Asymmetric Bioreduction of 4-hydroxy-2-butanone by Carbonyl Reductases PFODH and CpSADH Delivers 1,3-butanediol Enantiomers with Excellent R - and S -enantioselectivity

Chiral 1,3-butandiol (1,3-BD) is a very important intermediate for the synthesis of pharmaceutically valuable compounds. In this study, biocatalysts were developed to deliver optically pure R- and S- 1,3-BD enantiomers at high conversions. Based on the catalytic activity and enantioselectivity toward 4-hydroxy-2-butanone (4H2B), two carbonyl reductases, PFODH from Pichia finlandica and CpSADH from Candida parapsilosis were screened out from a library of 20 reductases. PFODH was discovered as a new biocatalyst for reducing the 4H2B to the corresponding R -product while CpSADH was used for the first time to reduce the target substrate with strict S selectivity, and both showed high substrate/product tolerance and good catalytic activity (with conversions of 81-90%) without requirement of external cofactors. Regeneration of cofactor NADH was facilitated by the substrate-coupled system using isopropyl alcohol as a cosubstrate. The substrate spectra of PFODH and CpSADH were further investigated by expanding to various substituted aryl ketones. Almost all of the analyzed ketones were reduced asymmetrically into their corresponding chiral alcohols with excellent ee values of 97-99%. PFODH easily reduced the substrates which are substituted adjacent to the carbonyl group or those substituted on the meta-position of the phenyl ring, while CpSADH hardly reduced those with substituents adjacent to the carbonyl group. These results demonstrate the industrial potential of PFODH and CpSADH in biosynthesis of optically pure 1,3-butanediol and other valuable chiral alcohols.