The hydrogenase from "Desulfovibrio vulgaris" and "D. gigas" have similar protein structures to each other and represent typical [Ni-Fe hydrogenases. The large subunit contains the active site buried deep in the core of the protein and the small subunit contains [Fe-S clusters. The Ni atom is coordinated to the Desulfovibrio hydrogenase by 4 cysteine ligands. Two of these same cysteine ligands also bridge the Fe of the active site. The Fe atom also contains three ligands, one CO and two CN that complete the active site. These additional ligands might contribute to the reactivity or help stabilize the Fe atom in the low spin +2 oxidation state. Typical [NiFe hydrogenases like those of "D. vulgaris" and "D. gigas" are poisoned by oxygen because an oxygen atom binds strongly to the NiFe active site.