A review—biosynthesis of optically pure ethyl (S)-4-chloro-3-hydroxybutanoate ester: recent advances and future perspectives

Advances in aldo-keto reductases immobilization for biocatalytic synthesis of chiral alcohols

Chiral alcohols are essential building blocks of numerous pharmaceuticals and fine chemicals. Aldo-keto reductases (AKRs) constitute a superfamily of oxidoreductases that catalyze the reduction of aldehydes and ketones to their corresponding alcohols using NAD(P)H as a coenzyme. Knowledge about the crucial roles of AKRs immobilization in the biocatalytic synthesis of chiral alcohols is expanding. Herein, we reviewed the characteristics of various AKRs immobilization approaches, the applications of different immobilization materials, and the prospects of continuous flow bioreactor construction by employing these immobilized biocatalysts for synthesizing chiral alcohols. Finally, the opportunities and ongoing challenges for AKR immobilization are discussed and the outlook for this emerging area is analyzed.

Catalytic Synthesis of (S)-CHBE by Directional Coupling and Immobilization of Carbonyl Reductase and Glucose Dehydrogenase

Ethyl (S)-4-chloro-3-hydroxybutyrate ((S)-CHBE) is an important chiral intermediate in the synthesis of the cholesterol-lowering drug atorvastatin. Studying the use of SpyTag/SpyCatcher and SnoopTag/SnoopCatcher systems for the asymmetric reduction reaction and directed coupling coenzyme regeneration is practical for efficiently synthesizing (S)-CHBE. In this study, Spy and Snoop systems were used to construct a double-enzyme directed fixation system of carbonyl reductase (BsCR) and glucose dehydrogenase (BsGDH) for converting 4-chloroacetoacetate (COBE) to (S)-CHBE and achieving coenzyme regeneration. We discussed the enzymatic properties of the immobilized enzyme and the optimal catalytic conditions and reusability of the double-enzyme immobilization system. Compared to the free enzyme, the immobilized enzyme showed an improved optimal pH and temperature, maintaining higher relative activity across a wider range. The double-enzyme immobilization system was applied to catalyze the asymmetric reduction reaction of COBE, and the yield of (S)-CHBE reached 60.1% at 30 °C and pH 8.0. In addition, the double-enzyme immobilization system possessed better operational stability than the free enzyme, and maintained about 50% of the initial yield after six cycles. In summary, we show a simple and effective strategy for self-assembling SpyCatcher/SnoopCatcher and SpyTag/SnoopTag fusion proteins, which inspires building more cascade systems at the interface. It provides a new method for facilitating the rapid construction of in vitro immobilized multi-enzyme complexes from crude cell lysate.

Converting the 3-quinuclidinone reductase from Agrobacterium tumefaciens into the ethyl 4-chloroacetoacetate reductase by site-directed mutagenesis

In this study, the 3-quinuclidinone reductase from Agrobacterium tumefaciens (AtQR) was modified by site-directed mutagenesis. And we further obtained a saturation mutant library in which the residue 197 was mutated. A single-point mutation converted the wild enzyme that originally had no catalytic activity in reduction of ethyl 4-chloroacetoacetate (COBE) into an enzyme with catalytic activity. The results of enzyme activity assays showed that the seven variants could asymmetrically reduce COBE to ethyl (S)-4-chloro-3-hydroxybutyrate ((S)-CHBE) with NADH as coenzyme. In the library, the variant E197N showed higher catalytic efficiency than others. The E197N was optimally active at pH 6.0 and 40°C, and the catalytic efficiency (k_cat /K_m ) for COBE was 51.36 s^-1 ·mM^-1 . This study showed that the substrate specificity of AtQR could be changed through site-directed mutagenesis at the residue 197.

Self-sufficient asymmetric reduction of β-ketoesters catalysed by a novel and robust thermophilic alcohol dehydrogenase co-immobilised with NADH

β-Hydroxyesters are essential building blocks utilised by the pharmaceutical and food industries in the synthesis of functional products. Beyond the conventional production methods based on chemical catalysis or whole-cell synthesis, the asymmetric reduction of β-ketoesters with cell-free enzymes is gaining relevance. To this end, a novel thermophilic (S)-3-hydroxybutyryl-CoA dehydrogenase from Thermus thermophilus HB27 (Tt27-HBDH) has been expressed, purified and biochemically characterised, determining its substrate specificity towards β-ketoesters and its dependence on NADH as a cofactor. The immobilization of Tt27-HBDH on agarose macroporous beads and its subsequent coating with polyethyleneimine has been found the best strategy to increase the stability and workability of the heterogeneous biocatalyst. Furthermore, we have embedded NADH in the cationic layer attached to the porous surface of the carrier. Since Tt27-HBDH catalyses cofactor recycling through 2-propanol oxidation, we achieve a self-sufficient heterogeneous biocatalyst where NADH is available for the immobilised enzymes but its lixiviation to the reaction bulk is avoided. Taking advantage of the autofluorescence of NADH, we demonstrate the activity of the enzyme towards the immobilised cofactor through single-particle analysis. Finally, we tested the operational stability in the asymmetric reduction of β-ketoesters in batch, succeeding in the reuse of both the enzyme and the co-immobilised cofactor up to 10 reaction cycles.

Efficient production of an ezetimibe intermediate using carbonyl reductase coupled with glucose dehydrogenase

Ezetimibe is a top-selling hypolipidemic drug for the treatment of cardiovascular diseases. Biosynthesis of (4S)-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-4-phenyl-1,3-oxazolidin-2-one ((S)-ET-5) using carbonyl reductase has shown advantages including high catalytic efficiency, excellent stereoselectivity, mild reaction conditions, and environmental friendness, and was considered as the key step for ezetimibe production. The regeneration efficiency of the cofactor, nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) is one of the main restricted factor. Recombinant Escherichia coli strain (smCR125) coexpressing carbonyl reductase (CR125) and glucose dehydrogenase were successfully constructed and applied for the production of (S)-ET-5 for the first time. Without extra addition of the coenzyme NADPH, the yield of 99.8% and the enantiomeric excess (e.e.) of 99.9% were achieved under ET-4 concentration of 200 g/L. Using a substrate fed-batch strategy, under the optimal conditions, the substrate ET-4 concentration was increased to 250 g/L with the yield of 98.9% and the e.e. of 99.9% after 12 hr reaction. The space-time yield of 494.5 g L^-1 d^-1 and the space-time yield per gram biocatalyst of 24.7 g L^-1 d^-1 g^-1 DCW were achieved, which were higher than ever reported for the biosynthesis of the ezetimibe intermediate.

Biochemical characterization of a novel azoreductase from Streptomyces sp.: Application in eco-friendly decolorization of azo dye wastewater

Azo dyes are the most widely applied chemical dyes that have also raised great concerns for environmental contamination and human health issues. There has been a growing interest in discovering bioremediation methods to degrade azo dyes for environmental and economic purposes. Azoreductases are key enzymes evolved in nature capable of degrading azo dyes. The current work reports the identification, expression, and properties of a novel azoreductase (AzoRed2) from Streptomyces sp. S27 which shows an excellent stability against pH change and organic solvents. To overcome the requirements of coenzyme while degrading azo dyes, we introduced a coenzyme regeneration enzyme, Bacillus subtilis glucose 1-dehydrogenase (BsGDH), to construct a recycling system in living cells. The whole-cell biocatalyst containing AzoRed2 and BsGDH was used to degrade a representative azo dye methyl red. The degradation rate of methyl red was up to 99% in 120 min with high substrate concentration (250 μM) and no external coenzyme added. The degradation rate was still 98% in the third batch trial. To sum up, a novel azoreductase with good properties was found, which was applied to construct whole-cell biocatalyst. Both the enzymes and whole-cell biocatalysts are good candidates for the industrial wastewater treatment and environmental restoration.

A novel carbonyl reductase with anti-Prelog stereospecificity for the production of t-butyl 6-cyano-(3R, 5R)-dihydroxyhexanoate

A novel gene (crc1) from Candida boidinii was cloned and then overexpressed in a recombinant strain BL21(DE3)/pET30a-crc1 of Escherichia coli. The resulting carbonyl reductase was prepared through fermentations using the recombinant strain. The purified enzyme showed an NADPH-dependent activity and specific activity was 4.65 U/mg using t-butyl 6-cyano-(5R)-hydroxy-3-oxohexanoate (ATS-6) as substrate. The enzyme was optimally active at 35 °C and pH 7, respectively. The apparent K _m and V _max of the enzyme for ATS-6 are 1.5 mM and 21.1 μmol/min mg, respectively, indicating excellent anti-Prelog stereospecificity. Under the optimum condition, t-butyl 6-cyano-(3R,5R)-dihydroxyhexanoate (ATS-7) was prepared with the enzyme with high d.e. value (99.9%) and good conversion (94%) in 4 h, indicating high stereoselectivity and conversion efficiency in biotransformation of ATS-6 to ATS-7.

Cloning, Expression and Characterization of a Highly Active Alcohol Dehydrogenase for Production of Ethyl (S)-4-Chloro-3-Hydroxybutyrate

A novel alcohol dehydrogenase from Bartonella apis (BaADH) was heterologous expressed in Escherichia coli. Its biochemical properties were investigated and used to catalyze the synthesis of ethyl (S)-4-chloro-3-hydroxybutanoate ((S)-CHBE), which is a chiral intermediate of the cholesterol-lowering drug atorvastatin. The purified recombinant BaADH displayed 182.4 U/mg of the specific activity using ethyl 4-chloroacetoacetate as substrate under the conditions of 50 °C in pH 7.0 Tris-HCl buffer. It was stable in storage buffers of pH 7 to 9 and retains up to 96.7% of the initial activity after 24 h. The K _m and V _max values of BaADH were 0.11 mM and 190.4 μmol min^-1 mg^-1, respectively. Synthesis of (S)-CHBE catalyzed by BaADH was performed with a cofactor regeneration system using a glucose dehydrogenase, and a conversion of 94.9% can be achieved after 1 h reaction. Homology modeling and substrate docking revealed that a typical catalytic triad is in contact with local water molecules to form a catalytic system. The results indicated this ADH could contribute to the further enzymatic synthesis of (S)-CHBE.

Biocatalyzed Synthesis of Statins: A Sustainable Strategy for the Preparation of Valuable Drugs

Statins, inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, are the largest selling class of drugs prescribed for the pharmacological treatment of hypercholesterolemia and dyslipidaemia. Statins also possess other therapeutic effects, called pleiotropic, because the blockade of the conversion of HMG-CoA to (R)-mevalonate produces a concomitant inhibition of the biosynthesis of numerous isoprenoid metabolites (e.g., geranylgeranyl pyrophosphate (GGPP) or farnesyl pyrophosphate (FPP)). Thus, the prenylation of several cell signalling proteins (small GTPase family members: Ras, Rac, and Rho) is hampered, so that these molecular switches, controlling multiple pathways and cell functions (maintenance of cell shape, motility, factor secretion, differentiation, and proliferation) are regulated, leading to beneficial effects in cardiovascular health, regulation of the immune system, anti-inflammatory and immunosuppressive properties, prevention and treatment of sepsis, treatment of autoimmune diseases, osteoporosis, kidney and neurological disorders, or even in cancer therapy. Thus, there is a growing interest in developing more sustainable protocols for preparation of statins, and the introduction of biocatalyzed steps into the synthetic pathways is highly advantageous—synthetic routes are conducted under mild reaction conditions, at ambient temperature, and can use water as a reaction medium in many cases. Furthermore, their high selectivity avoids the need for functional group activation and protection/deprotection steps usually required in traditional organic synthesis. Therefore, biocatalysis provides shorter processes, produces less waste, and reduces manufacturing costs and environmental impact. In this review, we will comment on the pleiotropic effects of statins and will illustrate some biotransformations nowadays implemented for statin synthesis.

Enhanced bioreduction synthesis of ethyl (R)-4-chloro-3-hydroybutanoate by alkalic salt pretreatment

In this study, biomass-hydrolysate was used for enhancing the bioreduction of ethyl 4-chloro-3-oxobutanoate (COBE). Firstly, dilute alkalic salt pretreatment was attempted to pretreat bamboo shoot shell (BSS). It was found that enzymatic in situ hydrolysis of 20-50 g/L BSS pretreated with dilute alkalic salts (0.4% Na₂CO₃, 0.032% Na₂S) at 7.5% sulfidity by autoclaving at 110 °C for 40 min gave sugar yields at 59.9%-73.5%. Moreover, linear relationships were corrected on solid recovery-total delignification-sugar yield. In BSS-hydrolysates, xylose and glucose could promote the reductase activity of recombinant E. coli CCZU-A13. Compared with glucose, hydrolysate could increase the reductase activity by 1.35-folds. Furthermore, the cyclohexane-hydrolysate (10:90, v/v) biphasic media containing ethylene diamine tetraacetic acid (EDTA, 40 mM) and l-glutamine (150 mM) was built for the effective biosynthesis of ethyl (R)-4-chloro-3-hydroxybutanoate [(R)-CHBE] (94.6% yield) from 500 mM COBE. In conclusion, this strategy has high potential for the effective biosynthesis of (R)-CHBE (>99% e.e.).

Engineering Streptomyces coelicolor Carbonyl Reductase for Efficient Atorvastatin Precursor Synthesis

Streptomyces coelicolor CR1 (ScCR1) has been shown to be a promising biocatalyst for the synthesis of an atorvastatin precursor, ethyl-(S)-4-chloro-3-hydroxybutyrate [(S)-CHBE]. However, limitations of ScCR1 observed for practical application include low activity and poor stability. In this work, protein engineering was employed to improve the catalytic efficiency and stability of ScCR1. First, the crystal structure of ScCR1 complexed with NADH and cosubstrate 2-propanol was solved, and the specific activity of ScCR1 was increased from 38.8 U/mg to 168 U/mg (ScCR1_I158V/P168S) by structure-guided engineering. Second, directed evolution was performed to improve the stability using ScCR1_I158V/P168S as a template, affording a triple mutant, ScCR1_{A60T/I158V/P168S}, whose thermostability (T₅₀¹⁵, defined as the temperature at which 50% of initial enzyme activity is lost following a heat treatment for 15 min) and substrate tolerance (C₅₀¹⁵, defined as the concentration at which 50% of initial enzyme activity is lost following incubation for 15 min) were 6.2°C and 4.7-fold higher than those of the wild-type enzyme. Interestingly, the specific activity of the triple mutant was further increased to 260 U/mg. Protein modeling and docking analysis shed light on the origin of the improved activity and stability. In the asymmetric reduction of ethyl-4-chloro-3-oxobutyrate (COBE) on a 300-ml scale, 100 g/liter COBE could be completely converted by only 2 g/liter of lyophilized ScCR1_{A60T/I158V/P168S} within 9 h, affording an excellent enantiomeric excess (ee) of >99% and a space-time yield of 255 g liter^-1 day^-1 These results suggest high efficiency of the protein engineering strategy and good potential of the resulting variant for efficient synthesis of the atorvastatin precursor.IMPORTANCE Application of the carbonyl reductase ScCR1 in asymmetrically synthesizing (S)-CHBE, a key precursor for the blockbuster drug Lipitor, from COBE has been hindered by its low catalytic activity and poor thermostability and substrate tolerance. In this work, protein engineering was employed to improve the catalytic efficiency and stability of ScCR1. The catalytic efficiency, thermostability, and substrate tolerance of ScCR1 were significantly improved by structure-guided engineering and directed evolution. The engineered ScCR1 may serve as a promising biocatalyst for the biosynthesis of (S)-CHBE, and the protein engineering strategy adopted in this work would serve as a useful approach for future engineering of other reductases toward potential application in organic synthesis.

Enzymatic synthesis of an ezetimibe intermediate using carbonyl reductase coupled with glucose dehydrogenase in an aqueous-organic solvent system

(4S)-3-[(5S)-5-(4-Fluorophenyl)-5-hydroxypentanoyl]-4-phenyl-1,3-oxazolidin-2-one ((S)-ET-5) is an important chiral intermediate in the synthesis of chiral side chain of ezetimibe. Recombinant Escherichia coli expressing carbonyl reductase (CBR) was successfully constructed in this study. The total E. coli biomass and the specific activity of recombinant CBR in 5L fermenter culture were 10.9gDCWL^-1 and 14900.3Ug^-1DCW, respectively. The dual-enzyme coupled biocatalytic process in an aqueous-organic biphasic solvent system was first constructed using p-xylene as the optimal organic phase under optimized reaction conditions, and 150gL^-1 (4S)-3-[5-(4-fluorophenyl)-1,5-dioxophentyl]-4-phenyl-1,3-oxazolidin-2-one (ET-4) was successfully converted to (S)-ET-5 with a conversion of 99.1% and diastereomeric excess of 99% after 24-h, which are the highest values reported to date for the production of (S)-ET-5.

Recent advances in biotechnological applications of alcohol dehydrogenases

Alcohol dehydrogenases (ADHs), which belong to the oxidoreductase superfamily, catalyze the interconversion between alcohols and aldehydes or ketones with high stereoselectivity under mild conditions. ADHs are widely employed as biocatalysts for the dynamic kinetic resolution of racemic substrates and for the preparation of enantiomerically pure chemicals. This review provides an overview of biotechnological applications for ADHs in the production of chiral pharmaceuticals and fine chemicals.

Efficient biosynthesis of ethyl (R)-4-chloro-3-hydroxybutyrate using a stereoselective carbonyl reductase from Burkholderia gladioli

Background

Ethyl (R)-4-chloro-3-hydroxybutyrate ((R)-CHBE) is a versatile chiral precursor for many pharmaceuticals. Although several biosynthesis strategies have been documented to convert ethyl 4-chloro-3-oxobutanoate (COBE) to (R)-CHBE, the catalytic efficiency and stereoselectivity are still too low to be scaled up for industrial applications. Due to the increasing demand of (R)-CHBE, it is essential to explore more robust biocatalyst capable of preparing (R)-CHBE efficiently.

Results

A stereoselective carbonyl reductase toolbox was constructed and employed into the asymmetric reduction of COBE to (R)-CHBE. A robust enzyme designed as BgADH3 from Burkholderia gladioli CCTCC M 2012379 exhibited excellent activity and enantioselectivity, and was further characterized and investigated in the asymmetric synthesis of (R)-CHBE. An economical and satisfactory enzyme-coupled cofactor recycling system was created using recombinant Escherichia coli cells co-expressing BgADH3 and glucose dehydrogenase genes to regenerate NADPH in situ. In an aqueous/octanol biphasic system, as much as 1200 mmol COBE was completely converted by using substrate fed-batch strategy to afford (R)-CHBE with 99.9 % ee at a space-time yield per gram of biomass of 4.47 mmol∙L⁻¹∙h⁻¹∙g DCW⁻¹.

Conclusions

These data demonstrate the promising of BgADH3 in practical synthesis of (R)-CHBE as a valuable chiral synthon. This study allows for the further application of BgADH3 in the biosynthesis of chiral alcohols, and establishes a preparative scale process for producing (R)-CHBE with excellent enantiopurity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-016-0301-x) contains supplementary material, which is available to authorized users.

Asymmetric synthesis of (S)-4-chloro-3-hydroxybutanoate by sorbose reductase from Candida albicans with two co-existing recombinant Escherichia coli strains

An NADPH-dependent sorbose reductase from Candida albicans was identified to catalyze the asymmetric reduction of ethyl 4-chloro-3-oxobutanoate (COBE). The activity of the recombinant enzyme toward COBE was 6.2 U/mg. The asymmetric reduction of COBE was performed with two coexisting recombinant Escherichia coli strains, in which the recombinant E. coli expressing glucose dehydrogenase was used as an NADPH regenerator. An optical purity of 99% (e.e.) and a maximum yield of 1240 mM (S)-4-chloro-3-hydroxybutanoate were obtained under an optimal biomass ratio of 1:2. A highest turnover number of 53,900 was achieved without adding extra NADP(+)/NADPH compared with those known COBE-catalytic systems.

Cloning, expression, and characterization of an anti-Prelog stereospecific carbonyl reductase from Gluconobacter oxydans DSM2343

A new anti-Prelog stereospecific carbonyl reductase (GoKR) from Gluconobacter oxydans DSM2343 was cloned and identified in Escherichia coli. This GoKR formed a homo-tetramer with a subunit size of approximately 27.0kDa. GoKR exhibited full activity with NADPH but not with NADH as a cofactor. The optimal pH and temperature were 9.0 and 30°C, respectively. GoKR reduced various ketones, including aliphatic and aromatic ketones, α- and β-keto esters. Aromatic ketones were reduced to (R)-enantiomers, whereas keto esters were reduced to (S)-hydroxy esters with different enantioselectivities. The data indicate that GoKR does not obey Prelog's rule and exhibits anti-Prelog enantiopreference. Enzyme-substrate-cofactor docking analysis showed that hydride transfer occurred at the si faces of carbonyl group for ethyl 4-chloro-3-oxobutanoate (COBE), which was then selectively reduced to the chiral (S)-alcohol. Excellent enantioselectivities were obtained for reducing COBE and ethyl 2-oxo-4-phenylbutyrate into the corresponding (S)-type products. These products are important for synthesizing HMG-CoA reductase (statins) and angiotensin-converting enzyme inhibitors, respectively.

Characterization of a putative stereoselective oxidoreductase from Gluconobacter oxydans and its application in producing ethyl (R)-4-chloro-3-hydroxybutanoate ester

A gene encoding an NADH-dependent short-chain dehydrogenase/reductase (gox2036) from Gluconobacter oxydans 621H was cloned and heterogeneously expressed in Escherichia coli. The protein (Gox2036) was purified to homogeneity and biochemically characterized. Gox2036 was a homotetramer with a subunit size of approximately 28 kDa. Gox2036 had a strict requirement for NAD⁺/NADH as the cofactor. Gox2036 displayed preference for oxidation of secondary alcohols and 2,3-diols as well as for reduction of α-diketones, hydroxy ketones, α-ketoesters, and β-ketoesters. However, Gox2036 was poorly active on 1,2-diols and acetoin and showed no activity on primary alcohols, polyols, and aldehydes. The optimum pH values for the oxidation and reduction reactions were 9 and 6, respectively. Gox2036 was highly selective in the reduction of various β-ketones and β-ketoesters. Among the substrates tested, ethyl 4-chloro acetoacetate was reduced to ethyl (R)-4-chloro-3-hydroxybutanoate ester with an excellent conversion yield of 96.9 % and optical purity of >99 % e.e. using an efficient in situ NADH-recycling system involving glucose and a glucose dehydrogenase from Bacillus subtilis (BsGDH).

Discovery of a reductase-producing strain recombinant E. coli CCZU-A13 using colorimetric screening and its whole cell-catalyzed biosynthesis of ethyl (R)-4-chloro-3-hydroxybutanoate

An NADH-dependent reductase (SsCR) was discovered by genome data mining. After SsCR was overexpressed in E. coli BL21, recombinant E. coli CCZU-A13 with high reductase activity and excellent stereoselectivity for the reduction of ethyl 4-chloro-3-oxobutanoate (COBE) into ethyl (R)-4-chloro-3-hydroxybutanoate ((R)-CHBE) was screened using one high-throughput colorimetric screening strategy. After the reaction optimization, a highly stereoselective bioreduction of COBE into (R)-CHBE (>99% ee) with the resting cells of E. coli CCZU-A13 was successfully demonstrated in n-butyl acetate-water (10:90, v/v) biphasic system. Biotransformation of 600mM COBE for 8h in the biphasic system, (R)-CHBE (>99% ee) could be obtained in the high yield of 100%. Moreover, the broad substrate specificity in the reduction of aliphatic and aromatic carbonyl compounds was also found. Significantly, E. coli CCZU-A13 shows high potential in the industrial production of (R)-CHBE (>99% ee) and its derivatives.

A genomic search approach to identify carbonyl reductases in Gluconobacter oxydans for enantioselective reduction of ketones

The versatile carbonyl reductases from Gluconobacter oxydans in the enantioselective reduction of ketones to the corresponding alcohols were exploited by genome search approach. All purified enzymes showed activities toward the tested ketoesters with different activities. In the reduction of 4-phenyl-2-butanone with in situ NAD(P)H regeneration system, (S)-alcohol was obtained with an e.e. of up to 100% catalyzed by Gox0644. Under the same experimental condition, all enzymes catalyzed ethyl 4-chloroacetoacetate to give chiral products with an excellent e.e. of up to 99%, except Gox0644. Gox2036 had a strict requirement for NADH as the cofactor and showed excellent enantiospecificity in the synthesis of ethyl (R)-4-chloro-3-hydroxybutanoate. For the reduction of ethyl 2-oxo-4-phenylbutyrate, excellent e.e. (>99%) and high conversion (93.1%) were obtained by Gox0525, whereas the other enzymes showed relatively lower e.e. and conversions. Among them, Gox2036 and Gox0525 showed potentials in the synthesis of chiral alcohols as useful biocatalysts.

Characterization of a newly synthesized carbonyl reductase and construction of a biocatalytic process for the synthesis of ethyl (S)-4-chloro-3-hydroxybutanoate with high space-time yield

A carbonyl reductase (SCR2) gene was synthesized and expressed in Escherichia coli after codon optimization to investigate its biochemical properties and application in biosynthesis of ethyl (S)-4-chloro-3-hydroxybutanoate ((S)-CHBE), which is an important chiral synthon for the side chain of cholesterol-lowering drug. The recombinant SCR2 was purified and characterized using ethyl 4-chloro-3-oxobutanoate (COBE) as substrate. The specific activity of purified enzyme was 11.9 U mg(-1). The optimum temperature and pH for enzyme activity were 45 °C and pH 6.0, respectively. The half-lives of recombinant SCR2 were 16.5, 7.7, 2.2, 0.41, and 0.05 h at 30 °C, 35 °C, 40 °C, 45 °C, and 50 °C, respectively, and it was highly stable in acidic environment. This SCR2 displayed a relatively narrow substrate specificity. The apparent K m and V max values of purified enzyme for COBE are 6.4 mM and 63.3 μmol min(-1) mg(-1), respectively. The biocatalytic process for the synthesis of (S)-CHBE was constructed by this SCR2 in an aqueous-organic solvent system with a substrate fed-batch strategy. At the final COBE concentration of 1 M, (S)-CHBE with yield of 95.3% and e.e. of 99% was obtained after 6-h reaction. In this process, the space-time yield per gram of biomass (dry cell weight, DCW) and turnover number of NADP(+) to (S)-CHBE were 26.5 mmol L(-1) h(-1) g(-1) DCW and 40,000 mol/mol, respectively, which were the highest values as compared with other works.

Development of a substrate-coupled biocatalytic process driven by an NADPH-dependent sorbose reductase from Candida albicans for the asymmetric reduction of ethyl 4-chloro-3-oxobutanoate

A substrate-coupled biocatalytic process was developed based on the reactions catalyzed by an NADPH-dependent sorbose reductase (SOU1) from Candida albicans in which ethyl 4-chloro-3-oxobutanoate (COBE) was reduced to (S)-4-chloro-3-hydroxybutanoate [(S)-CHBE], while NADPH was regenerated by the same enzyme via oxidation of sugar alcohols. (S)-CHBE yields of 1,140, 1,150, and 780 mM were obtained from 1,220 mM COBE when sorbitol, mannitol, and xylitol were used as co-substrates, respectively. Optimization of COBE and sorbitol proportions resulted in a maximum yield of (S)-CHBE (2,340 mM) from 2,500 mM COBE, and the enantiomeric excess was 99.6 %. The substrate-coupled system driven by SOU1 maintained a stable pH and a robust intracellular NADPH circulation; thus, pH adjustment and addition of extra coenzymes were unnecessary.

A novel reductase from Candida albicans for the production of ethyl (S)-4-chloro-3-hydroxybutanoate

A novel NADPH-dependent reductase (CaCR) from Candida albicans was cloned for the first time. It catalyzed asymmetric reduction to produce ethyl (S)-4-chloro-3-hydroxybutanoate ((S)-CHBE). It contained an open reading frame of 843 bp encoding 281 amino acids. When co-expressed with a glucose dehydrogenase in Escherichia coli, recombinant CaCR exhibited an activity of 5.7 U/mg with ethyl 4-chloro-3-oxobutanoate (COBE) as substrate. In the biocatalysis of COBE to (S)-CHBE, 1320 mM (S)-CHBE was obtained without extra NADP+/NADPH in a water/butyl acetate system, and the optical purity of the (S)-isomer was higher than 99% enantiomeric excess.