MopB_Res_Cmplx1_Nad11: The second domain of the Nad11/75-kDa subunit of the NADH-quinone oxidoreductase/respiratory complex I/NADH dehydrogenase-1(NDH-1) of eukaryotes and the Nqo3/G subunit of alphaproteobacteria NDH-1. The NADH-quinone oxidoreductase 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 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 Paracoccus denitrificans, this subunit is encoded by the nqo3 gene, and is part of the 14 distinct subunits constituting the 'minimal' functional enzyme. The Nad11/Nqo3 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 belongs to the molybdopterin_binding (MopB) superfamily of proteins.
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