Reversible H Atom Abstraction Catalyzed by the Radical S-Adenosylmethionine Enzyme HydG

[Image: see text] The organometallic H-cluster at the active site of [FeFe]-hydrogenases is synthesized by three accessory proteins, two of which are radical S-adenosylmethionine enzymes (HydE, HydG) and one of which is a GTPase (HydF). In this work we probed the specific role of H atom abstraction in HydG-catalyzed carbon monoxide and cyanide production from tyrosine. The isotope distributions of 5′-deoxyadenosine and p-cresol were evaluated using deuterium-labeled tyrosine substrates in H(2)O and D(2)O. The observation of multiply deuterated 5′-deoxyadenosine and deuterated S-adenosylmethionine when the reaction is carried out in D(2)O provides evidence for a 5′-deoxyadenosyl radical-mediated abstraction of a hydrogen atom from a solvent-exchangeable position as a reversible event.