<p>Cytochrome c oxidase (<db_xref db="EC" dbkey="1.9.3.1"/>) [<cite idref="PUB00000581"/>, <cite idref="PUB00002253"/>] is an oligomeric enzymatic complex which is a component of the respiratory chain and is involved in the transfer of electrons from cytochrome c to oxygen. In eukaryotes this enzyme complex is located in the mitochondrial inner membrane; in aerobic prokaryotes it is found in the plasma membrane. The enzyme complex consists of 3-4 subunits (prokaryotes) to up to 13 polypeptides (mammals).</p><p>Subunit 2 (CO II) transfers the electrons from cytochrome c to the catalytic subunit 1. It contains two adjacent transmembrane regions in its N terminus and the major part of the protein is exposed to the periplasmic or to the mitochondrial intermembrane space, respectively. CO II provides the substrate-binding site and contains a copper centre called Cu(A) (see <db_xref db="INTERPRO" dbkey="IPR001505"/>), probably the primary acceptor in cytochrome c oxidase. An exception is the corresponding subunit of the cbb3-type oxidase which lacks the copper A redox-centre. Several bacterial CO II have a C-terminal extension that contains a covalently bound haem c.</p><p> The N-terminal domain of cytochrome C oxidase contains two transmembrane alpha-helices. </p> Cytochrome C oxidase subunit II, transmembrane domain