a b s t r a c t[FeFe]-hydrogenases catalyze the protons/hydrogen interconversion through a unique di-iron active site consisting of three CO and two CN ligands, and a non-protein SCH 2 XCH 2 S (X = N or O) dithiolate bridge. Site assembly requires two ''Radical-S-adenosylmethionine (SAM or AdoMet)" iron-sulfur enzymes, HydE and HydG, and one GTPase, HydF. The sequence homology between HydG and ThiH, a Radical-SAM enzyme which cleaves tyrosine into p-cresol and dehydroglycine, and the finding of a similar cleavage reaction catalyzed by HydG suggests a mechanism for hydrogenase maturation. Here we propose that HydG is specifically involved in the synthesis of the dithiolate ligand, with two tyrosine-derived dehydroglycines as precursors along with an [FeS] cluster of HydG functioning both as electron shuttle and source of the sulfur atoms.
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