Repurposing haemoproteins for asymmetric metal-catalysed H atom transfer

Transition metal-hydrides have been widely exploited in catalysis for the hydrofunctionalization of unsaturated moieties, including carbonyls, alkenes and alkynes¹. To complement heterolytic metal-hydride bond cleavage, metal-hydride hydrogen atom transfer (MHAT) has recently gained attention, as a promising strategy for radical hydrofunctionalization of unactivated alkenes², thus enabling late-stage diversification of complex molecules^3,4. However, owing to the weak interactions between the prochiral organic radical and the enantiopure catalyst⁵, asymmetric MHAT⁶ remains challenging. Here we show that cytochrome P450 enzymes (CYPs) can be repurposed to catalyse asymmetric MHAT, a new-to-nature reaction. Directed evolution of P450_BM3 yielded a triple mutant that catalyses MHAT radical cyclization of unactivated alkenes, producing diverse cyclic compounds-including pyrrolidines and piperidines-with up to 98:2 enantiomeric ratio under aerobic whole-cell conditions. Apart from electron-deficient alkenes, alternative radical acceptors-including hydrazones, oximes and nitriles-were converted by repurposed P450_BM3 to enantioenriched cyclization products. Mechanistic investigations support an MHAT mechanism proceeding by homolytic cleavage of a fleeting iron(III)-hydride species^2,6. Starting from CYP119, directed evolution afforded a stereocomplementary MHATase, highlighting the potential of repurposed CYPs for MHAT biocatalysis. Our study highlights the prospect of integrating homolytic metal-hydride reactivity into metalloenzymes, thus expanding the scope of asymmetric radical biocatalysis.