Repurposing hemoproteins for metal-catalyzed H atom transfer in asymmetric radical biocatalysis

Transition metal–hydrides have been widely exploited in homogenous catalysis for hydrofunctionalization of unsaturated moieties, including carbonyls, alkenes and alkynes. As a complement to the well-established chemistry of these complexes involving heterolytic metal–hydride bond cleavage, metal–hydride hydrogen atom transfer (MHAT) has attracted increased interest, as it offers a promising strategy for radical hydrofunctionalziation of unactivated alkenes thus enabling late-stage diversification of complex molecules. However, due the weak interactions between the prochiral organic radical species and the enantiopure metal catalyst, achieving asymmetric MHAT remains challenging. Herein, we report our efforts to repurpose cytochrome P450 enzymes to catalyze asymmetric MHAT, a new-to-nature reaction. Directed evolution of the well-studied P450BM3 (CYP102A1) enzyme led to the identification of a triple mutant that catalyzes asymmetric MHAT radical cyclization of unactivated alkenes to afford diverse cyclic compounds, including pyrrolidines, in up to a 97:3 enantiomeric ratio under aerobic whole cell conditions. Mechanistic investigations support an MHAT mechanism proceeding via homolytic cleavage of a fleeting iron(III)hydride species. Directed evolution using CYP119 as hemoprotein scaffold led to the identification of a stereocomplementary MHATase, highlighting the potential of repurposed hemoproteins for MHAT biocatalysis. Our study showcases the potential of integrating abiotic metal–hydride activity into native metalloenzymes to expand the scope of asymmetric radical biocatalysis.