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

ABSTRACT

[FeFe] hydrogenases interconvert H2 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 H2 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 ( CrHydA1) 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 H2 production activity but giving significantly higher H2 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.