InterPro: IPR001268 NADH:ubiquinone oxidoreductase, 30kDa subunit

NADH:ubiquinone oxidoreductase (complex I) (EC: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], mitochondira, 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]. Mitochondrial complex I, which is located in the inner mitochondrial membrane, is the largest multimeric respiratory enzyme in the mitochondria, consisting of more than 40 subunits, one FMN co-factor and eight FeS clusters [4]. The assembly of mitochondrial complex I is an intricate process that requires the cooperation of the nuclear and mitochondrial genomes [4, 5]. Mitochondrial complex I can cycle between active and deactive forms that can be distinguished by the reactivity towards divalent cations and thiol-reactive agents. All redox prosthetic groups reside in the peripheral arm of the L-shaped structure. The NADH oxidation domain harbouring the FMN cofactor is connected via a chain of iron-sulphur clusters to the ubiquinone reduction site that is located in a large pocket formed by the PSST and 49kDa subunits of complex I [6].

The 30 kDa subunit from NADH:ubiquinone oxidoreductase is found in both eukaryotes and prokaryotes. In mammals and in Neurospora crassa, it is nuclear-encoded as a precursor form with a transit peptide, while in Paramecium (protein P1), in the Dictyostelium discoideum (Slime mold) it is mitochondrial-encoded and it is chloroplast-encoded in various higher plants. It is also present in bacteria.