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Towards a mechanistic understanding of the role of the iron-sulphur cluster-containing HypD protein in diatomic ligand biosynthesis of [NiFe]-hydrogenases
[NiFe]-hydrogenases are ancient iron-sulphur enzymes that have in their active site a bimetallic nickel-iron cofactor coordinated by four cysteinyl thiols. The cofactor is unusual in that the iron atom carries a carbon monoxide and two cyanides as diatomic ligands. Six conserved Hyp proteins are required to synthesize this NiFe(CN)2CO moiety. The Fe(CN)2CO portion of the cofactor is completed by the HypCDEF proteins and is inserted into the apo-catalytic subunit. Next, the nickel is inserted by HypA and HypB. The synthesis of the Fe(CN)2CO group is not completely understood. The HypEF proteins generate the cyanide ligands from the precursor carbamoylphosphate. Recent evidence from our group suggests that the CO is generated from endogenous CO2 bound to an iron atom that likely forms the basis of the cofactor. The key protein in Fe(CN)2CO group synthesis is the iron-sulphur enzyme HypD. HypD functions as a scaffold with which the other Hyp proteins interact. Moreover, it is the only Hyp protein with a redox function. This research proposal, therefore, will focus primarily on elucidating the biochemical function of HypD in generating the Fe(CN)2CO moiety. HypD functions together with a small ‚chaperone’-like protein HypC. HypC and HypD form a tight complex and as well as being responsible for recognition of the apo-catalytic subunit of the hydrogenase, HypC also binds iron bound with CO2. Our current working hypothesis is that CO2 bound by iron undergoes HypD-dependent reduction to an Fe(I)-CO intermediate to which the cyanide groups are transferred from the HypEF complex. We have recently demonstrated that HypD undergoes disulphide-thiol redox chemistry, explaining how CO2 reduction might proceed. Therefore, we aim to determine the physiological electron donor to HypD, the mechanism by which HypD catalyzes reduction of CO2 and how the HypCD complex transfers the Fe(CN)2CO group into the active site of the hydrogenase
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