MopB_NADH-Q-OR-NuoG2: The NuoG/Nad11/75-kDa subunit (second domain) of the NADH-quinone oxidoreductase (NADH-Q-OR)/respiratory complex I/NADH dehydrogenase-1 (NDH-1). The NADH-Q-OR is the first energy-transducting complex in the respiratory chains of many prokaryotes and eukaryotes. Mitochondrial complex I and its bacterial counterpart, NDH-1, function as a redox pump that uses the redox energy to translocate H+ ions across the membrane, resulting in a significant contribution to energy production. The atomic structure of complex I is not known and the mechanisms of electron transfer and proton pumping are not established. The nad11 gene codes for the largest (75-kDa) subunit of the mitochondrial NADH:ubiquinone oxidoreductase, it constitutes the electron input part of the enzyme, or the so-called NADH dehydrogenase fragment. In Escherichia coli, this subunit is encoded by the nuoG gene, and is part of the 14 distinct subunits constituting the 'minimal' functional enzyme. The nad11 gene is nuclear-encoded in animals, plants, and fungi, but is still encoded in the mitochondrial genome of some protists. The Nad11/NuoG subunit is made of two domains: the first contains three binding sites for FeS clusters (the fer2 domain), the second domain (this CD), is of unknown function or, as postulated, has lost an ancestral formate dehydrogenase activity that became redundant during the evolution of the complex I enzyme. Although only vestigial sequence evidence remains of a molybdopterin binding site, this protein domain family belongs to the molybdopterin_binding (MopB) superfamily of proteins. Bacterial type II NADH-quinone oxidoreductases and NQR-type sodium-motive NADH-quinone oxidoreductases are not homologs of this domain family.
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