-!- The enzyme, often referred to as the cytochrome bc1 complex or complex III, is the third complex in the electron transport chain. -!- It is present in the mitochondria of all aerobic eukaryotes and in the inner membranes of most bacteria. -!- The mammalian enzyme contains cytochromes b-562, b-566 and c(1), and a 2-iron ferredoxin. -!- Depending on the organism and physiological conditions, the enzyme extrudes either two or four protons from the cytoplasmic to the non- cytoplasmic compartment (cf. EC 1.6.99.3). -!- Formerly EC 1.10.2.2.