InterPro domain: IPR036927

Cytochrome c oxidase ( 1.9.3.1 ) is a key enzyme in aerobic metabolism. Proton pumping haem-copper oxidases represent the terminal, energy-transfer enzymes of respiratory chains in prokaryotes and eukaryotes. The CuB-haem a3 (or haem o) binuclear centre, associated with the largest subunit I of cytochrome c and ubiquinol oxidases ( 1.10.3.11 ), is directly involved in the coupling between dioxygen reduction and proton pumping [ 1 , 2 ].Some terminal oxidases generate a transmembrane proton gradient across the plasma membrane (prokaryotes) or the mitochondrial inner membrane (eukaryotes).

The enzyme complex consists of 3-4 subunits (prokaryotes) up to 13 polypeptides (mammals) of which only the catalytic subunit (equivalent to mammalian subunit I (CO I) is found in all haem-copper respiratory oxidases. The presence of a bimetallic centre (formed by a high-spin haem and copper B) as well as a low-spin haem, both ligated to six conserved histidine residues near the outer side of four transmembrane spans within CO I is common to all family members [ 3 , 4 , 5 ]. In contrast to eukaryotes the respiratory chain of prokaryotes is branched to multiple terminal oxidases. The enzyme complexes vary in haem and copper composition, substrate type and substrate affinity. The different respiratory oxidases allow the cells to customize their respiratory systems according to a variety of environmental growth conditions [ 6 ].

It has been shown that eubacterial quinol oxidase was derived from cytochrome c oxidase in Gram-positive bacteria and that archaebacterial quinol oxidase has an independent origin. A considerable amount of evidence suggests that proteobacteria (Purple bacteria) acquired quinol oxidase through a lateral gene transfer from Gram-positive bacteria [ 6 ].

This entry represents a structural domain superfamily found in subunit I of cytochrome c oxidase as well as related proteins, including quinol oxidase. Structurally, it is composed of twelve transmembrane helices in an approximate threefold rotational symmetric arrangement.

1. The superfamily of heme-copper respiratory oxidases. J. Bacteriol. 176, 5587-600
2. The proton pump of heme-copper oxidases. Cell Biol. Int. 18, 345-55
3. Evolution of cytochrome oxidase, an enzyme older than atmospheric oxygen. EMBO J. 13, 2516-25
4. Structure of cytochrome c oxidase. Biochim. Biophys. Acta 726, 135-48
5. Structural models of the redox centres in cytochrome oxidase. EMBO J. 6, 2819-23