Repurposing myoglobin into an abiological asymmetric ketoreductase

Thanks to recent advances in enzyme repurposing, hemoproteins have gained significant attention as versatile biocatalysts that catalyze a variety of transformations, ranging from oxidation to redox-neutral reactions. To complement these achievements, we report herein on our efforts to repurpose myoglobin (Mb) into an asymmetric ketoreductase, using PhSiH3 as reductant. Two rounds of mutagenesis afforded a double mutant capable of reducing with high enantioselectivity a broad range of prochiral aliphatic and aromatic ketones in the presence of whole cells. Additional rounds of directed evolution afforded a quintuple mutant with opposite enantioselectivity. Mechanistic investigations suggest that a fleeting Fe–H species undergoes heterolytic hydride transfer to afford enantiopure alcohols from the corresponding ketones. The excellent saturation kinetic profile, combined with the practicality of whole-cell biocatalysis under aerobic conditions, highlights the potential of repurposed Mb as an asymmetric ketoreductase with a broad substrate scope, thus expanding the reaction repertoire catalyzed by hemoproteins.