View source for Cytochrome c oxidase

{{Short description|Aerobix respiration enzyme}}
{{infobox enzyme
| Name       = Cytochrome c oxidase
| EC_number  = 1.9.3.1
| CAS_number = 9001-16-5
| GO_code    = 0009485
| image      = Cytochrome C Oxidase 1OCC in Membrane 2.png
| width      = 296px
| caption    = 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 homodimer in this structure)
| name       = Cytochrome c oxidase
}}
{{Infobox protein family
| Symbol = Cytochrome c oxidase
| Name = Cytochrome c oxidase
| image = Cmplx4.PNG
| width = 296px
| caption = Subunit I and II of Complex IV excluding all other subunits, {{PDB|2EIK}}
| InterPro= 
| SMART=
| PROSITE = 
| SCOP = 
| TCDB =
| OPM family= 4
| OPM protein= 2dyr
| Pfam= 
| PDB=
| Membranome superfamily= 257
}}
The [[enzyme]] '''cytochrome c oxidase''' or '''Complex IV''' (was {{EC number|1.9.3.1}}, now reclassified as a translocase {{EC number|7.1.1.9}}) is a large [[transmembrane protein]] complex found in [[bacteria]], [[archaea]], and the [[mitochondria]] of [[eukaryotes]].<ref name="EMBOJ">{{cite journal | vauthors = Castresana J, Lübben M, Saraste M, Higgins DG | title = Evolution of cytochrome oxidase, an enzyme older than atmospheric oxygen | language = en | journal = The EMBO Journal | volume = 13 | issue = 11 | pages = 2516–2525 | date = June 1994 | pmid = 8013452 | pmc = 395125 | doi=10.1002/j.1460-2075.1994.tb06541.x}}</ref>

It is the last enzyme in the [[Cellular respiration|respiratory]] [[electron transport chain]] of [[cell (biology)|cells]] located in the [[membrane]]. It receives an electron from each of four [[cytochrome c]] proteins and transfers them to one [[oxygen]] molecule and four [[proton]]s, producing two molecules of water. In addition to binding the four protons from the inner aqueous phase, it transports another four protons across the membrane, increasing the transmembrane difference of proton [[electrochemical potential]], which the [[ATP synthase]] then uses to synthesize [[Adenosine triphosphate|ATP]].

== Structure ==

=== The complex ===
The complex is a large [[integral membrane protein]] composed of several [[Cofactor (biochemistry)#Metal ions|metal prosthetic sites]] and 13<ref name="pmid22902835">{{cite journal |vauthors=Kadenbach B, Huttemann M |date=September 2015 |title=The subunit composition and function of mammalian cytochrome c oxidase |journal=Mitochondrion |volume=24 |issue= |pages=64–76 |doi=10.1016/j.mito.2015.07.002 |pmid=26190566 |doi-access=free}}</ref> protein subunits in mammals. In mammals, ten subunits are nuclear in origin, and three are synthesized in the mitochondria. The complex contains two [[heme]]s, a [[cytochrome a]] and [[cytochrome a|cytochrome a{{sub|3}}]], and two copper centers, the Cu{{sub|A}} and Cu{{sub|B}} centers.<ref name="pmid7652554">{{cite journal | vauthors = Tsukihara T, Aoyama H, Yamashita E, Tomizaki T, Yamaguchi H, Shinzawa-Itoh K, Nakashima R, Yaono R, Yoshikawa S | title = Structures of metal sites of oxidized bovine heart cytochrome c oxidase at 2.8 A | journal = Science | volume = 269 | issue = 5227 | pages = 1069–74 | date = August 1995 | pmid = 7652554 | doi = 10.1126/science.7652554 | bibcode = 1995Sci...269.1069T | s2cid = 27210776 }}</ref> In fact, the cytochrome a{{sub|3}} and Cu{{sub|B}} 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 Cu{{sub|A}} binuclear center and passes an electron to it, being oxidized back to cytochrome c containing Fe{{sup|3+}}. The reduced Cu{{sub|A}} binuclear center now passes an electron on to cytochrome a, which in turn passes an electron on to the cytochrome a{{sub|3}}>-Cu{{sub|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.

[[X-ray crystallography|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 a{{sub|3}}- Cu{{sub|B}} binuclear center to accept four electrons in reducing molecular oxygen and four protons 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{{ndash}}oxygen bond cleavage, avoiding any intermediate likely to form superoxide.<ref name = "Voest_2011" />{{rp|865–866}}

=== The conserved subunits ===
{| border=1 class="wikitable" style="text-align:center"
|+ Table of conserved subunits of cytochrome c oxidase complex<ref name="pmid9565029">{{cite journal | vauthors = Zhang Z, Huang L, Shulmeister VM, Chi YI, Kim KK, Hung LW, Crofts AR, Berry EA, Kim SH | title = Electron transfer by domain movement in cytochrome bc1 | journal = Nature | volume = 392 | issue = 6677 | pages = 677–84 | date = April 1998 | pmid = 9565029 | doi = 10.1038/33612 | bibcode = 1998Natur.392..677Z | s2cid = 4380033 }}</ref><ref name="pmid21211513">{{cite journal | vauthors = Kaila VR, Oksanen E, Goldman A, Bloch DA, Verkhovsky MI, Sundholm D, Wikström M | title = A combined quantum chemical and crystallographic study on the oxidized binuclear center of cytochrome c oxidase | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1807 | issue = 7 | pages = 769–78 | date = July 2011 | pmid = 21211513 | doi = 10.1016/j.bbabio.2010.12.016 | doi-access = free }}</ref>
!'''No.'''
!Subunit name
![[Human]] protein
!Protein description from [[UniProt]]
![[Pfam]] family with Human protein
|-
| 1 || Cox1 || [[MT-CO1|COX1_HUMAN]] || Cytochrome c oxidase subunit 1  || {{Pfam|PF00115}}
|-
| 2 || Cox2 || [[MT-CO2|COX2_HUMAN]] || Cytochrome c oxidase subunit 2 || {{Pfam|PF02790}}, {{Pfam|PF00116}}
|-
| 3 || Cox3 || [[MT-CO3|COX3_HUMAN]] || Cytochrome c oxidase subunit 3 || {{Pfam|PF00510}}
|-
| 4 || Cox4i1 || [[COX4I1|COX41_HUMAN]] || Cytochrome c oxidase subunit 4 isoform 1, mitochondrial || {{Pfam|PF02936}}
|-
| 5 || Cox4a2  || [[COX4I2|COX42_HUMAN]] || Cytochrome c oxidase subunit 4 isoform 2, mitochondrial || {{Pfam|PF02936}}
|-
| 6 || Cox5a || [[COX5A| COX5A_HUMAN ]] || Cytochrome c oxidase subunit 5A, mitochondrial || {{Pfam|PF02284}}
|-
| 7 || Cox5b || [[COX5B| COX5B_HUMAN ]] || Cytochrome c oxidase subunit 5B, mitochondrial || {{Pfam|PF01215}}
|-
| 8 || Cox6a1  || [[COX6A1|CX6A1_HUMAN]] ||  Cytochrome c oxidase subunit 6A1, mitochondrial || {{Pfam|PF02046}}
|-
| 9 || Cox6a2  || [[COX6A2|CX6A2_HUMAN]] ||   Cytochrome c oxidase subunit 6A2, mitochondrial || {{Pfam|PF02046}}
|-
| 10 || Cox6b1  || [[COX6B1|CX6B1_HUMAN]] ||   Cytochrome c oxidase subunit 6B1   || {{Pfam|PF02297}}
|-
| 11 || Cox6b2  || [[COX6B2|CX6B2_HUMAN]] ||  Cytochrome c oxidase subunit 6B2 || {{Pfam|PF02297}}
|-
| 12 || Cox6c  || [[COX6C| COX6C_HUMAN]]|| Cytochrome c oxidase subunit 6C || {{Pfam|PF02937}}
|-
| 13 || Cox7a1  || [[COX7A1|CX7A1_HUMAN]] ||  Cytochrome c oxidase subunit 7A1, mitochondrial || {{Pfam|PF02238}}
|-
| 14 || Cox7a2  || [[COX7A2|CX7A2_HUMAN]] ||  Cytochrome c oxidase subunit 7A2, mitochondrial || {{Pfam|PF02238}}
|-
| 15 || Cox7a3  || [[COX7A2P2|COX7S_HUMAN]] ||  Putative cytochrome c oxidase subunit 7A3, mitochondrial || {{Pfam|PF02238}}
|-
| 16 ||  Cox7b  || [[COX7B| COX7B_HUMAN]]|| Cytochrome c oxidase subunit 7B, mitochondrial || {{Pfam|PF05392}}
|-
| 17 ||  Cox7c  || [[COX7C| COX7C_HUMAN]]|| Cytochrome c oxidase subunit 7C, mitochondrial || {{Pfam|PF02935}}
|-
| 18 || Cox7r  || [[COX7A2L|COX7R_HUMAN]] ||   Cytochrome c oxidase subunit 7A-related protein, mitochondrial || {{Pfam|PF02238}}
|-
| 19 || Cox8a  || [[COX8A| COX8A_HUMAN]]|| Cytochrome c oxidase subunit 8A, mitochondrial P || {{Pfam|PF02285}}
|-
| 20 ||  Cox8c || [[COX8C| COX8C_HUMAN]]|| Cytochrome c oxidase subunit 8C, mitochondrial ||  {{Pfam|PF02285}}
|-
|-
|'''Assembly subunits'''<ref name="pmid22356826">{{cite journal | vauthors = Szklarczyk R, Wanschers BF, Cuypers TD, Esseling JJ, Riemersma M, van den Brand MA, Gloerich J, Lasonder E, van den Heuvel LP, Nijtmans LG, Huynen MA | title = 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 | journal = Genome Biology | volume = 13 | issue = 2 | article-number = R12 | date = February 2012 | pmid = 22356826 | pmc = 3334569 | doi = 10.1186/gb-2012-13-2-r12 | doi-access = free }}</ref><ref name="pmid23260140">{{cite journal | vauthors = Mick DU, Dennerlein S, Wiese H, Reinhold R, Pacheu-Grau D, Lorenzi I, Sasarman F, Weraarpachai W, Shoubridge EA, Warscheid B, Rehling P | title = MITRAC links mitochondrial protein translocation to respiratory-chain assembly and translational regulation | journal = Cell | volume = 151 | issue = 7 | pages = 1528–41 | date = December 2012 | pmid = 23260140 | doi = 10.1016/j.cell.2012.11.053 | doi-access = free | hdl = 11858/00-001M-0000-000E-DDDF-4 | hdl-access = free }}</ref><ref name="pmid24333015">{{cite journal | vauthors = Kozjak-Pavlovic V, Prell F, Thiede B, Götz M, Wosiek D, Ott C, Rudel T | title = C1orf163/RESA1 is a novel mitochondrial intermembrane space protein connected to respiratory chain assembly | journal = Journal of Molecular Biology | volume = 426 | issue = 4 | pages = 908–20 | date = February 2014 | pmid = 24333015 | doi = 10.1016/j.jmb.2013.12.001 }}</ref>
|-
|-
| 1 || Coa1 ||  [[COA1| COA1_HUMAN ]] || Cytochrome c oxidase assembly factor 1 homolog  ||  {{Pfam|PF08695}}
|-
| 2 || Coa3 ||  [[COA3| COA3_HUMAN ]] || Cytochrome c oxidase assembly factor 3 homolog, mitochondrial  ||  {{Pfam|PF09813}}
|-
| 3 || Coa4 ||  [[COA4| COA4_HUMAN ]] || Cytochrome c oxidase assembly factor 4 homolog, mitochondrial  ||  {{Pfam|PF06747}}
|-
| 4 || Coa5 ||  [[COA5| COA5_HUMAN ]] || Cytochrome c oxidase assembly factor 5 ||  {{Pfam|PF10203}}
|-
| 5 || Coa6 ||  [[COA6| COA6_HUMAN ]] || Cytochrome c oxidase assembly factor 6 homolog    ||  {{Pfam|PF02297}}
|-
| 6 || Coa7 ||  [[COA7| COA7_HUMAN ]] || Cytochrome c oxidase assembly factor 7, ||  {{Pfam|PF08238}}
|-
| 7 || Cox11 ||  [[COX11| COX11_HUMAN ]] || Cytochrome c oxidase assembly protein COX11 mitochondrial ||  {{Pfam|PF04442}}
|-
| 8 || Cox14 ||  [[COX14| COX14_HUMAN ]] || Cytochrome c oxidase assembly protein ||  {{Pfam|PF14880}}
|-
| 9 || Cox15 ||  [[COX15| COX15_HUMAN ]] || Cytochrome c oxidase assembly protein COX15 homolog ||  {{Pfam|PF02628}}
|-
| 10 || Cox16 ||  [[COX16| COX16_HUMAN ]] || Cytochrome c oxidase assembly protein COX16 homolog mitochondrial ||  {{Pfam|PF14138}}
|-
| 11 || Cox17 ||  [[COX17| COX17_HUMAN ]] || Cytochrome c oxidase copper chaperone ||  {{Pfam|PF05051}}
|-
| 12 || Cox18<ref name="pmid16911509">{{cite journal | vauthors = Gaisne M, Bonnefoy N | title = The COX18 gene, involved in mitochondrial biogenesis, is functionally conserved and tightly regulated in humans and fission yeast | journal = FEMS Yeast Research | volume = 6 | issue = 6 | pages = 869–82 | date = September 2006 | pmid = 16911509 | doi = 10.1111/j.1567-1364.2006.00083.x | doi-access = free }}</ref> ||  [[COX18| COX18_HUMAN ]] || Mitochondrial inner membrane protein (Cytochrome c oxidase assembly protein 18) ||  {{Pfam|PF02096}}
|-
| 13 || Cox19 ||  [[COX19| COX19_HUMAN ]] || Cytochrome c oxidase assembly protein ||  {{Pfam|PF06747}}
|-
| 14 || Cox20 ||  [[COX20| COX20_HUMAN ]] || Cytochrome c oxidase protein 20 homolog ||  {{Pfam|PF12597}}
|-
|}

== Assembly ==

COX assembly in [[S. cerevisiae|yeast]] are a complex process that is not entirely understood due to the rapid and irreversible aggregation of hydrophobic subunits that form the holoenzyme complex, as well as aggregation of mutant subunits with exposed hydrophobic patches.<ref name = "pmid16760263"/> COX subunits are encoded in both the nuclear and mitochondrial genomes. The three subunits that form the COX catalytic core are encoded in the mitochondrial genome. Over 30 different nuclear-encoded chaperone proteins are required for COX assembly.<ref>{{Cite journal |last1=Dickinson |first1=Elizabeth K. |last2=Adams |first2=Denise L. |last3=Schon |first3=Eric A. |last4=Glerum |first4=D. Moira |date=September 2000 |title=A Human SCO2 Mutation Helps Define the Role of Sco1p in the Cytochrome Oxidase Assembly Pathway |journal=Journal of Biological Chemistry |language=en |volume=275 |issue=35 |pages=26780–26785 |doi=10.1016/S0021-9258(19)61443-2|doi-access=free |pmid=10854440 }}</ref>

Cofactors, including hemes, are inserted into subunits I & II. The two heme molecules reside in subunit I, helping with transport to subunit II where two copper molecules aid with the continued transfer of electrons.<ref>{{cite web | first = Antony | last = Crofts | name-list-style = vanc | date = 1996 | title = Cytochrome oxidase: Complex IV | publisher = University of Illinois at Urbana-Champaign | url = http://www.life.illinois.edu/crofts/bioph354/cyt_ox.html | access-date = 2018-01-28 | archive-date = 2018-01-23 | archive-url = https://web.archive.org/web/20180123023311/http://www.life.illinois.edu/crofts/bioph354/cyt_ox.html | url-status = live }}</ref> Subunits I and IV initiate assembly. Different subunits may associate to form sub-complex intermediates that later bind to other subunits to form the COX complex.<ref name = "pmid16760263"/> In post-assembly modifications, COX will form a homodimer. This is required for activity. Dimers are connected by a [[cardiolipin]] molecule,<ref name="pmid16760263">{{cite journal | vauthors = Fontanesi F, Soto IC, Horn D, Barrientos A | title = Assembly of mitochondrial cytochrome c-oxidase, a complicated and highly regulated cellular process | journal = American Journal of Physiology. Cell Physiology | volume = 291 | issue = 6 | pages = C1129-47 | date = December 2006 | pmid = 16760263 | doi = 10.1152/ajpcell.00233.2006 }}</ref><ref name="pmid16199211">{{cite journal | vauthors = Khalimonchuk O, Rödel G | title = Biogenesis of cytochrome c oxidase | journal = Mitochondrion | volume = 5 | issue = 6 | pages = 363–88 | date = December 2005 | pmid = 16199211 | doi = 10.1016/j.mito.2005.08.002 }}</ref><ref name = "pmid26284624">{{cite journal | vauthors = Sedlák E, Robinson NC | title = Destabilization of the Quaternary Structure of Bovine Heart Cytochrome c Oxidase upon Removal of Tightly Bound Cardiolipin | journal = Biochemistry | volume = 54 | issue = 36 | pages = 5569–77 | date = September 2015 | pmid = 26284624 | doi = 10.1021/acs.biochem.5b00540 }}</ref> which has been found to play a key role in stabilization of the holoenzyme complex. The dissociation of subunits VIIa and III in conjunction with the removal of cardiolipin results in total loss of enzyme activity.<ref name = "pmid26284624"/> Subunits encoded in the nuclear genome are known to play a role in enzyme dimerization and stability. Mutations to these subunits eliminate COX function.<ref name = "pmid16760263"/>

Assembly is known to occur in at least three distinct rate-determining steps. The products of these steps have been found, though specific subunit compositions have not been determined.<ref name = "pmid16760263"/>

Synthesis and assembly of COX subunits I, II, and III are facilitated by translational activators, which interact with the 5’ untranslated regions of mitochondrial mRNA transcripts. Translational activators are encoded in the nucleus. They can operate through either direct or indirect interaction with other components of translation machinery, but exact molecular mechanisms are unclear due to difficulties associated with synthesizing translation machinery in-vitro.<ref name = "pmid22450032">{{cite journal | vauthors = Herrmann JM, Woellhaf MW, Bonnefoy N | title = Control of protein synthesis in yeast mitochondria: the concept of translational activators | journal = Biochimica et Biophysica Acta (BBA) - Molecular Cell Research | volume = 1833 | issue = 2 | pages = 286–94 | date = February 2013 | pmid = 22450032 | doi = 10.1016/j.bbamcr.2012.03.007 | doi-access = free }}</ref><ref name = "pmid21958598">{{cite journal | vauthors = Soto IC, Fontanesi F, Liu J, Barrientos A | title = Biogenesis and assembly of eukaryotic cytochrome c oxidase catalytic core | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1817 | issue = 6 | pages = 883–97 | date = June 2012 | pmid = 21958598 | pmc = 3262112 | doi = 10.1016/j.bbabio.2011.09.005 }}</ref> Though the interactions between subunits I, II, and III encoded within the mitochondrial genome make a lesser contribution to enzyme stability than interactions between bigenomic subunits, these subunits are more conserved, indicating potential unexplored roles for enzyme activity.<ref name = "pmid4255772">{{cite journal | vauthors = Aledo JC, Valverde H, Ruíz-Camacho M, Morilla I, López FD | title = Protein-protein interfaces from cytochrome c oxidase I evolve faster than nonbinding surfaces, yet negative selection is the driving force | journal = Genome Biology and Evolution | volume = 6 | issue = 11 | pages = 3064–76 | date = October 2014 | pmid = 25359921 | pmc = 4255772 | doi = 10.1093/gbe/evu240 }}</ref>

== Biochemistry ==
{{missing information|section|names of the six traditional intermediate states (APFOER); 2021 Cyro-EM result proposing an RPFOE mechanism with reversed assignment of red-ox phases ({{doi|10.1038/s41467-021-27174-y}} {{open access}})|date=December 2021}}
The overall reaction is
: 4 Fe{{sup|2+}} – cytochrome ''c'' + 4 H{{sup|+}} + O{{sub|2}} → 4 Fe{{sup|3+}} – cytochrome ''c'' + 2 H{{sub|2}}O                Δ''G''{{sup|o}}' = - 218 kJ/mol,  ''E''{{sup|o}}' = +565 mV

Two electrons are passed from two cytochrome c's, through the Cu{{sub|A}} and cytochrome a sites to the cytochrome a{{sub|3}}–Cu{{sub|B}} binuclear center, reducing the metals to the Fe{{sup|2+}} form and Cu{{sup|+}}. The hydroxide ligand is protonated and lost as water, creating a void between the metals that is filled by O{{sub|2}}. The oxygen is rapidly reduced, with two electrons coming from the Fe{{sup|2+}}-cytochrome a{{sub|3}}, which is converted to the ferryl oxo form (Fe{{sup|4+}}=O). The oxygen atom close to Cu{{sub|B}} picks up one electron from Cu{{sup|+}}, 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 from another cytochrome c is passed through the first two electron carriers to the cytochrome a{{sub|3}}–Cu{{sub|B}} binuclear center, and this electron and two protons convert the tyrosyl radical back to Tyr, and the hydroxide bound to Cu{{sub|B}}{{sup|2+}} to a water molecule. The fourth electron from another cytochrome c flows through Cu{{sub|A}} and cytochrome a to the cytochrome a{{sub|3}}–Cu{{sub|B}} binuclear center, reducing the Fe{{sup|4+}}=O to Fe{{sup|3+}}, with the oxygen atom picking up a proton simultaneously, regenerating this oxygen as a hydroxide ion coordinated in the middle of the cytochrome a{{sub|3}}–Cu{{sub|B}} center as it was at the start of this cycle. Overall, four reduced cytochrome c's are oxidized while O{{sub|2}} and four protons are reduced to two water molecules.<ref name = "Voest_2011">{{cite book |last1=Voet |first1=Donald |last2=Voet |first2=Judith G.  | name-list-style = vanc |title=Biochemistry |date=2011 |publisher=John Wiley & Sons |location=Hoboken, NJ |isbn=978-0-470-57095-1 |edition=4th}}</ref>{{rp|841–5}}

==Inhibition==

COX exists in three conformational states: fully oxidized (pulsed), partially reduced, and fully reduced. Each inhibitor has a high affinity to a different state. In the pulsed state, both the heme a{{sub|3}} and the Cu{{sub|B}} nuclear centers are oxidized; this is the conformation of the enzyme that has the highest activity. A two-electron reduction initiates a conformational change that allows oxygen to bind at the active site to the partially-reduced enzyme. Four electrons bind to COX to fully reduce the enzyme. Its fully reduced state, which consists of a reduced Fe{{sup|2+}} at the cytochrome a{{sub|3}} heme group and a reduced Cu{{sub|B}}{{sup|+}} binuclear center, is considered the inactive or resting state of the enzyme.<ref name="pmid17906319">{{cite journal | vauthors = Leavesley HB, Li L, Prabhakaran K, Borowitz JL, Isom GE | title = Interaction of cyanide and nitric oxide with cytochrome c oxidase: implications for acute cyanide toxicity | journal = Toxicological Sciences | volume = 101 | issue = 1 | pages = 101–11 | date = January 2008 | pmid = 17906319 | doi = 10.1093/toxsci/kfm254 | doi-access = free }}</ref>

[[Cyanide]], [[azide]], and [[carbon monoxide]]<ref name="pmid12969439">{{cite journal | vauthors = Alonso JR, Cardellach F, López S, Casademont J, Miró O | title = Carbon monoxide specifically inhibits cytochrome c oxidase of human mitochondrial respiratory chain | journal = Pharmacology & Toxicology | volume = 93 | issue = 3 | pages = 142–6 | date = September 2003 | pmid = 12969439 | doi = 10.1034/j.1600-0773.2003.930306.x | doi-access = free }}</ref> all bind to cytochrome c oxidase, inhibiting the protein from functioning and leading to the chemical [[asphyxiation]] of cells. Higher concentrations of molecular oxygen are needed to compensate for increasing inhibitor concentrations, leading to an overall decrease in metabolic activity in the cell in the presence of an inhibitor. Other ligands, such as nitric oxide and hydrogen sulfide, can also inhibit COX by binding to regulatory sites on the enzyme, reducing the rate of cellular respiration.<ref name = "pmid24059525">{{cite journal | vauthors = Nicholls P, Marshall DC, Cooper CE, Wilson MT | s2cid = 11554252 | title = Sulfide inhibition of and metabolism by cytochrome c oxidase | journal = Biochemical Society Transactions | volume = 41 | issue = 5 | pages = 1312–6 | date = October 2013 | pmid = 24059525 | doi = 10.1042/BST20130070 }}</ref>

Cyanide is a non-competitive inhibitor for COX,<ref>{{Cite book|url=https://books.google.com/books?id=HHaDGynAz1EC&q=cyanide+cytochrome+competitive&pg=PA130|title=Advanced Biology|last1=Roberts|first1=Michael|last2=Reiss|first2=Michael Jonathan|last3=Monger|first3=Grace|name-list-style=vanc|date=2000|publisher=Nelson Thornes|isbn=9780174387329|language=en|access-date=2020-10-25|archive-date=2022-02-24|archive-url=https://web.archive.org/web/20220224105346/https://books.google.com/books?id=HHaDGynAz1EC&q=cyanide+cytochrome+competitive&pg=PA130|url-status=live}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=ASADBUVAiDUC&q=cyanide+cytochrome+competitive&pg=PA92|title=Biology: A Functional Approach|vauthors=Roberts MB|date=1986|publisher=Nelson Thornes|isbn=9780174480198|language=en|access-date=2020-10-25|archive-date=2022-02-24|archive-url=https://web.archive.org/web/20220224105345/https://books.google.com/books?id=ASADBUVAiDUC&q=cyanide+cytochrome+competitive&pg=PA92|url-status=live}}</ref> binding with high affinity to the partially-reduced state of the enzyme and hindering further reduction of the enzyme. In the pulsed state, cyanide binds slowly, but with high affinity. The ligand is posited to electrostatically stabilize both metals at once by positioning itself between them. A high nitric oxide concentration, such as one added exogenously to the enzyme, reverses cyanide inhibition of COX.<ref name= "pmid1144519">{{cite journal | vauthors = Jensen P, Wilson MT, Aasa R, Malmström BG | title = Cyanide inhibition of cytochrome c oxidase. A rapid-freeze e.p.r. investigation | journal = The Biochemical Journal | volume = 224 | issue = 3 | pages = 829–37 | date = December 1984 | pmid = 6098268 | pmc = 1144519 | doi = 10.1042/bj2240829 }}</ref>

[[Nitric oxide]] can reversibly<ref name= "pmid19461104">{{cite journal | vauthors = Gladwin MT, Shiva S | title = The ligand binding battle at cytochrome c oxidase: how NO regulates oxygen gradients in tissue | journal = Circulation Research | volume = 104 | issue = 10 | pages = 1136–8 | date = May 2009 | pmid = 19461104 | doi = 10.1161/CIRCRESAHA.109.198911 | doi-access = free }}</ref> bind to either metal ion in the binuclear center to be oxidized to nitrite. NO and CN{{sup|−}} will compete with oxygen to bind at the site, reducing the rate of cellular respiration. Endogenous NO, however, which is produced at lower levels, augments CN{{sup|−}} inhibition. Higher levels of NO, which correlate with the existence of more enzyme in the reduced state, lead to a greater inhibition of cyanide.<ref name = "pmid17906319"/> At these basal concentrations, NO inhibition of Complex IV is known to have beneficial effects, such as increasing oxygen levels in blood vessel tissues. The inability of the enzyme to reduce oxygen to water results in a buildup of oxygen, which can diffuse deeper into surrounding tissues.<ref name = "pmid19461104"/> NO inhibition of Complex IV has a larger effect at lower oxygen concentrations, increasing its utility as a vasodilator in tissues of need.<ref name = "pmid19461104"/>

[[Hydrogen sulfide]] will bind COX in a noncompetitive fashion at a regulatory site on the enzyme, similar to carbon monoxide. Sulfide has the highest affinity to either the pulsed or partially reduced states of the enzyme, and is capable of partially reducing the enzyme at the heme a{{sub|3}} center. It is unclear whether endogenous H{{sub|2}}S levels are sufficient to inhibit the enzyme. There is no interaction between hydrogen sulfide and the fully reduced conformation of COX.<ref name = "pmid24059525"/>

[[Methanol]] in [[denatured alcohol|methylated spirits]] is converted into [[formic acid]], which also inhibits the same oxidase system. High levels of ATP can [[allosteric inhibition|allosterically]] inhibit cytochrome c oxidase, binding from within the mitochondrial matrix.<ref name="pmid9363790">{{cite journal |vauthors=Arnold S, Kadenbach B | title = Cell respiration s controlled by ATP, an allosteric inhibitor of cytochrome-c oxidase. | journal = Eur J Biochem | pages = 350–354| date = October 1997 | doi = 10.1111/j.1432-1033.1997.t01-1-00350.x | pmid = 9363790 | volume=249| issue = 1 | doi-access = free }}</ref>

==Extramitochondrial and subcellular localizations==

[[File:Map of the human mitochondrial genome.svg|thumb|320px|Location of the 3 cytochrome c oxidase subunit genes in the human mitochondrial genome: ''[[MT-CO1|COXI]]'', ''[[MT-CO2|COXII]]'', and ''[[MT-CO3|COXIII]]'' (orange boxes).]]

Cytochrome c oxidase has 3 subunits which are encoded by [[mitochondrial DNA]] (cytochrome c oxidase [[Cytochrome c oxidase subunit I|subunit I]], [[Cytochrome c oxidase subunit II|subunit II]], and [[Cytochrome c oxidase subunit III|subunit III]]). 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.<ref name="Sadacharan">{{cite journal | vauthors = Sadacharan SK, Singh B, Bowes T, Gupta RS | title = Localization of mitochondrial DNA encoded cytochrome c oxidase subunits I and II in rat pancreatic zymogen granules and pituitary growth hormone granules | journal = Histochemistry and Cell Biology | volume = 124 | issue = 5 | pages = 409–21 | date = November 2005 | pmid = 16133117 | doi = 10.1007/s00418-005-0056-2 | s2cid = 24440427 }}</ref> 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.<ref name="pmid18575266">{{cite book | vauthors = Gupta RS, Ramachandra NB, Bowes T, Singh B  | title = The Biology of Extracellular Molecular Chaperones | chapter = Unusual Cellular Disposition of the Mitochondrial Molecular Chaperones Hsp60, Hsp70 and Hsp10 | series = Novartis Foundation Symposia | volume = 291 | pages = 59–68; discussion 69–73, 137–40 | date = 2008 | pmid = 18575266 | doi = 10.1002/9780470754030.ch5  | isbn = 9780470754030 }}</ref><ref name="Soltysb">{{cite book | vauthors = Soltys BJ, Gupta RS | title = Mitochondrial Proteins at Unexpected Cellular Locations: Export of Proteins from Mitochondria from an Evolutionary Perspective | series = International Review of Cytology | volume = 194 | pages = 133–96 | year = 1999 | pmid = 10494626 | doi = 10.1016/S0074-7696(08)62396-7 | isbn = 9780123645982 }}</ref> This raises the possibility about existence of yet unidentified specific mechanisms for protein translocation from mitochondria to other cellular destinations.<ref name="Sadacharan" /><ref name="Soltysb" /><ref>{{cite journal | vauthors = Soltys BJ, Gupta RS | title = Mitochondrial-matrix proteins at unexpected locations: are they exported? | journal = Trends in Biochemical Sciences | volume = 24 | issue = 5 | pages = 174–7 | date = May 1999 | pmid = 10322429 | doi = 10.1016/s0968-0004(99)01390-0 }}</ref>

==Genetic defects and disorders==

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.<ref name="pmid15119951">{{cite journal | vauthors = Pecina P, Houstková H, Hansíková H, Zeman J, Houstek J | title = Genetic defects of cytochrome c oxidase assembly | journal = Physiological Research | volume = 53 | pages = S213-23 | year = 2004 | issue = Suppl 1 | doi = 10.33549/physiolres.930000.53.S213 | pmid = 15119951 | s2cid = 8119738 | url = http://www.biomed.cas.cz/physiolres/pdf/53%20Suppl%201/53_S213.pdf | access-date = 2010-11-17 | archive-date = 2011-07-18 | archive-url = https://web.archive.org/web/20110718170246/http://www.biomed.cas.cz/physiolres/pdf/53%20Suppl%201/53_S213.pdf | url-status = live }}</ref>

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.<ref name="pmid17215873">{{cite journal | vauthors = Zee JM, Glerum DM | title = Defects in cytochrome oxidase assembly in humans: lessons from yeast | journal = Biochemistry and Cell Biology | volume = 84 | issue = 6 | pages = 859–69 | date = December 2006 | pmid = 17215873 | doi = 10.1139/o06-201 | bibcode = 2006BCB....84..201Z }}</ref>

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]].

==Histochemistry==

The increased reliance of neurons on oxidative phosphorylation for energy<ref name = "pmid24032355">{{cite journal | vauthors = Johar K, Priya A, Dhar S, Liu Q, Wong-Riley MT | title = Neuron-specific specificity protein 4 bigenomically regulates the transcription of all mitochondria- and nucleus-encoded cytochrome c oxidase subunit genes in neurons | journal = Journal of Neurochemistry | volume = 127 | issue = 4 | pages = 496–508 | date = November 2013 | pmid = 24032355 | pmc = 3820366 | doi = 10.1111/jnc.12433 }}</ref> facilitates the use of COX histochemistry in mapping regional brain metabolism in animals, since it establishes a direct and positive correlation between enzyme activity and neuronal activity.<ref name="pmid:2469224">{{cite journal | vauthors = Wong-Riley MT | title = Cytochrome oxidase: an endogenous metabolic marker for neuronal activity | journal = Trends in Neurosciences | volume = 12 | issue = 3 | pages = 94–101 | date = March 1989 | pmid = 2469224 | doi = 10.1016/0166-2236(89)90165-3 | s2cid = 42996304 }}</ref> This can be seen in the correlation between COX enzyme amount and activity, which indicates the regulation of COX at the level of gene expression. COX distribution is inconsistent across different regions of the animal brain, but its pattern of its distribution is consistent across animals. This pattern has been observed in the monkey, mouse, and calf brain. One isozyme of COX has been consistently detected in histochemical analysis of the brain.<ref name = "pmid2555458">{{cite journal | vauthors = Hevner RF, Wong-Riley MT | title = Brain cytochrome oxidase: purification, antibody production, and immunohistochemical/histochemical correlations in the CNS | journal = The Journal of Neuroscience | volume = 9 | issue = 11 | pages = 3884–98 | date = November 1989 | pmid = 2555458 | pmc = 6569932 | doi=10.1523/jneurosci.09-11-03884.1989}}</ref> Such brain mapping has been accomplished in spontaneous mutant mice with cerebellar disease such as [[reeler]]<ref name="pmid16511878">{{cite journal | vauthors = Strazielle C, Hayzoun K, Derer M, Mariani J, Lalonde R | title = Regional brain variations of cytochrome oxidase activity in Relnrl-orl mutant mice | journal = Journal of Neuroscience Research | volume = 83 | issue = 5 | pages = 821–31 | date = April 2006 | pmid = 16511878 | doi = 10.1002/jnr.20772 | s2cid = 45787322 }}</ref> and a transgenic model of [[Alzheimer's disease]].<ref name="pmid:12732258">{{cite journal | vauthors = Strazielle C, Sturchler-Pierrat C, Staufenbiel M, Lalonde R | title = Regional brain cytochrome oxidase activity in beta-amyloid precursor protein transgenic mice with the Swedish mutation | journal = Neuroscience | volume = 118 | issue = 4 | pages = 1151–63 | year = 2003 | pmid = 12732258 | doi = 10.1016/S0306-4522(03)00037-X | s2cid = 9366458 }}</ref> This technique has also been used to map learning activity in the animal brain.<ref name="pmid:19969098">{{cite journal | vauthors = Conejo NM, González-Pardo H, Gonzalez-Lima F, Arias JL | title = Spatial learning of the water maze: progression of brain circuits mapped with cytochrome oxidase histochemistry | journal = Neurobiology of Learning and Memory | volume = 93 | issue = 3 | pages = 362–71 | date = March 2010 | pmid = 19969098 | doi = 10.1016/j.nlm.2009.12.002 | s2cid = 24271956 | hdl = 10651/7300 | hdl-access = free }}</ref>

==Additional images==
<gallery>
 Image:Mitochondrial electron transport chain (annotated diagram).svg|ETC
 Image:Komplex IV.png|Complex IV
</gallery>

== See also ==
* [[Cytochrome c oxidase subunit I]]
* [[Cytochrome c oxidase subunit II]]
* [[Cytochrome c oxidase subunit III]]
* [[Heme a]]

== References ==
{{Reflist}}

== External links ==
* [https://web.archive.org/web/20021013113802/http://www-bioc.rice.edu/~graham/CcO.html The Cytochrome Oxidase home page] at [[Rice University]]
* [https://web.archive.org/web/20090111220321/http://www2.ufp.pt/~pedros/anim/2frame-iven.htm Interactive Molecular model of cytochrome c oxidase] (Requires [https://web.archive.org/web/20060320002451/http://www.mdl.com/products/framework/chime/ MDL Chime])
* {{UMichOPM|families|superfamily|4}}
* {{MeshName|Cytochrome-c+Oxidase}}

{{Electron transport chain}}
{{Proton pumps}}
{{Mitochondrial DNA}}
{{Authority control}}

{{DEFAULTSORT:Cytochrome C Oxidase}}
[[Category:Cellular respiration]]
[[Category:EC 1.9.3]]
[[Category:Hemoproteins]]
[[Category:Integral membrane proteins]]
[[Category:Copper enzymes]]