Characterization of the [FeFe]-hydrogenase maturation protein HydF by EPR techniques: insights into the catalytic mechanism.

The catalytic site of [FeFe]-hydrogenase, the “H-cluster”, composed by a [4Fe-4S] unit connected by a cysteinyl residue to a [2Fe] center coordinated by three CO, two CN- and a bridging dithiolate, is assembled in a complex maturation pathway, at present not fully characterized, involving three conserved proteins, HydG, HydE and HydF. In this contribution we review our studies on HydF, a protein which acts as scaffold and carrier for the [2Fe] unit of the H-cluster. HydF is a complex enzyme which contains one [4Fe-4S] cluster binding site, with three conserved cysteine residues and a non-Cys ligand. We have exploited EPR, HYSCORE and PELDOR spectroscopies to get insight into the structure and chemical role of HydF. On the basis of the results we discuss the possibility that the non-Cys ligated Fe atom of the [4Fe-4S] cluster, is the site where the [2Fe] subcluster precursor is anchored and finally processed to be delivered to the hydrogenase (HydA). Our PELDOR experiments on the isolated GTPase domain of HydF, have also suggested that interactions with HydG and HydE proteins may be regulated by the binding of the nucleotide.