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Electron transport chain. Complex IV is at the right.

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)

The enzyme cytochrome c oxidase or Complex IV (PDB 2OCC, EC 1.9.3.1) is a large transmembrane protein complex found in bacteria and the mitochondrion.

It is the last protein in the electron transport chain. 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 translocates four protons, helping to establish a chemiosmotic potential that the ATP synthase then uses to synthesize ATP.

Summary reaction:

4 Fe²⁺-cytochrome c + 8 H⁺_in + O₂ → 4 Fe³⁺-cytochrome c + 2 H₂O + 4 H⁺_out

The complex is a large integral membrane protein composed of several metal prosthetic sites and 13 protein subunits in mammals. In mammals, ten 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 Cu_A and Cu_B centers^[1]. In fact, the cytochrome a₃ and Cu_B form a binuclear center that is the site of oxygen reduction. Reduced cytochrome c coming from complex III docks near the Cu_A binuclear center, passing an electron to it and being oxidized back to cytochrome c containing Fe⁺³. (This binuclear center has six amino acids coordinating the two Cu ions; it resembles a 2Fe-2S cluster). The reduced Cu_A binuclear center now passes an electron on to cytochrome a, which in turn passes an electron on to the cytochrome a₃- Cu_B 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- Cu_B 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 ^[2].

First two electrons are passed from two cytochrome c's, through the Cu_A and cytochrome a sites to the cytochrome a₃- Cu_B binuclear center, reducing the metals to the Fe⁺² form and Cu⁺¹. The hydroxide ligand is protonated and lost as water, creating a void between the metals which is filled by O₂. The oxygen is rapidly reduced, with two electrons coming from the Fe⁺²cytochrome a₃, which is converted to the ferryl oxo form (Fe⁺⁴=O). The oxygen atom close to Cu_B picks up one electron from Cu⁺¹, 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- Cu_B binuclear center, and this electron and two protons convert the tyrosyl radical back to Tyr, and the hydroxide bound to Cu_B⁺² to a water molecule. The fourth electron from another cytochrome c flows through Cu_A and cytochrome a to the cytochrome a₃- Cu_B binuclear center, reducing the Fe⁺⁴=O to Fe⁺³, with the oxygen atom picking up a proton simultaneously, regenerating this oxygen as a hydroxide ion coordinated in the middle of the cytochrome a₃- Cu_B 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^[3] all bind to cytochrome c oxidase, thus inhibiting the protein from functioning which results in chemical suffocation of cells.

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 predominantly affect 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^[4]

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. ^[5]

Currently, mutations have been identified in six COX assembly factors: SURF1, SCO1, SCO2, COX10, COX15, 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.

ETC	Complex IV

1. ^ Tsukihara T., Aoyama H., Yamashita E., Tomizaki T., Yamaguchi H., Shinzawa-Itoh K., Nakashima R., Yaono R., Yoshikawa S. (1995) Structures of metal sites of oxidized bovine heart cytochrome c oxidase at 2.8 Å. Science 269, 1069-1074
2. ^ Voet D., Voet JG (2004) Biochemistry, 3rd Edition. John Wiley & Sons, pps. 818-820
3. ^ Alonso J, Cardellach F, López S, Casademont J, Miró O (2003). "Carbon monoxide specifically inhibits cytochrome c oxidase of human mitochondrial respiratory chain". Pharmacol Toxicol 93 (3): 142-6. PMID 12969439. 
4. ^ Pecina, P., Houstkova, H., Hansikova, H., Zeman, J., Houstek, J. (2004). Genetic Defects of Cytocrhome c Oxidase Assembly. Physiol. Res. 53(Suppl. 1): S213-S223.
5. ^ Zee, J.M., and Glerum, D.M. (2006). Defects in cytochrome oxidase assembly in humans: lessons from yeast. Biochem. Cell Biol. 84: 859-869.

• The Cytochrome Oxidase home page at Rice University
• Interactive Molecular model of cytochrome c oxidase (Requires MDL Chime)
• UMich Orientation of Proteins in Membranes families/superfamily-4
• MeSH Cytochrome-c+Oxidase