Stepwise [FeFe]-hydrogenase H-cluster assembly revealed in the structure of HydAΔEFG

Complex enzymes containing Fe-S clusters are ubiquitous in nature, where they are involved in a number of fundamental processes including carbon dioxide fixation, nitrogen fixation and hydrogen metabolism(1,2). Hydrogen metabolism is facilitated by the activity of three evolutionarily and structurally unrelated enzymes: the [NiFe]-hydrogenases, [FeFe]-hydrogenases and [Fe]-hydrogenases(3,4) (Hmd). The catalytic core of the [FeFe]-hydrogenase (HydA), termed the H-cluster, exists as a [4Fe-4S] subcluster linked by a cysteine thiolate to a modified 2Fe subcluster with unique non-protein ligands(5,6). The 2Fe subcluster and non-protein ligands are synthesized by the hydrogenase maturation enzymes HydE, HydF and HydG; however, the mechanism, synthesis and means of insertion of H-cluster components remain unclear(7-10). Here we show the structure of HydA(Delta EFG) (HydA expressed in a genetic background devoid of the active site H-cluster biosynthetic genes hydE, hydF and hydG) revealing the presence of a [4Fe-4S] cluster and an open pocket for the 2Fe subcluster. The structure indicates that H-cluster synthesis occurs in a stepwise manner, first with synthesis and insertion of the [4Fe-4S] subcluster by generalized host-cell machinery(11,12) and then with synthesis and insertion of the 2Fe subcluster by specialized hydE-, hydF- and hydG-encoded maturation machinery(7-10). Insertion of the 2Fe subcluster presumably occurs through a cationically charged channel that collapses following incorporation, as a result of conformational changes in two conserved loop regions. The structure, together with phylogenetic analysis, indicates that HydA emerged within bacteria most likely from a Nar1-like ancestor lacking the 2Fe subcluster, and that this was followed by acquisition in several unicellular eukaryotes.