Engineering Non-haem Enzymes for Nickel-Catalyzed C(sp2)‒S Coupling via Ligand-to-Metal Charge Transfer Photocatalysis

Integrating new metal-catalysed transformations into enzymes is a key objective in biocatalysis. This study introduces photoinduced ligand-to-metal charge transfer (LMCT) as a new strategy for enabling abiotic cross-coupling reactions in metalloenzymes. By tailoring the primary coordination sphere to establish a 2-histidine metal binding site and replacing the iron center with nickel, the ethylene-forming enzyme from Pseudomonas savastanoi (PsEFE) was activated for nickel-catalysed C(sp2)‒S cross-coupling between aryl bromides and thiols. Directed evolution of PsEFE produced highly active variants capable of generating over 50 thioether products in up to 98% yield and 530 total turnover numbers. Mechanistic investigations suggest that this photoenzymatic reaction involves a Ni(II)/Ni(I)/Ni(III) catalytic cycle with generation of a reactive Ni(I) species and thiyl radical via photoinduced LMCT. We anticipate that these findings will inspire further exploration of integrating abiotic cross-coupling transformations into enzymatic catalysis.