Engineering the phenylacetaldehyde reductase mutant for improved substrate conversion in the presence of concentrated 2-propanol

Phenylacetaldehyde reductase (PAR) from Rhodococcus sp. ST-10 is useful for chiral alcohol production because of its broad substrate specificity and high stereoselectivity. The conversion of ketones into alcohols by PAR requires the coenzyme NADH. PAR can regenerate NADH by oxidizing additional alcohols, especially 2-propanol. However, substrate conversion by wild-type PAR is suppressed in concentrated 2-propanol. Previously, we developed the Sar268 mutant of PAR, which can convert several substrates in the presence of concentrated 2-propanol. In this paper, further mutational engineering of Sar268 was performed to achieve higher process yield. Each of nine amino acid positions that had been examined for generating Sar268 was subjected to saturation mutagenesis. Two novel substitutions at the 42nd amino acid position increased m-chlorophenacyl chloride (m-CPC) conversion. Moreover, several nucleotide substitutions identified from libraries of random mutations around the start codon also improved the PAR activity. E. coli cells harboring plasmid pHAR1, which has the integrated sequence of the top clones from the above selections, provided greater conversion of m-CPC and ethyl 4-chloro-3-oxobutanoate than the Sar268 mutant, with very high optical purity of products. This mutant is a promising novel biocatalyst for efficient chiral alcohol production.