NADH:ubiquinone oxidoreductase (complex I) is one of the respiratory chain components (complexes I-IV and ATP synthase) in the mitochondrial inner membrane of eukaryotes or the plasma membrane of prokaryotes.

Structural domains comprising this superfamily share the structure of the 30kDa subunit of this complex, of which the bacterial homologue is denoted Nqo5. The subunit composes the hydrogenase-like module together with the Nqo4, Nqo6, and Nqo9 subunits. Homologous proteins of the Nqo5 subunit are also contained in membrane-bound multisubunit hydrogenases, while soluble-type hydrogenases are not. This subunit is essential in complex I assembly. This entry represents the Nqo5 subunit PMID:27398308.