InterPro domain: IPR037232

Respiratory-chain NADH dehydrogenase ( 1.6.5.3 ) (also known as complex I or NADH-ubiquinone oxidoreductase) is an oligomeric enzymatic complex located in the inner mitochondrial membrane which also seems to exist in the chloroplast and in cyanobacteria (as a NADH-plastoquinone oxidoreductase). Among the 25 to 30 polypeptide subunits of this bioenergetic enzyme complex there is one with a molecular weight of 30 Kd (in mammals) which has been found to be [ 1 ]:

• Nuclear encoded, as a precursor form with a transit peptide in mammals, and in Neurospora crassa.
• Mitochondrial encoded in Paramecium (protein P1), and in the slime mold Dictyostelium discoideum (ORF 209).
• Chloroplast encoded in various higher plants (ORF 159).

It is also present in bacteria:

• In the cyanobacteria Synechocystis strain PCC 6803 (gene ndhJ).
• Subunit C of Escherichia coli NADH-ubiquinone oxidoreductase (gene nuoC).
• Subunit Nqo5 of Paracoccus denitrificans NADH-ubiquinone oxidoreductase. Nqo5 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 [ 2 ].

1. The respiratory-chain NADH dehydrogenase (complex I) of mitochondria. Eur. J. Biochem. 197, 563-76
2. Characterization of the Nqo5 subunit of bacterial complex I in the isolated state. FEBS Open Bio 6, 687-95