NADH:ubiquinone oxidoreductase (complex I) (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) [1]. 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 [2], mitochondria, 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 [3].

This entry represents subunit 1 NADH:ubiquinone oxidoreductase [4]. Among the many polypeptide subunits that make up complex I, there are fifteen which are located in the membrane part, seven of which are encoded by the mitochondrial and chloroplast genomes of most species. The most conserved of these organelle-encoded subunits is known as subunit 1 (gene ND1 in mitochondrion, and NDH1 in chloroplast) and seems to contain the ubiquinone binding site.

The ND1 subunit is highly similar to subunit 4 of Escherichia coli formate hydrogenlyase (gene hycD), subunit C of hydrogenase-4 (gene hyfC). Paracoccus denitrificans NQO8 and Escherichia coli nuoH NADH-ubiquinone oxidoreductase subunits also belong to this family [5].

1. The NADH:ubiquinone oxidoreductase (complex I) of respiratory chains. Q. Rev. Biophys. 25, 253-324
2. The respiratory complex I of bacteria, archaea and eukarya and its module common with membrane-bound multisubunit hydrogenases. FEBS Lett. 479, 1-5
3. Assembly of the Escherichia coli NADH:ubiquinone oxidoreductase (complex I). Biochim. Biophys. Acta 1777, 735-9
4. The respiratory-chain NADH dehydrogenase (complex I) of mitochondria. Eur. J. Biochem. 197, 563-76
5. The gene locus of the proton-translocating NADH: ubiquinone oxidoreductase in Escherichia coli. Organization of the 14 genes and relationship between the derived proteins and subunits of mitochondrial complex I. J. Mol. Biol. 233, 109-22

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