NADH:ubiquinone oxidoreductase, also known as complex I, is a respiratory-chain enzyme that catalyses the transfer of two electrons from NADH to ubiquinone, reaction associated with proton translocation across the membrane PMID:1470679. Complex I is found in bacteria, cyanobacteria (as a NADH-plastoquinone oxidoreductase), archaea PMID:10940377, mitochondria, and in the hydrogenosome, a mitochondria-derived organelle. In the majority of cases, the bacterial complex consists of 14 different subunits, while the mitochondrial complex contains homologues to these subunit together with approximately 31 additional proteins PMID:18394423.

Structural domains comprising this superfamily share the structure of chain 3 of the NADH-ubiquinone / plastoquinone oxidoreductase.