Cytochrome c oxidase (E.C:7.1.1.9) 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 (E.C:1.10.3.11), is directly involved in the coupling between dioxygen reduction and proton pumping [1,2,3,4]. 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 (COXI) 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 [2,3].

1: The whole structure of the 13-subunit oxidized cytochrome c oxidase at 2.8 A. Tsukihara T, Aoyama H, Yamashita E, Tomizaki T, Yamaguchi H, Shinzawa-Itoh K, Nakashima R, Yaono R, Yoshikawa S; Science 1996;272:1136-1144. PMID:8638158

2: Evolution of cytochrome oxidase, an enzyme older than atmospheric oxygen. Castresana J, Lubben M, Saraste M, Higgins DG; EMBO J. 1994;13:2516-2525. PMID:8013452

3: Dimeric structures of quinol-dependent nitric oxide reductases (qNORs) revealed by cryo-electron microscopy. Gopalasingam CC, Johnson RM, Chiduza GN, Tosha T, Yamamoto M, Shiro Y, Antonyuk SV, Muench SP, Hasnain SS; Sci Adv. 2019;5:eaax1803. PMID:31489376

4: The proton pump of heme-copper oxidases. Papa S, Capitanio N, Glaser P, Villani G; Cell Biol Int. 1994;18:345-355. PMID:8049679