NADH + H+ + ubiquinone + 4H+(in) => NAD+ + ubiquinol + 4H+(out)
This multi-subunit enzyme is much the largest component of the mitochondrial electron transport chain. It and contains more than 30 polypeptide chains, and is comparable in size and complexity with a large ribosomal subunit. Seven of these peptides are coded by the mitochondrial DNA, and the remainder are imported from the cytosol.
The prosthetic groups include a single molecule of flavin mononucleotide (FMN) and about seven non-heme iron-sulphur clusters, only four of which have been partly characterised. There is some evidence for a strongly bound, non-exchangeable molecule of ubiquinone which functions in proton tranlocation, but this remains speculative at present. Although oxidation and reduction of the flavin can be observed using optical spectroscopy, the non-heme iron can only be studied using tedious low temperature electron spin resonance (EPR) spectroscopy and progress has been correspondingly slow.
The enzyme spans the mitochondrial inner membrane, and appears to consist of distinct hydrophilic and hydrophobic domains. The hydrophilic domain (containing FMN and at least three of the non-heme iron centres) extends far into the aqueous matrix space. The separate hydrophobic domain is embedded in the membrane lipids, and contains at least one additional non-heme iron cluster.
The enzyme is strongly inhibited by rotenone, an insecticide derived from derris root.