His-Ligation to the [4Fe-4S] Subcluster Tunes the Catalytic Bias of [FeFe] Hydrogenase

[FeFe] hydrogenases interconvert H-2 into protons and electrons reversibly and efficiently. The active site H-cluster is composed of two sites: a unique [2Fe] subcluster ([2Fe](H)) covalently linked via cysteine to a canonical [4Fe-4S] cluster ([4Fe-4S](H)). Both sites are redox active and electron transfer is proton-coupled, such that the potential of the H-cluster lies very close to the H-2 thermodynamic potential, which confers the enzyme with the ability to operate quickly in both directions without energy losses. Here, one of the cysteines coordinating [4Fe-4S](H) (Cys362) in the [FeFe] hydrogenase from the green algae Chlamydomonas reinhardtii (CrHydAl) was exchanged with histidine and the resulting C362H variant was shown to contain a [4Fe-4S] cluster with a more positive redox potential than the wild-type. The change in the [4Fe-4S] cluster potential resulted in a shift of the catalytic bias, diminishing the H-2 production activity but giving significantly higher H-2 oxidation activity, albeit with a 200 mV overpotential requirement. These results highlight the importance of the [4Fe-4S] cluster as an electron injection site, modulating the redox potential and the catalytic properties of the H-cluster.