A [4Fe-4S]-Fe(CO)(CN)-L-cysteine intermediate is the first organometallic precursor in [FeFe] hydrogenase H-cluster bioassembly

Biosynthesis of the [FeFe] hydrogenase active site (the “H-cluster”) requires the interplay of multiple proteins and small molecules. Among them, the radical S-adenosyl-methionine enzyme HydG, a tyrosine lyase, has been proposed to generate an Fe(CO)(2)(CN) moiety-containing complex that is eventually incorporated into the H-cluster. Here we describe the characterization of an intermediate in the HydG reaction: a [4Fe-4S][(Cys)Fe(CO)(CN)] species, termed “Complex A”, in which a CO, a CN(−) and a cysteine (Cys) molecule bind to the unique “dangler” Fe site of the auxiliary [5Fe-4S] cluster of HydG. Identification of this intermediate—the first organometallic precursor to the H-cluster—validates the previously hypothesized HydG reaction cycle and provides a basis for elucidating the biosynthetic origin of other moieties of the H-cluster.