Kinetic resolution of α-methylbenzylamine by recombinant <Emphasis Type="Italic">Pichia pastoris</Emphasis> expressing ω-transaminase

Recombinant Pichia pastoris expressing ω-transaminase (TA) was used as a whole-cell biocatalyst to kinetically resolve α-methylbenzylamine (MBA). To overcome product inhibition of ω-TA by acetophenone (deaminated product of α-MBA), the reaction condition of endogenous oxidoreductases, which can catalyze the reduction of acetophenone into non-inhibitory 1-phenylethanol, was optimized. When the whole-cell reaction was carried out using recombintat P. pastoris in 100 mM Tris/HCl buffer (pH 9.0) containing 2.5% glucose and 1% methanol, 100 mM α-MBA was successfully resolved to (R)-α-MBA (> 99% ee) at a conversion of 52.2%.     

Simultaneous synthesis of enantiomerically pure (S)-amino acids and (R)-amines using α/ω-aminotransferase coupling reactions with two-liquid phase reaction system

An efficient simultaneous synthesis of enantiopure (S)-amino acids and chiral (R)-amines was achieved using α/ω-aminotransferase (α/ω-AT) coupling reaction with two-liquid phase system. As, among the enzyme components in the α/ω-AT coupling reaction systems, only ω-AT is severely hampered by product inhibition by ketone product, the coupled reaction cannot be carried out above 60 mM substrates. To overcome this problem, a two-liquid phase reaction was chosen, where dioctylphthalate was selected as the solvent based upon biocompatibility, partition coefficient and effect on enzyme activity. Using 100 mM of substrates, the AroAT/ω-AT and the AlaAT/ω-AT coupling reactions asymmetrically synthesized (S)-phenylalanine and (S)-2-aminobutyrate with 93% (>99% ee^S) and 95% (>99% ee^S) of conversion yield, and resolved the racemic α-methylbenzylamine with 56% (95% ee^R) and 54% (96% ee^R) of conversion yield, respectively. Moreover, using 300 mM of 2-oxobutyrate and 300 mM of racemic α-methylbenzylamine as substrates, the coupling reactions yielded 276 mM of (S)-2-aminobutyrate (>99% ee) and 144 mM of (R)-α-methylbenzylamine (>96% ee) in 9 h. Here, most of the reactions take place in the aqueous phase, and acetophenone mainly moved to the organic phase according to its partition coefficient.     

Bioproduction of chiral mandelate by enantioselective deacylation of α-acetoxyphenylacetic acid using whole cells of newly isolated Pseudomonas sp. ECU1011

Substrate-directed screening was carried out to find bacteria that could deacylate O-acetylated mandelic acid from environmental samples. From more than 200 soil isolates, we identified for the first time that Pseudomonas sp. ECU1011 biocatalytically deacylated (S)-α-acetoxyphenylacetic acid with high enantioselectivity (E > 200), yielding (S)-mandelic acid with 98.1% enantiomeric excess (ee) at a 45.5% conversion rate. The catalytic deacylation of (S)-α-acetoxyphenylacetic acid by the resting cell was optimized using a single-factor method to yield temperature and pH optima of 30°C and 6.5, respectively. These optima help to reduce the nonselective spontaneous hydrolysis of the racemic substrate. It was found that substrate concentrations up to 60 mM could be used. 2-Propanol was used as a moderate cosolvent to help the substrate disperse in the aqueous phase. Under optimized reaction conditions, the ee of the residual (R)-α-acetoxyphenylacetic acid could be improved further, to greater than 99%, at a 60% conversion rate. Furthermore, using this newly isolated strain of Pseudomonas sp. ECU1011, three kinds of optically pure analogs of (S)-mandelic acid and (R)-α-acetoxyphenylacetic acid were successfully prepared at high enantiomeric purity.     

Microbial production of optically active β-phenylalanine ethyl ester through stereoselective hydrolysis of racemic β-phenylalanine ethyl ester

The ability to produce (R)- or (S)-β-phenylalanine ethyl ester (3-amino-3-phenylpropionic acid ethyl ester, BPAE) from racemic BPAE through stereoselective hydrolysis was screened for in BPAE-assimilating microorganisms. Sphingobacterium sp. 238C5 and Arthrobacter sp. 219D2 were found to be potential catalysts for (R)- and (S)-BPAE production, respectively. On a 24-h reaction, with 2.5% (w/v) racemic BPAE (130 mM) as the substrate and wet cells of Sphingobacterium sp. 238C5 as the catalyst, 1.15% (w/v) (R)-BPAE (60 mM) with enantiomeric purity of 99% e.e. was obtained, the molar yield as to racemic BPAE being 46%. On a 48-h reaction, with 2.5% (w/v) racemic BPAE (130 mM) as the substrate and wet cells of Arthrobacter sp. 219D2 as the catalyst, 0.87% (w/v) (S)-BPAE (45 mM) with enantiomeric purity of 99% e.e. was obtained, the molar yield as to racemic BPAE being 35%. The enzyme stereoselectively hydrolyzing (S)-BPAE was purified to homogeneity from the cell-free extract of Sphingobacterium sp. 238C5. The enzyme was a monomeric protein with a molecular mass of about 42,000. The enzyme catalyzed hydrolysis of β-phenylalanine esters, while the common aliphatic and aromatic carboxylate esters were not catalyzed.     

Simultaneous synthesis of enantiomerically pure (R)-1-phenylethanol and (R)-alpha-methylbenzylamine from racemic alpha-methylbenzylamine using omega-transaminase/alcohol dehydrogenase/glucose dehydrogenase coupling reaction

A simultaneous synthesis of (R)-1-phenylethanol and (R)--methylbenzylamine from racemic -methylbenzyl- amine was achievied using an -transaminase, alcohol dehydrogenase, and glucose dehydrogenase in a coupled reaction. Racemic -methylbenzylamine (100 mM) was converted to 49 mM (R)-1-phenylethanol (> 99% ee) and 48 mM (R)--methylbenzylamine (> 98% ee) in 18 h at 37 °C. This method was also used to overcome product inhibition of -TA by the ketone product in the kinetic resolution of racemic amines at high concentration.     

Kinetic resolution of α-lipoic acid via enzymatic differentiation of a remote stereocenter

Kinetic resolution of α-lipoic acid, a case of remote stereocenter discrimination, was accomplished using lipase from Aspergillus oryzae WZ007. Performance of this lipase was investigated for enantioselective esterification of (S)-α-lipoic acid, leaving the target product (R)-α-lipoic acid in unreacted form. The effects of chain length of alcohol, type of solvent, molar ratio of alcohol:acid, and reaction temperature were studied. The optimum reaction conditions were found to be esterification with n-octanol at 50°C in heptane with an alcohol:acid molar ratio of 5:1. The conversion rate of α-lipoic acid was 75.2%, with an enantiomeric excess of 92.5% towards unreacted substrate in a reaction time of 48 h.     

Active site model of (R)-selective ω-transaminase and its application to the production of d-amino acids

ω-Transaminase (ω-TA) is one of the important biocatalytic toolkits owing to its unique enzyme property which enables the transfer of an amino group between primary amines and carbonyl compounds. In addition to preparation of chiral amines, ω-TA reactions have been exploited for the asymmetric synthesis of l-amino acids using (S)-selective ω-TAs. However, despite the availability of (R)-selective ω-TAs, catalytic utility of the ω-TAs has not been explored for the production of d-amino acids. Here, we investigated the substrate specificity of (R)-selective ω-TAs from Aspergillus terreus and Aspergillus fumigatus and demonstrated the asymmetric synthesis of d-amino acids from α-keto acids. Substrate specificity toward d-amino acids and α-keto acids revealed that the two (R)-selective ω-TAs possess strict steric constraints in the small binding pocket that precludes the entry of a substituent larger than an ethyl group, which is reminiscent of (S)-selective ω-TAs. Molecular models of the active site bound to an external aldimine were constructed and used to explain the observed substrate specificity and stereoselectivity. α-Methylbenzylamine (α-MBA) showed the highest amino donor reactivity among five primary amines (benzylamine, α-MBA, α-ethylbenzylamine, 1-aminoindan, and isopropylamine), leading us to employ α-MBA as an amino donor for the amination of 5 reactive α-keto acids (pyruvate, 2-oxobutyrate, fluoropyruvate, hydroxypyruvate, and 2-oxopentanoate) among 17 ones tested. Unlike the previously characterized (S)-selective ω-TAs, the enzyme activity of the (R)-selective ω-TAs was not inhibited by acetophenone (i.e., a deamination product of α-MBA). Using racemic α-MBA as an amino donor, five d-amino acids (d-alanine, d-homoalanine, d-fluoroalanine, d-serine, and d-norvaline) were synthesized with excellent product enantiopurity (enantiomeric excess >99.7 %).     

Cloning,expression and enantioselective hydrolytic catalysis of a microsomal epoxide hydrolase from a marine fish, <Emphasis Type="Italic">Mugil cephalus</Emphasis>

The cDNA of a marine fish microsomal epoxide hydrolase (mEH) gene from Mugil cephalus was cloned by rapid amplification of cDNA ends (RACE) techniques. The homology model for the mEH of M. cephalus showed a characteristic structure of α/β-hydrolase-fold main domain with a lid domain over the active site. The characteristic catalytic triad, consisting of Asp(238), His(444), and Glu(417), was highly conserved. The cloned mEH gene was expressed in Escherichia coli and the recombinant mEH exhibited (R)-preferred hydrolysis activity toward racemic styrene oxide. We obtained enantiopure (S)-styrene oxide with a high enantiopurity of more than 99% enantiomeric excess and yield of 15.4% by batch kinetic resolution of 20 mM racemic styrene oxide.     

The novel inhibitors of serine proteases

Thirty optically active nonprotein α-amino acids and peptides based thereon have been screened for their ability to interact with bovine trypsin and proteinase K from Tritirachium album Limber, which belong to the group of serine proteases. Both structure-based drug design approach and determination of enzyme activity have been used to identify low molecular weight inhibitors of trypsin and proteinase K. Compounds have been selected that according to the docking analysis were able to interact with trypsin and proteinase K. Following the docking analysis measurement of enzymes activity (2R,3S)-β-hydroxyleucine and (2S,3R)-β-hydroxyleucine inhibited both enzymes activity, whereas (S)-α-methyl-β-phenylalanine, (R)-α-methyl-β-phenylalanine, (S)-allylglycine, (R)-allylglycine, (S)-α-allylalanine, (R)-α-allylalanine and allo-O-ethylthreonine inhibited only proteinase K; and N-formyl-(S)-methionyl-(2S,3R)-hydroxyleucine, N-formyl-(S)-methionyl-(2R,3S)-hydroxyleucine, N-formyl-(S)-methionyl-(S)-allylglycine and N-formyl-(S)-methionyl-(R)-allylglycine inhibited trypsin. It has been shown that inhibition of trypsin by (2R,3S)-β-hydroxyleucine and N-formyl-(S)-methionyl-(2R,3S)-hydroxyleucine is of a competitive mode.     

Production of enantiomerically pure (<Emphasis Type="Italic">S</Emphasis>)-β-phenylalanine and (<Emphasis Type="Italic">R</Emphasis>)-β-phenylalanine by penicillin G acylase from <Emphasis Type="Italic">Escherichia coli</Emphasis> in aqueous medium

A new approach has been developed for the production of enantiomerically pure (S)-β-phenylalanine (S-BPA) and (R)-β-phenylalanine in aqueous medium based on enantioselective acylation and hydrolysis properties of penicillin G acylase from Escherichia coli. The acylation reaction was highly preferential for the acylation of (R)-BPA to form N-phenylacetyl-(R)-BPA using phenylacetamide as an acyl donor, which was separated and then hydrolyzed to (R)-BPA by the same enzyme at pH 7.5. The optimal acylation reaction was at pH 10, 25°C with a 2:1 molar ratio of phenylacetamide to BPA, 8 IU ml⁻¹ enzyme and 150 mM BPA. These resulted in a conversion of about 50% BPA; enantiomeric excess of (S)-BPA and (R)-BPA separated were 98 and 99%, respectively.     

Synthesis and binding properties of endomorphin-2analogs containing α-hydroxymethyl amino acids

Summary Novel endomorphin-2 analogs containing the unusual amphiphilic amino acid (R)- and (S)-α-hydroxymethyltyrosine in position 1 and (R)- and (S)-α-hydroxymethylphenylalanine in the positions 3 and 4 were synthesized via the solid-phase method. The binding characteristics of the synthetic analogs may suggest that α-hydroxymethyl substitution of amino acid residues influences the conformation of a peptide much more than simply increasing the local amphiphilic character of the peptide.     

Histamine H<Subscript>3</Subscript> Receptor Agonists Decrease Hypothalamic Histamine Levels and Increase Stereotypical Biting in Mice Challenged with Methamphetamine

The effects of the histamine H₃ receptor agonists (R)-α-methylhistamine, imetit and immepip on methamphetamine (METH)-induced stereotypical behavior were examined in mice. The administration of METH (10 mg/kg, i.p.) to male ddY mice induced behaviors including persistent locomotion and stereotypical behaviors, which were classified into four categories: stereotypical head-bobbing (1.9%), circling (1.7%), sniffing (14.3%), and biting (82.1%). Pretreatment with (R)-α-methylhistamine (3 and 10 mg/kg, i.p.) significantly decreased stereotypical sniffing, but increased stereotypical biting induced by METH, in a dose-dependent manner. This effect of (R)-α-methylhistamine on behavior was mimicked by imetit or immepip (brain-penetrating selective histamine H₃ receptor agonists; 10 mg/kg, i.p. for each drug). Hypothalamic histamine levels 1 h after METH challenge were significantly increased in mice pretreated with saline. These increases in histamine levels were significantly decreased by pretreatment with histamine H₃ receptor agonists, effects which would appear to underlie the shift from METH-induced stereotypical sniffing to biting.     

Cloning,functional expression and biochemical characterization of a stereoselective alcohol dehydrogenase from Pseudomonas fluorescens DSM50106

Sequencing of a genomic library prepared from Pseudomonas fluorescens DSM 50106 identified an orf showing 29% identity to a C α-dehydrogenase of Pseudomonas paucimobilis and high homology to several sequences with unknown functions derived from genome projects. The corresponding gene adhF1 encodes a dehydrogenase of 296 amino acids with a calculated molecular mass of 31.997 kDa. The gene was functionally expressed in E. coli using a rhamnose inducible expression system. The resulting recombinant enzyme was active in the pH range 6–10 (best pH 8) and at 5–25 °C. This dehydrogenase converts cyclic ketones to the corresponding alcohols utilizing the cofactor NADH. The highest activity was found for cyclohexanone. The enzyme also exhibits high stereoselectivity in the desymmetrization of the prochiral ketone acetophenone, producing optically pure (R)-α-phenyl ethanol (>99%ee) at high conversion (95%). Electronic Publication     

Necessary and sufficient conditions for the asymmetric synthesis of chiral amines using ω‐aminotransferases

The half reactions of ω‐aminotransferase (ω‐AT) from Vibrio fluvialis JS17 (ω‐ATVf) were carried out using purified pyridoxal 5′‐phosphate‐enzyme (PLP‐Enz) and pyridoxamine 5′‐phosphate‐enzyme (PMP‐Enz) complexes to investigate the relative activities of substrates. In the reaction generating PMP‐Enz from PLP‐Enz using L ‐alanine as an amine donor, L ‐alanine showed about 70% of the initial reaction rate of (S)‐α‐methylbenzylamine ((S)‐α‐MBA). However, in the subsequent half reaction recycling PLP‐Enz from PMP‐Enz using acetophenone as an amine acceptor, acetophenone showed nearly negligible reactivity compared to pyruvate. These results indicate that the main bottleneck in the asymmetric synthesis of (S)‐α‐MBA lies not in the amination of PLP by alanine, but in the amination of acetophenone by PMP‐Enz, where conformational restraints of the enzyme structure is likely to be the main reason for limiting the amine group transfer from PMP‐Enz to acetophenone. Based upon those half reaction experiments using the two amino acceptors of different activity, it appears that the relative activities of the two amine donors and the two acceptors involved in the ω‐AT reactions can roughly determine the asymmetric synthesis yield of the target chiral amine compound. Predicted conversion yields of several target chiral amines were calculated and compared with the experimental conversion yields. Approximately, a positive linear correlation (Pearson's correlation coefficient = 0.92) was observed between the calculated values and the experimental conversion yields. To overcome the low (S)‐α‐MBA productivity of ω‐ATVf caused by the possible disadvantageous structural constraints for acetophenone, new ω‐ATs showing higher affinity to benzene ring of acetophenone than ω‐ATVf were computationally screened using comparative modeling and protein‐ligand docking. ω‐ATs from Streptomyces avermitilis MA‐4680 (SAV2612) and Agrobacterium tumefaciens str. C58 (Atu4761) were selected, and the two screened ω‐ATs showed higher asymmetric synthesis reaction rate of (S)‐α‐MBA and lower (S)‐α‐MBA degradation reaction rate than ω‐ATVf. To verify the higher conversion yield of the variants of ω‐ATs, the reaction with 50 mM acetophenone and 50 mM alanine was performed with coupling of lactate dehydrogenase and two‐phase reaction system. SAV2612 and Atu4761 showed 70% and 59% enhanced yield in the synthesis of (S)‐α‐MBA compared to that of ω‐ATVf, respectively. Biotechnol. Bioeng. 2011;108: 253–263. © 2010 Wiley Periodicals, Inc.     

Synthesis of (R)-2-phenylpropanoic acid from its racemate through an isomerase-involving reaction by Nocardia diaphanozonaria

(R)-2-Phenylpropanoic acid was synthesized from the racemic acid through an isomerization reaction involving resting cells of Nocardia diaphanozonaria JCM3208. The isomerization activity of the cells was enhanced 25-fold by adding 5.5 mM racemic 2-phenylpropanoic acid to the culture medium. When 5 mM racemic 2-phenylpropanoic acid was included in the reaction mixture (4 ml) containing resting cells (100 mg dry cell wt) in 25 mM K₂HPO₄/KH₂PO₄ buffer (pH 7.0) at 30 °C for 8 h, 4.56 mM (R)-2-phenylpropanoic acid (95.8% e.e.) was formed with a 91% molar conversion yield.     

Asymmetric biocatalytic reduction of 3,5-bis(trifluoromethyl) acetophenone to (<Emphasis Type="Italic">1R</Emphasis>)-[3,5-bis(trifluoromethyl)phenyl] ethanol using whole cells of newly isolated <Emphasis Type="Italic">Leifsonia xyli</Emphasis> HS0904

A novel bacterial strain HS0904 was isolated from a soil sample using 3,5-bis(trifluoromethyl) acetophenone as the sole carbon source. This bacterial isolate can asymmetrically reduce 3,5-bis(trifluoromethyl) acetophenone to (1R)-[3,5-bis(trifluoromethyl)phenyl] ethanol with high enantiometric excess (ee) value. Based on its morphological, physiological characteristics, Biolog, 16S rDNA sequence and phylogenetic analysis, strain HS0904 was identified as Leifsonia xyli HS0904. To our knowledge, this is the first reported case on the species L. xyli exhibited R-stereospecific carbonyl reductase and used for the preparation of chiral (1R)-[3,5-bis(trifluoromethyl)phenyl] ethanol. The optimization of parameters for microbial transformation of 3,5-bis(trifluoromethyl) acetophenone to (1R)-[3,5-bis(trifluoromethyl)phenyl] ethanol catalyzed by whole cells of L. xyli HS0904 was carried out by examining some key factors including buffer pH, reaction temperature, shaking speed, substrate concentration, and reaction time. The obtained optimized conditions for the bioreduction are as follows: buffer pH 8.0, 70 mM of 3,5-bis(trifluoromethyl) acetophenone, 100 g l⁻¹ of glucose as co-substrate, 200 g l⁻¹ of resting cells as biocatalyst, reaction for 30 h at 30 °C and 200 rpm. Under above conditions, 99.4% of product ee and best yield of 62% were obtained, respectively. The results indicated that isolate L. xyli HS0904 is a novel potential biocatalyst for the production of (1R)-[3,5-bis(trifluoromethyl)phenyl] ethanol.     

Microbial reduction of alpha-chloroketone to alpha-chlorohydrin

Microbial reduction of α-chloroketone to α-chlorohydrin was studied as one of the approaches for construction of the chiral center of the corresponding epoxide. About 100 microorganisms covering many species of Candida, Pichia, Hansenula, Geotrichum, Rhodococcus and Aureobasidium were screened to reduce the α-chloroketone stereospecifically. Many strains provided the R-α-chlorohydrin with 100% enantiomeric excess (ee), e.g., Candida sonorensis SC 16117, Geotrichum candidum SC 5469, Rhodotorula glutinis SC 16293, Sphingomonas paucimobilis SC 16113, Pichia silvicola SC 16159 and Rhodococcus equi SC 15835. Few microorganisms showed preferential formation of S-α-chlorohydrin after reduction. Among them, Pichia pinus SC 13864 and two Pichia methanolica strains SC 16116 and SC 13860 were the best, providing the S-α-chlorohydrin with ee of 88%, 79% and 78%, respectively. The enantiospecificity of the reduction by these Pichia species can be modified by changing the pH or prior heat treatment of the cells and S-α-chlorohydrin with ≥95% ee was obtained by appropriate modification of reaction conditions. Journal of Industrial Microbiology & Biotechnology (2001) 26, 259–262. Received 23 June 2000/ Accepted in revised form 06 November 2000     

Enantioselective hydrolysis of (R)-2, 2-dimethylcyclopropane carboxamide by immobilized cells of an R-amidase-producing bacterium, Delftia tsuruhatensis CCTCC M 205114, on an alginate capsule carrier

Immobilized cells of Delftia tsuruhatensis CCTCC M 205114 harboring R-amidase were applied in asymmetric hydrolysis of (R)-2, 2-dimethylcyclopropane carboxamide (R − 1) from racemic (R, S)-2, 2-dimethylcyclopropane carboxamide to accumulate (S)-2, 2-dimethylcyclopropane carboxamide (S − 1). Maximum R-amidase activity of 13.1 U/g wet cells (0.982 U/g beads) was obtained under conditions of 3% sodium alginate, 2.5% CaCl₂, 15 h crosslinking and 2 mm bead size, which was 53.9% of that of free cells (24.3 U/g wet cells). In addition, characterization of the immobilized cells was examined. The optimum R − 1 hydrolysis conditions were identified as follows: substrate concentration 10 mM, pH 8.5, temperature 35°C and time course 40 min. Under optimum conditions, the maximum yield and enantiomeric excess of (R)-2, 2-dimethylcyclopropanecarboxylic acid were 49.5% and >99%, respectively. This afforded S − 1 with a yield >49% and an e.e. of 97.7%. With good operational stability and excellent enanotioselectivity, the immobilized cells could be potentially utilized in industrial production of S − 1.     

Three new polyhydroxysteroids from the tropical starfish <Emphasis Type="Italic">Asteropsis carinifera</Emphasis>

Thirteen steroidal compounds including three new polyhydroxysteroids, (24R,25S)-24-methyl-5α-cholestane-3β,6α,8,15β,16β,26-hexaol, (22E,24R,25S)-24-methyl-5α-cholest-22-ene-3β,6α,8,15β,16β,26-hexaol, and (22E,24R,25S)-24-methyl-5α-cholest-22-ene-3β,4β,6α,8,15β,16β,26-heptaol, have been isolated along with ten previously known polyhydroxysteroids from the tropical starfish Asteropsis carinifera collected near the coast of Vietnam. The structures of the new compounds were elucidated by spectroscopic methods (mainly 2D NMR and ESI mass spectrometry).     

Polar steroids from <Emphasis Type="Italic">Solaster endeca</Emphasis> starfish and the physiological activity of polar steroids from three starfish species

Four polyhydroxylated steroids, new (20R)-5α-cholestan-3β,6α,8,15α,24,26-hexaol (I) and known (20R,25S)-5α-cholestan-3β,6α,8,15β,16β,26-hexaol, (20R,25S)-5α-cholestan-3β,6α,15β,16β,26-pentaol, and marthasterone sulfate were isolated from the Solaster endeca starfish inhabiting the Sea of Okhotsk and characterized. Steroid (I) contains a 24,26-dihydroxylated side chain, which is unique for starfish polyols. The isolated steroids and related metabolites from two starfish species of the Evasterias genus (in total, 15 compounds) were weakly cytotoxic in a human HeLa cell culture and some of them were inhibitors of non-specific esterase from mouse Ehrlich carcinoma. The effects of these compounds on the p53 protein activity were studied in a yeast two-hybrid test system and both inhibitors and stimulators of this activity were found among them.