<p> The iron-only hydrogenases (<db_xref db="EC" dbkey="1.18.99.1"/>) catalyse the two-electron reduction of two protons to yield dihydrogen, as part of an energy cycle. Fe-only hydrogenases are restricted to strictly anaerobic microbes, and are often very sensitive to molecular oxygen. The cytoplasmic monomeric Fe hydrogenases are involved in hydrogen production, while the periplasmic, heterodimeric Fe hydrogenases are involved in hydrogen uptake. Fe hydrogenases consist of two, intertwined domains, the catalytic domain [<cite idref="PUB00011734"/>] and the larger subunit C domain [<cite idref="PUB00006430"/>]. The larger subunit C domain can be divided into three subdomains. There are five distinct metal clusters, one in the catalytic domain. This entry represents the two intertwined domains, the catalytic domain and the large subunit C domain.</p>