Proximal histidine redox potential

The [4Fe-4S] clusters in D. gigas hydrogenase, unusually, show pH-dependent redox potentials, which means that they transfer protons and electrons at the same time. The distal cluster has a histidine ligand near the surface and so it presumably acquires a proton from the solution on reduction. The proximal cluster is distant from the surface, but there is a potential channel for protons leading to the [NiFe]-center, where protons are involved in the cataljdic reaction. Thus when the [NiFe]-center reduces the proximal cluster, both an electron and a proton can be transferred, at least as far as the [3Fe-4S] cluster. 

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