IUBMB Enzyme Nomenclature


Accepted name: cytochrome-c oxidase

Reaction: 4 ferrocytochrome c + O2 + 8 H+[side 1] = 4 ferricytochrome c + 2 H2O + 4 H+[side 2]

For diagram of reaction click here.

Other name(s): cytochrome aa3; cytochrome caa3; cytochrome bb3; cytochrome cbb3; cytochrome ba3; cytochrome a3; Warburg's respiratory enzyme; indophenol oxidase; indophenolase; complex IV (mitochondrial electron transport); ferrocytochrome c oxidase; cytochrome oxidase (ambiguous); NADH cytochrome c oxidase (incorrect)

Systematic name: ferrocytochrome-c:oxygen oxidoreductase

Comments: An oligomeric membrane heme-Cu:O2 reductase-type enzyme that terminates the respiratory chains of aerobic and facultative aerobic organisms. The reduction of O2 to water is accompanied by the extrusion of four protons. The cytochrome-aa3 enzymes of mitochondria and many bacterial species are the most abundant group, but other variations, such as the bacterial cytochrome-cbb3 enzymes, also exist. All of the variants have a conserved catalytic core subunit (subunit I) that contains a low-spin heme (of a- or b-type), a binuclear metal centre composed of a high-spin heme iron (of a-, o-, or b-type heme, referred to as a3, o3 or b3 heme), and a Cu atom (CuB). Besides subunit I, the enzyme usually has at least two other core subunits: subunit II is the primary electron acceptor; subunit III usually does not contain any cofactors, but in the case of cbb3-type enzymes it is a diheme c-type cytochrome. While most bacterial enzymes consist of only 3-4 subunits, the mitochondrial enzyme is much more complex and contains 14 subunits.

Links to other databases: BRENDA, EXPASY, GTD, KEGG, MetaCyc, PDB, CAS registry number: 9001-16-5


1. Keilin, D. and Hartree, E.F. Cytochrome oxidase. Proc. R. Soc. Lond. B Biol. Sci. 125 (1938) 171-186.

2. Keilin, D. and Hartree, E.F. Cytochrome and cytochrome oxidase. Proc. R. Soc. Lond. B Biol. Sci. 127 (1939) 167-191.

3. Wainio, W.W., Eichel, B. and Gould, A. Ion and pH optimum for the oxidation of ferrocytochrome c by cytochrome c oxidase in air. J. Biol. Chem. 235 (1960) 1521-1525.

4. Yonetani, T. Studies on cytochrome oxidase. II. Steady state properties. J. Biol. Chem. 235 (1960) 3138-3243. [PMID: 13787372]

5. Yonetani, T. Studies on cytochrome oxidase. III. Improved purification and some properties. J. Biol. Chem. 236 (1961) 1680-1688. [PMID: 13787373]

6. Henning, W., Vo, L., Albanese, J. and Hill, B.C. High-yield purification of cytochrome aa3 and cytochrome caa3 oxidases from Bacillus subtilis plasma membranes. Biochem. J. 309 (1995) 279-283. [PMID: 7619069]

7. Keightley, J.A., Zimmermann, B.H., Mather, M.W., Springer, P., Pastuszyn, A., Lawrence, D.M. and Fee, J.A. Molecular genetic and protein chemical characterization of the cytochrome ba3 from Thermus thermophilus HB8. J. Biol. Chem. 270 (1995) 20345-20358. [PMID: 7657607]

8. Ducluzeau, A.L., Ouchane, S. and Nitschke, W. The cbb3 oxidases are an ancient innovation of the domain bacteria. Mol. Biol. Evol. 25 (2008) 1158-1166. [PMID: 18353797]

[EC created 1961 as EC, modified 2000, transferred 2019 to EC, modified 2021]

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