Cytochrome c oxidase, subunit I domain Cytochrome c oxidase (<db_xref db="EC" dbkey="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 (<db_xref db="EC" dbkey="1.10.3"/>), is directly involved in the coupling between dioxygen reduction and proton pumping [<cite idref="PUB00002253"/>, <cite idref="PUB00006485"/>].Some terminal oxidases generate a transmembrane proton gradient across the plasma membrane (prokaryotes) or the mitochondrial inner membrane (eukaryotes). <p> 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 [<cite idref="PUB00001256"/>, <cite idref="PUB00000581"/>, <cite idref="PUB00001153"/>]. 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 [<cite idref="PUB00002253"/>]. </p><p>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 [<cite idref="PUB00002253"/>].</p><p>Nitric oxide reductase (NOR) (<db_xref db="EC" dbkey="1.7.99.7"/>) exists in denitrifying species of archae and eubacteria and is a heterodimer of cytochromes b and c. Phenazine methosulphate can act as acceptor.</p>This entry represents the structural domain of the cytochrome c oxidase subunit I.