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Chiral alcohols are valuable as diverse chemicals and synthetic intermediate materials. Phenylacetaldehyde reductase (PAR) is an enzyme that converts a wide variety of ketones into chiral alcohols with high optical purity. When an alcohol such as 2-propanol is used as a hydrogen donor, PAR itself will also mediate the regeneration of the coenzyme NADH in situ. Perceiving a capacity for improvement, we sought to develop a PAR that is able to convert higher concentrations of substrates in the presence of high concentrations of 2-propanol. The selection procedure for mutants was re-examined and a procedure able to select an effective amino acid substitution was established. Two advantageous amino acid substitutions were successfully selected using the procedure. When high-concentration substrate conversion reaction was subjected with a mutant that integrated both the two amino acid substitutions, near-complete conversions of m-chlorophenacyl chloride (m-CPC) (2.1 mmol/ml) and ethyl 4-chloro-3-oxobutanoate (ECOB) (1.9 mmol/ml) were achieved.

We describe an efficient method for producing both enantiomers of chiral alcohols by asymmetric hydrogen-transfer bioreduction of ketones in a 2-propanol (IPA)-water medium with E. coli biocatalysts expressing phenylacetaldehyde reductase (PAR: wild-type and mutant enzymes) from Rhodococcus sp. ST-10 and alcohol dehydrogenase from Leifsonia sp. S749 (LSADH). We also describe the detailed properties of mutant PARs, Sar268, and HAR1, which were engineered to have high activity and productivity in media composed of polar organic solvent and water, and the construction of three-dimensional structure of PAR by homology modeling. The K(m) and V(max) values for some substrates and the substrate specificity of mutant PARs were quite different from those of wild-type PAR. The results well explained the increased productivity of engineered PARs in IPA-water medium.