Asymmetric synthesis of (S)-ethyl-4-chloro-3-hydroxy butanoate using a Saccharomyces cerevisiae reductase: enantioselectivity and enzyme-substrate docking studies

Electron withdrawing group-dependent substrate inhibition of an α-ketoamide reductase from Saccharomyces cerevisiae

Aldo-keto reductases remain enzymes of interest in biocatalysis due to their ability to reduce carbonyls to alcohols stereospecifically. Based on genomic sequence, we identified aldo-keto reductases of a S. cerevisiae strain extracted from an ancient amber sample. One of the putative enzymes, AKR 163, displays 99% identity with α-amide ketoreductases from the S288C and YJM248 S. cerevisiae strains, which have been investigated for biocatalytic applications. To further investigate AKR 163, we successfully cloned, expressed in E.coli as a glutathione-S-transferase fusion protein, and affinity purified AKR 163. Kinetic studies revealed that AKR 163 experiences strong substrate inhibition by substrates containing halogen atoms or other electron withdrawing groups adjacent to the reactive carbonyl, with Ki values ranging from 0.29 to 0.6 mM and KM values ranging from 0.38 to 0.9 mM at pH 8.0. Substrates without electron withdrawing groups do not display substrate inhibition kinetics and possess much larger KM values between 83 and 260 mM under the same conditions. The kcat values ranged from 0.5 to 2.5s-1 for substrates exhibiting substrate inhibition and 0.22 to 0.52s-1 for substrates that do not engage in substrate inhibition. Overall, the results are consistent with rate-limiting dissociation of the NADP+ cofactor after hydride transfer when electron withdrawing groups are present and activating the reduction step. This process leads to a buildup of enzyme-NADP+ complex that is susceptible to binding and inhibition by a second substrate molecule.

Nonionic surfactants and their effects on asymmetric reduction of 2-octanone with Saccharomyces cerevisiae

In an aqueous buffer system, serious reverse and side reactions were found in the asymmetric reduction of 2-octanone with Saccharomyces cerevisiae. However, some nonionic surfactants added to the aqueous buffer system improved the bioreduction process by decreasing the reverse and side reaction rates in addition to effectively increasing the average positive reaction rate. Further, a shorter carbon chain length of hydrophilic or hydrophobic moieties in surfactants resulted in a higher yield of (S)-2-octanol. The alkylphenol ethoxylate surfactants had a less influence than polyoxyethylenesorbitan trialiphatic surfactants on the product e.e. It suggested that the product e.e. resulting from the change of carbon chain length of the hydrophobic moieties varied markedly compared with the change of carbon chain length of the hydrophilic moiety. Emulsifier OP-10 and Tween 20 markedly enhanced the yield and product e.e. at the concentration of 0.4 mmol L⁻¹ with a yield of 73.3 and 93.2%, and the product e.e. of 99.2 and 99.3%, respectively, at the reaction time of 96 h.

Production of precursors for anti-Alzheimer drugs: Asymmetric bioreduction in a packed-bed bioreactor using immobilized D. carota cells

(S)-1-Phenylethanol derivatives, which are the precursors of many pharmacological products, have also been used as anti-Alzheimer drugs. Bioreduction experiments were performed in a batch and packed-bed bioreactor. Then, the kinetics constants were determined by examining the reaction kinetics in the batch system with free and immobilized carrot cells. Also, the effective diffusion coefficient (D_e) of acetophenone in calcium alginate-immobilized carrot cells was investigated. Kinetics constants for free cells, which are intrinsic values, are reaction rate V_max = 0.052 mmol L^-1 min^-1, and constants of the Michaelis-Menten K_M = 2.31 mmol L^-1. Kinetics constants for immobilized cells, which are considered apparent values, are V_{max, app} = 0.0407 mmol L^-1 min^-1, K_{M, app} = 3.0472 mmol L^-1 for 2 mm bead diameter, and V_{max, app} = 0.0453 mmol L^-1 min^-1, K_{M, app} = 4.9383 mmol L^-1 for 3 mm bead diameter. Average value of effective diffusion coefficient of acetophenone in immobilized beads was determined as 1.97 × 10^-6 cm² s^-1. Using immobilized carrot cells in an up-flow packed-bed reactor, continuous production of (S)-1-phenylethanol through asymmetric bioreduction of acetophenone was performed. The effects of the residence time and concentrations of substrate were investigated at pH 7.6 and 33°C. Enantiomerically pure (S)-1-phenylethanol (ee > 99%) was produced with 75% conversion at 4-hr residence time.

Biosynthesis of ethyl (S)-4-chloro-3-hydroxybutanoate with an NADH-dependent reductase (ClCR) discovered by genome data mining using a modified colorimetric screening strategy

An NADH-dependent reductase (ClCR) was discovered by genome data mining. After ClCR was overexpressed in E. coli BL21, recombinant E. coli CCZU-T15 with high reductase activity and excellent stereoselectivity for the reduction of ethyl 4-chloro-3-oxobutanoate (COBE) into ethyl (S)-4-chloro-3-hydroxybutanoate [(S)-CHBE] was screened using a modified high-throughput colorimetric screening strategy. After the reaction optimization, a highly stereoselective bioreduction of COBE into (S)-CHBE (>99% ee) with the resting cells of E. coli CCZU-T15 was successfully demonstrated in toluene-water (50:50, v/v) biphasic system. Biotransformation of 1000 mM COBE for 24 h in the biphasic system, (S)-CHBE (>99% ee) could be obtained in the high yield of 96.4%. Significantly, E. coli CCZU-T15 shows high potential in the industrial production of (S)-CHBE (>99% ee).

Identification of important residues in diketoreductase from Acinetobacter baylyi by molecular modeling and site-directed mutagenesis

Diketoreductase (DKR) from Acinetobacter baylyi exhibits a unique property of double reduction of a β, δ-diketo ester with excellent stereoselectivity, which can serve as an efficient biocatalyst for the preparation of an important chiral intermediate for cholesterol lowering statin drugs. Taken the advantage of high homology between DKR and human heart 3-hydroxyacyl-CoA dehydrogenase (HAD), a molecular model was created to compare the tertiary structures of DKR and HAD. In addition to the possible participation of His-143 in the enzyme catalysis by pH profile, three key amino acid residues, Ser-122, His-143 and Glu-155, were identified and mutated to explore the possibility of involving in the catalytic process. The catalytic activities for mutants S122A/C, H143A/K and E155Q were below detectable level, while their binding affinities to the diketo ester substrate and cofactor NADH did not change obviously. The experimental results were further supported by molecular docking, suggesting that Ser-122 and His-143 were essential for the proton transfer to the carbonyl functional groups of the substrate. Moreover, Glu-155 was crucial for maintaining the proper orientation and protonation of the imidazole ring of His-143 for efficient catalysis.