Bacterial cytochrome c oxidase is found bound to the to the cell membrane, where it is involved in the generation of the transmembrane proton electrochemical gradient. It is composed of four subunits. Subunit IV consists of one transmembrane helix that does not interact directly with the other subunits, but maintains its position by indirect contacts via phospholipid molecules found in the structure. The function of subunit IV is as yet unknown [1].

1: The X-ray crystal structures of wild-type and EQ(I-286) mutant cytochrome c oxidases from Rhodobacter sphaeroides. Svensson-Ek M, Abramson J, Larsson G, Tornroth S, Brzezinski P, Iwata S; J Mol Biol 2002;321:329-339. PMID:12144789