Structural Insight into Enantioselective Inversion of an Alcohol Dehydrogenase Reveals a Polar Gate in Stereorecognition of Diaryl Ketones

Diaryl ketones are important building blocks for synthesizing pharmaceuticals and are generally regarded as difficult-to-reduce ketones due to the large steric hindrance of their two bulky aromatic side chains. Alcohol dehydrogenase from Kluyveromyces polyspora (KpADH) has been identified as a robust biocatalyst due to its high conversion of diaryl ketone substrate (4-chlorophenyl)(pyridine-2-yl)ketone (CPMK) with a moderate R-selectivity of 82% ee. To modulate the stereoselectivity of KpADH, a polarity scanning strategy was proposed, in which six key residues inside and at the entrance of the substrate binding pocket were identified. After iterative combinatorial mutagenesis, variants Mu-R2 and Mu-S5 with enhanced (99.2% ee, R) and inverted (97.8% ee, S) stereoselectivity were obtained. The crystal structures of KpADH and two mutants in complex with NADPH were resolved to elucidate the evolution of enantioselective inversion. Based on MD simulation, Mu-R2-CPMKProR and Mu-S5-CPMKProS were more favorable in the formation of prereaction states. Interestingly, a quadrilateral plane formed by alpha-carbons of four residues (N136, V161, C237, and G214) was identified at the entrance of the substrate binding pocket of Mu-S5; this plane acts as a polar gate for substrates. Due to the discrepancy in charge characteristics between chlorophenyl and pyridine substituents, the pro-S orientation of CPMK is defined when it passes through the polar gate in Mu-S5, whereas the similar plane in wild-type is blocked by several aromatic residues. Our result paves the way for engineering stereocomplementary ADH toward bulky diaryl ketones and provides structural insight into the mechanism of stereoselective inversion.