allyl glycidyl ether and styrene oxide by over 50% to E-values of 10 and 16, respectively. The study underlines that single amino acid exchanges in the substrate tunnel region can lead to significant improvements in enantioselectivity and activity of the epoxide hydrolase from A. niger M200.

Enantioconvergent production of (R)-1-phenyl-1,2-ethanediol from styrene oxide by combining the Solanum tuberosum and an evolved Agrobacterium radiobacter AD1 epoxide hydrolases

Soluble epoxide hydrolase (EH) from the potato Solanum tuberosum and an evolved EH of the bacterium Agrobacterium radiobacter AD1, EchA-I219F, were purified for the enantioconvergent hydrolysis of racemic styrene oxide into the single product (R)-1-phenyl-1,2-ethanediol, which is an important intermediate for pharmaceuticals. EchA-I219F has enhanced enantioselectivity (enantiomeric ratio of 91 based on products) for converting (R)-styrene oxide to (R)-1-phenyl-1,2-ethanediol (2.0 +/- 0.2 micromol/min/mg), and the potato EH converts (S)-styrene oxide primarily to the same enantiomer, (R)-1-phenyl-1,2-ethanediol (22 +/- 1 micromol/min/mg), with an enantiomeric ratio of 40 +/- 17 (based on substrates). By mixing these two purified enzymes, inexpensive racemic styrene oxide (5 mM) was converted at 100% yield to 98% enantiomeric excess (R)-1-phenyl-1,2-ethanediol at 4.7 +/- 0.7 micromol/min/mg. Hence, at least 99% of substrate is converted into a single stereospecific product at a rapid rate.

Enzymatic resolution of racemic phenyloxirane by a novel epoxide hydrolase from Aspergillus niger SQ-6 and its fed-batch fermentation

A microorganism with the ability to catalyze the resolution of racemic phenyloxirane was isolated and identified as Aspergillus niger SQ-6. Chiral capillary electrophoresis was successfully applied to separate both phenyloxirane and phenylethanediol. The epoxide hydrolase (EH) involved in this resolution process was (R)-stereospecific and constitutively expressed. When whole cells were used during the biotransformation process, the optimum temperature and pH for stereospecific vicinal diol production were 35 degrees C and 7.0, respectively. After a 24-h conversion, the enantiomer excess of (R)-phenylethanediol produced was found to be >99%, with a conversion rate of 56%. In fed-batch fermentations at 30 degrees C for 44 h, glycerol (20 g L(-1)) and corn steep liquor (CSL) (30 g L(-1)) were chosen as the best initial carbon and nitrogen sources, and EH production was markedly improved by pulsed feeding of sucrose (2 g L(-1) h(-1)) and continuous feeding of CSL (1 g L(-1) h(-1)) at a fermentation time of 28 h. After optimization, the maximum dry cell weight achieved was 24.5+/-0.8 g L(-1); maximum EH production was 351.2+/-13.1 U L(-1) with a specific activity of 14.3+/-0.5 U g(-1). Partially purified EH exhibited a temperature optimum at 37 degrees C and pH optimum at 7.5 in 0.1 M phosphate buffer. This study presents the first evidence for the existence of a predicted epoxide racemase, which might be important in the synthesis of epoxide intermediates.

Protein engineering of epoxide hydrolase from Agrobacterium radiobacter AD1 for enhanced activity and enantioselective production of (R)-1-phenylethane-1,2-diol

DNA shuffling and saturation mutagenesis of positions F108, L190, I219, D235, and C248 were used to generate variants of the epoxide hydrolase of Agrobacterium radiobacter AD1 (EchA) with enhanced enantioselectivity and activity for styrene oxide and enhanced activity for 1,2-epoxyhexane and epoxypropane. EchA variant I219F has more than fivefold-enhanced enantioselectivity toward racemic styrene oxide, with the enantiomeric ratio value (E value) for the production of (R)-1-phenylethane-1,2-diol increased from 17 for the wild-type enzyme to 91, as well as twofold-improved activity for the production of (R)-1-phenylethane-1,2-diol (1.96 +/- 0.09 versus 1.04 +/- 0.07 micromol/min/mg for wild-type EchA). Computer modeling indicated that this mutation significantly alters (R)-styrene oxide binding in the active site. Another three variants from EchA active-site engineering, F108L/C248I, I219L/C248I, and F108L/I219L/C248I, also exhibited improved enantioselectivity toward racemic styrene oxide in favor of production of the corresponding diol in the (R) configuration (twofold enhancement in their E values). Variant F108L/I219L/C248I also demonstrated 10-fold- and 2-fold-increased activity on 5 mM epoxypropane (24 +/- 2 versus 2.4 +/- 0.3 micromol/min/mg for the wild-type enzyme) and 5 mM 1,2-epoxyhexane (5.2 +/- 0.5 versus 2.6 +/- 0.0 micromol/min/mg for the wild-type enzyme). Both variants L190F (isolated from a DNA shuffling library) and L190Y (created from subsequent saturation mutagenesis) showed significantly enhanced activity for racemic styrene oxide hydrolysis, with 4.8-fold (8.6 +/- 0.3 versus 1.8 +/- 0.2 micromol/min/mg for the wild-type enzyme) and 2.7-fold (4.8 +/- 0.8 versus 1.8 +/- 0.2 micromol/min/mg for the wild-type enzyme) improvements, respectively. L190Y also hydrolyzed 1,2-epoxyhexane 2.5 times faster than the wild-type enzyme.

The Saccharomyces cerevisiae ORF YNR064c protein has characteristics of an ‘orphaned’ epoxide hydrolase

The open reading frame YNR064c in Saccharomyces cerevisiae encodes a protein tentatively assigned as similar to a bacterial dehalogenase. In this study we conclude that the YNR064c protein displays characteristics of an epoxide hydrolase belonging to the alpha/beta-hydrolase fold family of enzymes. Endogenous expression of the protein in S. cerevisiae was confirmed and a His-tagged variant of the protein was heterologously expressed in both Escherichia coli and Pichia pastoris for isolation and characterization. The YNR064c protein displayed low but reproducible epoxide hydrolase activity with racemic phenanthrene 9,10-oxide and trans- or cis-stilbene oxide. Phylogenetic analysis of related gene products found in various microorganisms suggested that the YNR064c protein is a member of a new subclass of alpha/beta-hydrolase fold enzymes.

Enhancing the enantioselectivity of an epoxide hydrolase by directed evolution

[reaction: see text] The epoxide hydrolase (EH) from Aspergillus niger, which shows a selectivity factor of only E = 4.6 in the hydrolytic kinetic resolution of glycidyl phenyl ether, has been subjected to directed evolution for the purpose of enhancing enantioselectivity. After only one round of error-prone polymerase chain reaction (epPCR), enantioselectivity was more than doubled (E = 10.8). The improved mutant enzyme contains three amino acid exchanges, two of which are spatially far from the catalytically active center.

Microsomal Epoxide Hydrolase, Endotoxin, and Lung Function Decline in Cotton Textile Workers

Occupational exposure to endotoxin in organic dust may induce lung function decline. Microsomal epoxide hydrolase (mEH) detoxifies reactive oxygen species generated by endotoxin exposure, and polymorphisms of the mEH gene are associated with altered enzyme activity. We investigated the associations between mEH polymorphisms, endotoxin exposure, and lung function decline in a 20-year prospective study of 265 workers exposed to endotoxin and 234 control subjects. mEH Tyr113His and His139Arg polymorphisms were genotyped by the 5' nuclease assay, and data were analyzed using multivariate linear regression models, adjusting for important covariates. Overall, the annual decline rate of FEV1 was 29.47 ml during the 20-year follow-up. Endotoxin exposure was associated with faster lung function decline among genotypes associated with slower enzyme activity: estimates (SE) of annual FEV1 decline rates for endotoxin exposure were -2.33 (2.07), -2.81 (1.66), and -6.73 (2.83) ml for Tyr/Tyr, Tyr/His, and His/His genotype groups, respectively, for the Tyr113His polymorphism; and -1.82 (2.58) and -4.27 (1.33) ml for Arg/Arg + His/Arg and His/His genotypes, respectively, for the His139Arg polymorphism. We conclude that mEH polymorphisms modify the association between occupational endotoxin exposure and longitudinal lung function decline.

Epoxide Hydrolases: Structure, Function, Mechanism, and Assay

Epoxide hydrolases are a class of enzymes important in the detoxification of genotoxic compounds, as well as in the control of physiological signaling molecules. This chapter gives an overview on the function, structure, and enzymatic mechanism of structurally characterized epoxide hydrolases and describes selected assays for the quantification of epoxide hydrolase activity.

Active Site Engineering of the Epoxide Hydrolase from Agrobacterium radiobacter AD1 to Enhance Aerobic Mineralization of cis-1,2-Dichloroethylene in Cells Expressing an Evolved Toluene ortho-Monooxygenase

Chlorinated ethenes are the most prevalent ground-water pollutants, and the toxic epoxides generated during their aerobic biodegradation limit the extent of transformation. Hydrolysis of the toxic epoxide by epoxide hydrolases represents the major biological detoxification strategy; however, chlorinated epoxyethanes are not accepted by known bacterial epoxide hydrolases. Here, the epoxide hydrolase from Agrobacterium radiobacter AD1 (EchA), which enables growth on epichlorohydrin, was tuned to accept cis-1,2-dichloroepoxyethane as a substrate by accumulating beneficial mutations from three rounds of saturation mutagenesis at three selected active site residues, Phe-108, Ile-219, and Cys-248 (no beneficial mutations were found at position Ile-111). The EchA F108L/I219L/C248I variant coexpressed with a DNA-shuffled toluene ortho-monooxygenase, which initiates attack on the chlorinated ethene, enhanced the degradation of cis-dichloroethylene (cis-DCE) an infinite extent compared with wild-type EchA at low concentrations (6.8 microm) and up to 10-fold at high concentrations (540 microm). EchA variants with single mutations (F108L, I219F, or C248I) enhanced cis-DCE mineralization 2.5-fold (540 microm), and EchA variants with double mutations, I219L/C248I and F108L/C248I, increased cis-DCE mineralization 4- and 7-fold, respectively (540 microm). For complete degradation of cis-DCE to chloride ions, the apparent Vmax/Km for the Escherichia coli strain expressing recombinant the EchA F108L/I219L/C248I variant was increased over 5-fold as a result of the evolution of EchA. The EchA F108L/I219L/C248I variant also had enhanced activity for 1,2-epoxyhexane (2-fold) and the natural substrate epichlorohydrin (6-fold).

Fungal epoxide hydrolases: new landmarks in sequence-activity space

Epoxide hydrolases are useful catalysts for the hydrolytic kinetic resolution of epoxides, which are sought after intermediates for the synthesis of enantiopure fine chemicals. The epoxide hydrolases from Aspergillus niger and from the basidiomycetous yeasts Rhodotorula glutinis and Rhodosporidium toruloides have demonstrated potential as versatile, user friendly biocatalysts for organic synthesis. A recombinant A. niger epoxide hydrolase, produced by an overproducing A. niger strain, is already commercially available and recombinant yeast epoxide hydrolases expressed in Escherichia coli have shown excellent results. Within the vast body of activity information on the one hand and gene sequence information on the other hand, the epoxide hydrolases from the Rhodotorula spp. and A. niger stand out because we have sequence information as well as activity information for both the wild-type and recombinant forms of these enzymes.

Overexpression of Arabidopsis thaliana soluble epoxide hydrolase 1 in Pichia pastoris and characterisation of the recombinant enzyme

Epoxide hydrolases are enzymes involved in metabolism and defense of plants. Genome scanning suggested the presence of several genes encoding epoxide hydrolase in Arabidopsis thaliana. To assure that the predicted genes are functional and the translated products have epoxide hydrolase activity analysis at the protein level is needed. We have started to clone the cDNAs and overexpress them for catalytic and physico-chemical analysis. We here report that Pichia pastoris serves as an efficient system for overexpression of soluble epoxide hydrolase 1 (AtsEH1) from A. thaliana. A tag containing six histidine residues was added to the N-terminus to enable efficient one-step purification on nickel-agarose. The enzyme was expressed at levels >18 mg.L(-1) of culture and a French Press was found to be effective to achieve cell lysis. The recombinant enzyme had a molecular mass of 37 or 38 kDa based on SDS-PAGE or MALDI-TOF analysis, respectively. The enzyme was highly active towards the substrate trans-stilbene oxide (TSO) and had a pH optimum at 7 and a temperature optimum at 54 degrees C. Using TSO as substrate the K(m) and V(max) values were determined to 5 micro M and 2 micromol min(-1) mg protein(-1), respectively. The activity was 50-fold lower towards cis-stilbene oxide. The stability over time was tested from 20 to 54 degrees C and the enzyme lost activity at varying degrees at the temperatures tested but was stable for several months at 4 degrees C.

Cloning and expression of the haloalkane dehalogenase gene dhmA from Mycobacterium avium N85 and preliminary characterization of DhmA

Haloalkane dehalogenases are microbial enzymes that catalyze cleavage of the carbon-halogen bond by a hydrolytic mechanism. Until recently, these enzymes have been isolated only from bacteria living in contaminated environments. In this report we describe cloning of the dehalogenase gene dhmA from Mycobacterium avium subsp. avium N85 isolated from swine mesenteric lymph nodes. The dhmA gene has a G+C content of 68.21% and codes for a polypeptide that is 301 amino acids long and has a calculated molecular mass of 34.7 kDa. The molecular masses of DhmA determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by gel permeation chromatography are 34.0 and 35.4 kDa, respectively. Many residues essential for the dehalogenation reaction are conserved in DhmA; the putative catalytic triad consists of Asp123, His279, and Asp250, and the putative oxyanion hole consists of Glu55 and Trp124. Trp124 should be involved in substrate binding and product (halide) stabilization, while the second halide-stabilizing residue cannot be identified from a comparison of the DhmA sequence with the sequences of three dehalogenases with known tertiary structures. The haloalkane dehalogenase DhmA shows broad substrate specificity and good activity with the priority pollutant 1,2-dichloroethane. DhmA is significantly less stable than other currently known haloalkane dehalogenases. This study confirms that a hydrolytic dehalogenase is present in the facultative pathogen M. avium. The presence of dehalogenase-like genes in the genomes of other mycobacteria, including the obligate pathogens Mycobacterium tuberculosis and Mycobacterium bovis, as well as in other bacterial species, including Mesorhizobium loti, Xylella fastidiosa, Photobacterium profundum, and Caulobacter crescentus, led us to speculate that haloalkane dehalogenases have some other function besides catalysis of hydrolytic dehalogenation of halogenated substances.

Highly selective hydrolytic kinetic resolution of terminal epoxides catalyzed by chiral (salen)Co(III) complexes. Practical synthesis of enantioenriched terminal epoxides and 1,2-diols

The hydrolytic kinetic resolution (HKR) of terminal epoxides catalyzed by chiral (salen)Co(III) complex 1 x OAc affords both recovered unreacted epoxide and 1,2-diol product in highly enantioenriched form. As such, the HKR provides general access to useful, highly enantioenriched chiral building blocks that are otherwise difficult to access, from inexpensive racemic materials. The reaction has several appealing features from a practical standpoint, including the use of H(2)O as a reactant and low loadings (0.2-2.0 mol %) of a recyclable, commercially available catalyst. In addition, the HKR displays extraordinary scope, as a wide assortment of sterically and electronically varied epoxides can be resolved to > or = 99% ee. The corresponding 1,2-diols were produced in good-to-high enantiomeric excess using 0.45 equiv of H(2)O. Useful and general protocols are provided for the isolation of highly enantioenriched epoxides and diols, as well as for catalyst recovery and recycling. Selectivity factors (k(rel)) were determined for the HKR reactions by measuring the product ee at ca. 20% conversion. In nearly all cases, k(rel) values for the HKR exceed 50, and in several cases are well in excess of 200.

Synthetic applications of epoxide hydrolases

There have been several recent advances in the area of biocatalysed hydrolytic kinetic resolution of epoxides using 'newly discovered' enzymes (i.e. epoxide hydrolases). These biocatalysts, two of which will become commercially available in the near future, appear to be highly promising tools for fine organic synthesis, as they enable the preparation of various epoxides and/or their corresponding diols in enantiopure form.

Microbiological transformations. 47. A step toward a green chemistry preparation of enantiopure (S)-2-, -3-, and -4-pyridyloxirane via an epoxide hydrolase catalyzed kinetic resolution

The biocatalyzed hydrolytic kinetic resolution of 2-, 3-, and 4-pyridyloxirane by the Aspergillus niger epoxide hydrolase (EH) has been explored. This was used to perform a gram scale preparation of these epoxides of (S) absolute configuration using a process performed at a concentration as high as 10 g/L (82 mM). All three epoxides have been obtained in a nearly enantiopure form (ee > 98%). Interestingly, it was shown that this biotransformation could be achieved using plain water instead of buffer solution, an important improvement as far as downstream processing of an eventual industrial process is concerned. Neither of these substrates could be obtained in reasonable enantiomeric purity and yield using the nowadays most efficient metal-based catalysts.