Respiratory-chain NADH dehydrogenase (EC 1.6.5.3) [1,2] (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:

• 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.

This protein, in its mature form, consists of from 157 to 266 amino acid residues. The best conserved region is located in the C-terminal section and can be used as a signature pattern.