<p> NADH:ubiquinone oxidoreductase (complex I) (<db_xref db="EC" dbkey="1.6.5.3"/>) is a respiratory-chain enzyme that catalyses the transfer of two electrons from NADH to ubiquinone in a reaction that is associated with proton translocation across the membrane (NADH + ubiquinone = NAD+ + ubiquinol) [<cite idref="PUB00005074"/>]. Complex I is a major source of reactive oxygen species (ROS) that are predominantly formed by electron transfer from FMNH(2). Complex I is found in bacteria, cyanobacteria (as a NADH-plastoquinone oxidoreductase), archaea [<cite idref="PUB00043561"/>], mitochondira, and in the hydrogenosome, a mitochondria-derived organelle. In general, the bacterial complex consists of 14 different subunits, while the mitochondrial complex contains homologues to these subunits in addition to approximately 31 additional proteins [<cite idref="PUB00045437"/>]. Mitochondrial complex I, which is located in the inner mitochondrial membrane, is the largest multimeric respiratory enzyme in the mitochondria, consisting of more than 40 subunits, one FMN co-factor and eight FeS clusters [<cite idref="PUB00045438"/>]. The assembly of mitochondrial complex I is an intricate process that requires the cooperation of the nuclear and mitochondrial genomes [<cite idref="PUB00045438"/>, <cite idref="PUB00045439"/>]. Mitochondrial complex I can cycle between active and deactive forms that can be distinguished by the reactivity towards divalent cations and thiol-reactive agents. All redox prosthetic groups reside in the peripheral arm of the L-shaped structure. The NADH oxidation domain harbouring the FMN cofactor is connected via a chain of iron-sulphur clusters to the ubiquinone reduction site that is located in a large pocket formed by the PSST and 49kDa subunits of complex I [<cite idref="PUB00045440"/>].</p><p>This entry describes the G subunit (one of 14 subunits, A to N) of the NADH-quinone oxidoreductase complex I which generally couples NADH and ubiquinone oxidation/reduction in bacteria and mammalian mitochondria while translocating protons, but may act on NADPH and/or plastoquinone in cyanobacteria and plant chloroplasts. This family does not contain related subunits from formate dehydrogenase complexes.</p>