Structure of Q-cytochrome c oxidoreductase. This enzyme is a homodimer with each monomer consisting of 11 distinct polypeptide chains. Some of the more prominent components in one monomer are colored while the other monomer is white. Although each monomer contains the same components, some are identified in one monomer or the other for ease of viewing. Notice that the major prosthetic groups, three hemes and a 2Fe-2S cluster, are located either near the cytoplasmic edge of the complex bordering the intermembrane space (top) or in the region embedded in the membrane (α helices represented by tubes). They are well positioned to mediate the electron-transfer reactions between quinones in the membrane and cytochrome c in the intermembrane space.

[Drawn from 1BCC.pdb.]