Complex III is a multisubunit transmembrane protein encoded by both the mitochondrial (cytochrome b) and the nuclear genomes (all other subunits).

Complex III is present in the mitochondria of all animals and all aerobic eukaryotes and the inner membranes of most bacteria.

This enzyme belongs to the family of oxidoreductases, specifically those acting on diphenols and related substances as donor with a cytochrome as acceptor.

Some have been commercialized as fungicides (the strobilurin derivatives, best known of which is azoxystrobin; QoI inhibitors) and as anti-malaria agents (atovaquone).

[11] Electron leakage occurs mainly at the Qo site and is stimulated by antimycin A. Antimycin A locks the b hemes in the reduced state by preventing their re-oxidation at the Qi site, which, in turn, causes the steady-state concentrations of the Qo semiquinone to rise, the latter species reacting with oxygen to form superoxide.

[12] Superoxide produced at the Qo site can be released both into the mitochondrial matrix[13][14] and into the intermembrane space, where it can then reach the cytosol.

[21] The extent to which these various pathologies are due to bioenergetic deficits or overproduction of superoxide is presently unknown.