<p>[NiFe] hydrogenases function in H2 metabolism in a variety of microorganisms, enabling them to use H2 as a source of reducing equivalent under aerobic and anaerobic conditions [NiFe] hydrogenases consist of two subunits, hydrogenase large and hydrogenase small. The large subunit contains the binuclear [NiFe] active site, while the small subunit binds at least one [4Fe-4S] cluster [<cite idref="PUB00035518"/>].</p><p>Energy-converting [NiFe] hydrogenases (or [NiFe]-hydrogenase-3-type) form a distinct group within the [NiFe] hydrogenase family [<cite idref="PUB00035516"/>, <cite idref="PUB00035517"/>]. Members of this subgroup include:</p><ul><li>Hydrogenase 3 and 4 (Hyc and Hyf) from <taxon tax_id="562">Escherichia coli</taxon> </li><li>CO-induced hydrogenase (Coo) from <taxon tax_id="1085">Rhodospirillum rubrum</taxon> </li><li>Mbh hydrogenase from <taxon tax_id="2261">Pyrococcus furiosus</taxon> </li><li>Eha and Ehb hydrogenases from Methanothermobacter species</li><li>Ech hydrogenase from <taxon tax_id="2208">Methanosarcina barkeri</taxon> </li></ul><p>Energy-converting [NiFe] hydrogenases are membrane-bound enzymes with a six-subunit core: the large and small hydrogenase subunits, plus two hydrophilic proteins and two integral membrane proteins. Their large and small subunits show little sequence similarity to other [NiFe] hydrogenases, except for key conserved residues coordinating the active site and [FeS] cluster. However, they show considerable sequence similarity to the six-subunit, energy-conserving NADH:quinone oxidoreductases (complex I), which are present in cytoplasmic membranes of many bacteria and in inner mitochondrial membranes. However, the reactions they catalyse differ significantly from complex I. Energy-converting [NiFe] hydrogenases function as ion pumps.</p><p>This entry represents a [NiFe]-hydrogenase-3-type complex, small subunit/NADH:quinone oxidoreductase (complex I), subunit PSST/NdhK/NuoB.</p> [NiFe]-hydrogenase-3-type complex, small subunit/NADH:quinone oxidoreductase, subunit NuoB