<p> Many microorganisms, such as methanogenic, acetogenic, nitrogen-fixing, photosynthetic, or sulphate-reducing bacteria, metabolise hydrogen. Hydrogen activation is mediated by a family of enzymes, termed hydrogenases, which either provide these organisms with reducing power from hydrogen oxidation, or act as electron sinks. There are two hydrogenases families that differ functionally from each other: NiFe hydrogenases tend to be more involved in hydrogen oxidation, while Iron-only FeFe (Fe only) hydrogenases in hydrogen production. </p><p>Fe only hydrogenases (<db_xref db="EC" dbkey="1.12.7.2"/>) show a common core structure, which contains a moiety, deeply buried inside the protein, with an Fe-Fe dinuclear centre, nonproteic bridging, terminal CO and CN- ligands attached to each of the iron atoms, and a dithio moiety, which also bridges the two iron atoms and has been tentatively assigned as a di(thiomethyl)amine. This common core also harbours three [4Fe-4S] iron-sulphur clusters [<cite idref="PUB00010634"/>]. In FeFe hydrogenases, as in NiFe hydrogenases, the set of iron-sulphur clusters is dispersed regularly between the dinuclear Fe-Fe centre and the molecular surface. These clusters are distant by about 1.2 nm from each other but the [4Fe-4S] cluster closest to the dinuclear centre is covalently bound to one of the iron atoms though a thiolate bridging ligand. The moiety including the dinuclear centre, the thiolate bridging ligand, and the proximal [4Fe-4S] cluster is known as the H-cluster. A channel, lined with hydrophobic amino acid side chains, nearly connects the dinuclear centre and the molecular surface. Furthermore hydrogen-bonded water molecule sites have been identified at the interior and at the surface of the protein.</p><p>The small subunit is comprised of alternating random coil and alpha helical structures that encompass the large subunit in a novel protein fold [<cite idref="PUB00006430"/>].</p>