Chiral alcohol synthesis with yeast carbonyl reductases

Semi-rational engineering of a thermostable aldo-keto reductase from Thermotoga maritima for synthesis of enantiopure ethyl-2-hydroxy-4-phenylbutyrate (EHPB)

A novel aldo-keto reductase Tm1743 characterized from Thermotoga maritima was explored as an effective biocatalyst in chiral alcohol production. Natural Tm1743 catalyzes asymmetric reduction of ethyl 2-oxo-4-phenylbutyrate (EOPB) at high efficiency, but the production of, ethyl (S)-2-hydroxy-4-phenylbutyrate ((S)-EHPB), which is less desirable, is preferred with an enantiomeric excess (ee) value of 76.5%. Thus, altering the enantioselectivity of Tm1743 to obtain the more valuable product (R)-EHPB for angiotensin drug synthesis is highly desired. In this work, we determined the crystal structure of Tm1743 in complex with its cofactor NADP+ at 2.0 Å resolution, and investigated the enantioselectivity of Tm1743 through semi-rational enzyme design. Molecular simulations based on the crystal structure obtained two binding models representing the pro-S and pro-R conformations of EOPB. Saturation mutagenesis studies revealed that Trp21 and Trp86 play important roles in determining the enantioselectivity of Tm1743. The best (R)- and (S)-EHPB preferring Tm1743 mutants, denoted as W21S/W86E and W21L/W118H, were identified; their ee values are 99.4% and 99.6% and the catalytic efficiencies are 0.81 and 0.12 mM-1s-1, respectively. Our work presents an efficient strategy to improve the enantioselectivity of a natural biocatalyst, which will serve as a guide for further exploration of new green catalysts for asymmetric reactions.

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.

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).

Characterization and site-directed mutation of a novel aldo-keto reductase from Lodderomyces elongisporus NRRL YB-4239 with high production rate of ethyl (R)-4-chloro-3-hydroxybutanoate

A novel aldo-keto reductase (LEK) from Lodderomyces elongisporus NRRL YB-4239 (ATCC 11503) was discovered by genome database mining for carbonyl reduction. LEK was overexpressed in Escherichia coli BL21 (DE3), purified to homogeneity and the catalytic properties were studied. Among the substrates, ethyl 4-chloro-3-oxobutanoate was converted to ethyl (R)-4-chloro-3- hydroxybutanoate ((R)-CHBE), an important pharmaceutical intermediate, with an excellent enantiomeric excess (e.e.) (>99 %). The mutants W28A and S209G obtained by site-directed mutation were identified with much higher molar conversion yields and lower Km values. Further, the constructed coenzyme regeneration system with glucose as co-substrate resulted in a yield of 100 %, an enantioselectivity of >99 %, and the calculated production rate of 56.51 mmol/L/H. These results indicated the potential of LEK for the industrial production of (R)-CHBE and other valuable chiral alcohols.

Diversity of 4-chloroacetoacetate ethyl ester-reducing enzymes in yeasts and their application to chiral alcohol synthesis

Enzymes which reduce 4-chloroacetoacetate ethyl ester (CAAE) to (R)- or (S)-4-chloro-3-hydroxybutanoate ethyl ester (CHBE) were investigated. Several microorganisms which can reduce CAAE with high yields were discovered. An NADPH-dependent aldehyde reductase, ARI, and an NADPH-dependent carbonyl reductase, S1, were isolated from Sporobolomyces salmonicolor and Candida magnoliae, respectively, and enzymatic synthesis of chiral CHBE was performed through the reduction of CAAE. When ARI-overproducing Escherichia coli transformant cells or C. magnoliae cells were incubated in an organic solvent-water diphasic system. CAAE was stoichiometrically converted to (R)- or (S)-CHBE (> 92% enantiomeric excess), respectively. Multiple CAAE-reducing enzymes were present in S. salmonicolor, C. magnoliae and bakers' yeast. Comparison of the primary structures of these CAAE-reducing enzymes with other protein sequences showed that CAAE-reducing enzymes are widely distributed in various protein families, and various physiological roles of these enzymes in the cell were speculated.

Applying the Taguchi robust design to the optimization of the asymmetric reduction of ethyl 4-chloro acetoacetate by bakers' yeast

This study examined the characteristics and operational parameters of the asymmetric reduction of ethyl 4-chloro acetoacetate by bakers' yeast in order to produce S-4-chloro-3-hydroxybutyric acid ethyl ester. Eight operational variables were also optimized using the Taguchi method with consideration of the freshness of yeast cells as a noise factor. An L(18) orthogonal array was used to design the experiments. The reaction yield and the product's optical purity were considered as two product quality variables. A desirability function was applied to combine these two qualities as a single objective function. Additionally, the signal-to-noise (SN) ratio was used to estimate the variability in product quality. Optimization was undertaken not only to yield the best performance, but also to minimize the variation in quality. The confirmation experiments indicated that the reaction performance and the robustness of the product quality under the optimized conditions were higher than those obtained in other experiments in this study. Our results further demonstrate that the product's optical purity could be increased to >95% by adjusting the operational level of the main factors.

Production of fine chemicals using biocatalysis

Presently, a large number of biotransformations are carried out on an industrial scale and are discussed in a fast increasing number of reviews. Besides this, a significant number of biotransformations have been investigated over the past year, from degrading to transforming and synthetic reactions. The development of more specific and stable biocatalysts, either isolated enzymes or whole cells, generated by the new methods of genetic engineering and improved by reaction engineering have led to new industrial biotransformations.