Compared to the other major proton-pumping subunits of the electron transport chain, the number of subunits found can be small, as small as three polypeptide chains. This number does increase, and eleven subunits are found in higher animals. Three subunits have prosthetic groups. The cytochrome b subunit has two b-type hemes (b and b), the cytochrome c subunit has one c-type heme (c), and the Rieske Iron Sulfur Protein subunit (ISP) has a two iron, two sulfur iron-sulfur cluster (2Fe•2S).
Structures of complex III: ,