Cytochrome c oxidase subunit 2, also known as cytochrome c oxidase polypeptide II, is a protein that in humans is encoded by the MT-CO2 gene. Cytochrome c oxidase subunit II, abbreviated COXII, COX2, COII, or MT-CO2, is the second subunit of cytochrome c oxidase. It is also one of the three mitochondrial DNA (mtDNA) encoded subunits (MT-CO1, MT-CO2, MT-CO3) of respiratory complex IV. The MT-CO2 gene is located on the p arm of mitochondrial DNA at position 12 and it spans 683 base pairs.[5] The MT-CO2 gene produces a 25.6 kDa protein composed of 227 amino acids.MT-CO2 is a subunit of the enzyme Cytochrome c oxidase (EC 1.9.3.1) (Complex IV), an oligomeric enzymatic complex of the mitochondrial respiratory chain 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). The N-terminal domain of cytochrome C oxidase contains two transmembrane alpha-helices. The structure of MT-CO2 is known to contain one redox center and a binuclear copper A center (CuA). The CuA is located in a conserved cysteine loop at 196 and 200 amino acid positions and conserved histidine at 204. Several bacterial MT-CO2 have a C-terminal extension that contains a covalently bound haem c.