Cytochrome c oxidase

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Cytochrome c oxidase
Cytochrome C Oxidase 1OCC in Membrane 2.png
The crystal structure of bovine cytochrome c oxidase in a phospholipid bilayer. The intermembrane space lies to top of the image. Adapted from PDB 1OCC (It is a homo dimer in this structure)
EC number1.9.3.1
CAS number9001-16-5
IntEnzIntEnz view
ExPASyNiceZyme view
MetaCycmetabolic pathway
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / EGO
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Cytochrome c oxidase
Cytochrome C Oxidase 1OCC in Membrane 2.png
The crystal structure of bovine cytochrome c oxidase in a phospholipid bilayer. The intermembrane space lies to top of the image. Adapted from PDB 1OCC (It is a homo dimer in this structure)
EC number1.9.3.1
CAS number9001-16-5
IntEnzIntEnz view
ExPASyNiceZyme view
MetaCycmetabolic pathway
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / EGO

The enzyme cytochrome c oxidase or Complex IV, EC is a large transmembrane protein complex found in bacteria and the mitochondrion of eukaryotes.

It is the last enzyme in the respiratory electron transport chain of mitochondria (or bacteria) located in the mitochondrial (or bacterial) membrane. It receives an electron from each of four cytochrome c molecules, and transfers them to one oxygen molecule, converting molecular oxygen to two molecules of water. In the process, it binds four protons from the inner aqueous phase to make water, and in addition translocates four protons across the membrane, helping to establish a transmembrane difference of proton electrochemical potential that the ATP synthase then uses to synthesize ATP.


Subunit I and II of Complex IV excluding all other subunits, PDB 2EIK

The complex is a large integral membrane protein composed of several metal prosthetic sites and 14 [1] protein subunits in mammals. In mammals, eleven subunits are nuclear in origin, and three are synthesized in the mitochondria. The complex contains two hemes, a cytochrome a and cytochrome a3, and two copper centers, the CuA and CuB centers.[2] In fact, the cytochrome a3 and CuB form a binuclear center that is the site of oxygen reduction. Cytochrome c, which is reduced by the preceding component of the respiratory chain (cytochrome bc1 complex, complex III), docks near the CuA binuclear center and passes an electron to it, being oxidized back to cytochrome c containing Fe3+. The reduced CuA binuclear center now passes an electron on to cytochrome a, which in turn passes an electron on to the cytochrome a3-CuB binuclear center. The two metal ions in this binuclear center are 4.5 Å apart and coordinate a hydroxide ion in the fully oxidized state.

Crystallographic studies of cytochrome c oxidase show an unusual post-translational modification, linking C6 of Tyr(244) and the ε-N of His(240) (bovine enzyme numbering). It plays a vital role in enabling the cytochrome a3- CuB binuclear center to accept four electrons in reducing molecular oxygen to water. The mechanism of reduction was formerly thought to involve a peroxide intermediate, which was believed to lead to superoxide production. However, the currently accepted mechanism involves a rapid four-electron reduction involving immediate oxygen-oxygen bond cleavage, avoiding any intermediate likely to form superoxide.[3]


Site of assembly is believed to occur near TOM/TIM, where complex intermediates are accessible to bind with subunits imported from cytosol. Hemes and cofactors are inserted into subunits I & II. Subunits I and IV initiate assembly. Other subunits may form sub-complex intermediates that later bind to others to form COX complex. In post-assembly modifications, the enzyme is dimerized, which is required for active/efficient enzyme action. Dimers are connected by a cardiolipin molecule.[4][5]

Table of conserved subunits of cytochrome oxidase c complex[6][7][edit]

No.Subunit nameHuman proteinProtein description from UniProtPfam family with Human protein

1Cox1COX1_HUMANCytochrome c oxidase subunit 1Pfam PF00115
2Cox2COX2_HUMANCytochrome c oxidase subunit 2Pfam PF02790, Pfam PF00116
3Cox3COX3_HUMANCytochrome c oxidase subunit 3Pfam PF00510
4Cox4i1COX41_HUMANCytochrome c oxidase subunit 4 isoform 1, mitochondrialPfam PF02936
5Cox4a2COX42_HUMANCytochrome c oxidase subunit 4 isoform 2, mitochondrialPfam PF02936
6Cox5aCOX5A_HUMANCytochrome c oxidase subunit 5A, mitochondrialPfam PF02284
7Cox5bCOX5B_HUMANCytochrome c oxidase subunit 5B, mitochondrialPfam PF01215
8Cox6a1CX6A1_HUMANCytochrome c oxidase subunit 6A1, mitochondrialPfam PF02046
9Cox6a2CX6A2_HUMANCytochrome c oxidase subunit 6A2, mitochondrialPfam PF02046
10Cox6b1CX6B1_HUMANCytochrome c oxidase subunit 6B1Pfam PF02297
11Cox6b2CX6B2_HUMANCytochrome c oxidase subunit 6B2Pfam PF02297
12Cox6cCOX6C_HUMANCytochrome c oxidase subunit 6CPfam PF02937
13Cox7a1CX7A1_HUMANCytochrome c oxidase subunit 7A1, mitochondrialPfam PF02238
14Cox7a2CX7A2_HUMANCytochrome c oxidase subunit 7A2, mitochondrialPfam PF02238
15Cox7a3COX7S_HUMANPutative cytochrome c oxidase subunit 7A3, mitochondrialPfam PF02238
16Cox7bCOX7B_HUMANCytochrome c oxidase subunit 7B, mitochondrialPfam PF05392
17Cox7cCOX7C_HUMANCytochrome c oxidase subunit 7C, mitochondrialPfam PF02935
18Cox7rCOX7R_HUMANCytochrome c oxidase subunit 7A-related protein, mitochondrialPfam PF02238
19Cox8aCOX8A_HUMANCytochrome c oxidase subunit 8A, mitochondrial PPfam PF02285
20Cox8cCOX8C_HUMANCytochrome c oxidase subunit 8C, mitochondrialPfam PF02285
Assembly subunits[8][9][10]
1Coa1COA1_HUMANCytochrome c oxidase assembly factor 1 homologPfam PF08695
2Coa3COA3_HUMANCytochrome c oxidase assembly factor 3 homolog, mitochondrialPfam PF09813
3Coa4COA4_HUMANCytochrome c oxidase assembly factor 4 homolog, mitochondrialPfam PF06747
4Coa5COA5_HUMANCytochrome c oxidase assembly factor 5Pfam PF10203
5Coa6COA6_HUMANCytochrome c oxidase assembly factor 6 homologPfam PF02297
6Coa7COA7_HUMANCytochrome c oxidase assembly factor 7,Pfam PF08238
7Cox11COX11_HUMANCytochrome c oxidase assembly protein COX11 mitochondrialPfam PF04442
8Cox14COX14_HUMANCytochrome c oxidase assembly proteinPfam PF14880
9Cox15COX15_HUMANCytochrome c oxidase assembly protein COX15 homologPfam PF02628
10Cox16COX16_HUMANCytochrome c oxidase assembly protein COX16 homolog mitochondrialPfam PF14138
11Cox17COX17_HUMANCytochrome c oxidase copper chaperonePfam PF05051
12Cox18[11]COX18_HUMANMitochondrial inner membrane protein (Cytochrome c oxidase assembly protein 18)Pfam PF02096
13Cox19COX19_HUMANCytochrome c oxidase assembly proteinPfam PF06747
14Cox20COX20_HUMANCytochrome c oxidase protein 20 homologPfam PF12597


Summary reaction:

4 Fe2+-cytochrome c + 8 H+in + O2 → 4 Fe3+-cytochrome c + 2 H2O + 4 H+out

Two electrons are passed from two cytochrome c's, through the CuA and cytochrome a sites to the cytochrome a3- CuB binuclear center, reducing the metals to the Fe2+ form and Cu+. The hydroxide ligand is protonated and lost as water, creating a void between the metals that is filled by O2. The oxygen is rapidly reduced, with two electrons coming from the Fe2+cytochrome a3, which is converted to the ferryl oxo form (Fe4+=O). The oxygen atom close to CuB picks up one electron from Cu+, and a second electron and a proton from the hydroxyl of Tyr(244), which becomes a tyrosyl radical: The second oxygen is converted to a hydroxide ion by picking up two electrons and a proton. A third electron arising from another cytochrome c is passed through the first two electron carriers to the cytochrome a3- CuB binuclear center, and this electron and two protons convert the tyrosyl radical back to Tyr, and the hydroxide bound to CuB2+ to a water molecule. The fourth electron from another cytochrome c flows through CuA and cytochrome a to the cytochrome a3- CuB binuclear center, reducing the Fe4+=O to Fe3+, with the oxygen atom picking up a proton simultaneously, regenerating this oxygen as a hydroxide ion coordinated in the middle of the cytochrome a3- CuB center as it was at the start of this cycle. The net process is that four reduced cytochrome c's are used, along with 4 protons, to reduce O2 to two water molecules.


Cyanide, sulfide, azide, and carbon monoxide[12] all bind to cytochrome c oxidase, thus competitively inhibiting the protein from functioning, which results in chemical asphyxiation of cells. Methanol in methylated spirits is converted into formic acid, which also inhibits the same oxidase system. High levels of ATP can allosterically inhibit cytochrome c oxidase, binding from within the mitochondrial matrix.[13]

Subcellular Localization and Presence at Extramitochondrial Sites[edit]

Cytochrome c oxidase has 3 subunits which are encoded by mitochondrial DNA. Of these 3 subunits encoded by mitochondrial DNA, two have been identified in extramitochondrial locations. In pancreatic acinar tissue, these subunits were found in zymogen granules. Additionally, in the anterior pituitary, relatively high amounts of these subunits were found in growth hormone secretory granules.[14] The extramitochondrial function of these cytochrome c oxidase subunits has not yet been characterized. Besides cytochrome c oxidase subunits, extramitochondrial localization has also been observed for large numbers of other mitochondrial proteins.,[15][16] This raises the possibility about existence of yet unidentified specific mechanisms for protein translocation from mitochondria to other cellular destinations.[14][16][17]

Genetic defects and disorders[edit]

Defects involving genetic mutations altering cytochrome c oxidase (COX) functionality or structure can result in severe, often fatal metabolic disorders. Such disorders usually manifest in early childhood and affect predominantly tissues with high energy demands (brain, heart, muscle). Among the many classified mitochondrial diseases, those involving dysfunctional COX assembly are thought to be the most severe.[18]

The vast majority of COX disorders are linked to mutations in nuclear-encoded proteins referred to as assembly factors, or assembly proteins. These assembly factors contribute to COX structure and functionality, and are involved in several essential processes, including transcription and translation of mitochondrion-encoded subunits, processing of preproteins and membrane insertion, and cofactor biosynthesis and incorporation.[19]

Currently, mutations have been identified in seven COX assembly factors: SURF1, SCO1, SCO2, COX10, COX15, COX20, COA5 and LRPPRC. Mutations in these proteins can result in altered functionality of sub-complex assembly, copper transport, or translational regulation. Each gene mutation is associated with the etiology of a specific disease, with some having implications in multiple disorders. Disorders involving dysfunctional COX assembly via gene mutations include Leigh syndrome, cardiomyopathy, leukodystrophy, anemia, and sensorineural deafness.


COX histochemistry is used for mapping regional brain metabolism in animals, since there is a direct relation between enzyme activity and neuronal activity.[20] Such brain mapping has been accomplished in spontaneous mutant mice with cerebellar disease such as reeler[21] and a transgenic model of Alzheimer's disease.[22] This technique has also been used to map learning activity in animal brain.[23]

Additional images[edit]

See also[edit]


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  3. ^ Voet, Donald (2010). Biochemistry. New York: J. Wiley & Sons. pp. 865–866. ISBN 0-470-57095-4. 
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  8. ^ Szklarczyk R, Wanschers BF, Cuypers TD, Esseling JJ, Riemersma M, van den Brand MA et al. (2012). "Iterative orthology prediction uncovers new mitochondrial proteins and identifies C12orf62 as the human ortholog of COX14, a protein involved in the assembly of cytochrome c oxidase.". Genome Biol 13 (2): R12. doi:10.1186/gb-2012-13-2-r12. PMC 3334569. PMID 22356826. 
  9. ^ Mick DU, Dennerlein S, Wiese H, Reinhold R, Pacheu-Grau D, Lorenzi I et al. (2012). "MITRAC links mitochondrial protein translocation to respiratory-chain assembly and translational regulation.". Cell 151 (7): 1528–41. doi:10.1016/j.cell.2012.11.053. PMID 23260140. 
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External links[edit]