Summary: Ubiquinol cytochrome reductase transmembrane region
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Coenzyme Q â€“ cytochrome c reductase Edit Wikipedia article
|SCOPe||1be3 / SUPFAM|
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / QuickGO|
The coenzyme Q : cytochrome c â€“ oxidoreductase, sometimes called the cytochrome bc1 complex, and at other times complex III, is the third complex in the electron transport chain (EC 22.214.171.124), playing a critical role in biochemical generation of ATP (oxidative phosphorylation). Complex III is a multisubunit transmembrane protein encoded by both the mitochondrial (cytochrome b) and the nuclear genomes (all other subunits). Complex III is present in the mitochondria of all animals and all aerobic eukaryotes and the inner membranes of most eubacteria. Mutations in Complex III cause exercise intolerance as well as multisystem disorders. The bc1 complex contains 11 subunits, 3 respiratory subunits (cytochrome B, cytochrome C1, Rieske protein), 2 core proteins and 6 low-molecular weight proteins.
Ubiquinolâ€”cytochrome-c reductase catalyzes the chemical reaction
- QH2 + 2 ferricytochrome c Q + 2 ferrocytochrome c + 2 H+
This enzyme belongs to the family of oxidoreductases, specifically those acting on diphenols and related substances as donor with a cytochrome as acceptor. This enzyme participates in oxidative phosphorylation. It has four cofactors: cytochrome c1, cytochrome b-562, cytochrome b-566, and a 2-Iron ferredoxin of the Rieske type.
- 1 Nomenclature
- 2 Structure
- 3 Composition of complex
- 4 Reaction
- 5 Reaction mechanism
- 6 Inhibitors of complex III
- 7 Oxygen free radicals
- 8 Human gene names
- 9 Mutations in complex III genes in human disease
- 10 See also
- 11 Additional images
- 12 References
- 13 Further reading
- 14 External links
The systematic name of this enzyme class is ubiquinol:ferricytochrome-c oxidoreductase. Other names in common use include:
Compared to the other major proton-pumping subunits of the electron transport chain, the number of subunits found can be small, as small as three polypeptide chains. This number does increase, and eleven subunits are found in higher animals. Three subunits have prosthetic groups. The cytochrome b subunit has two b-type hemes (bL and bH), the cytochrome c subunit has one c-type heme (c1), and the Rieske Iron Sulfur Protein subunit (ISP) has a two iron, two sulfur iron-sulfur cluster (2Feâ€¢2S).
Structures of complex III:,
Composition of complex
In vertebrates the bc1 complex, or Complex III, contains 11 subunits: 3 respiratory subunits, 2 core proteins and 6 low-molecular weight proteins. Proteobacterial complexes may contain as few as three subunits.
Table of subunit composition of complex III
|No.||Subunit name||Human protein||Protein description from UniProt||Pfam family with Human protein|
|Respiratory subunit proteins|
|1||MT-CYB / Cyt b||CYB_HUMAN||Cytochrome b||Pfam PF13631|
|2||CYC1 / Cyt c1||CY1_HUMAN||Cytochrome c1, heme protein, mitochondrial||Pfam PF02167|
|3||Rieske / UCR1||UCRI_HUMAN||Cytochrome b-c1 complex subunit Rieske, mitochondrial EC 126.96.36.199||Pfam PF02921 , Pfam PF00355|
|Core protein subunits|
|4||QCR1 / SU1||QCR1_HUMAN||Cytochrome b-c1 complex subunit 1, mitochondrial||Pfam PF00675, Pfam PF05193|
|5||QCR2 / SU2||QCR2_HUMAN||Cytochrome b-c1 complex subunit 2, mitochondrial||Pfam PF00675, Pfam PF05193|
|Low-molecular weight protein subunits|
|6||QCR6 / SU6||QCR6_HUMAN||Cytochrome b-c1 complex subunit 6, mitochondrial||Pfam PF02320|
|7||QCR7 / SU7||QCR7_HUMAN||Cytochrome b-c1 complex subunit 7||Pfam PF02271|
|8||QCR8 / SU8||QCR8_HUMAN||Cytochrome b-c1 complex subunit 8||Pfam PF02939|
|9||QCR9 / SU9 / UCRC||QCR9_HUMANa||Cytochrome b-c1 complex subunit 9||Pfam PF09165|
|10||QCR10 / SU10||QCR10_HUMAN||Cytochrome b-c1 complex subunit 10||Pfam PF05365|
|11||QCR11 / SU11||QCR11_HUMAN||Cytochrome b-c1 complex subunit 11||Pfam PF08997|
- a In vertebrates, a cleavage product of 8 kDa from the N-terminus of the Rieske protein (Signal peptide) is retained in the complex as subunit 9. Thus subunits 10 and 11 correspond to fungal QCR9p and QCR10p.
- QH2 + 2 cytochrome c (FeIII) + 2 H+
in â†’ Q + 2 cytochrome c (FeII) + 4 H+
The reaction mechanism for complex III (cytochrome bc1, coenzyme Q: cytochrome C oxidoreductase) is known as the ubiquinone ("Q") cycle. In this cycle four protons get released into the positive "P" side (inter membrane space), but only two protons get taken up from the negative "N" side (matrix). As a result, a proton gradient is formed across the membrane. In the overall reaction, two ubiquinols are oxidized to ubiquinones and one ubiquinone is reduced to ubiquinol. In the complete mechanism, two electrons are transferred from ubiquinol to ubiquinone, via two cytochrome c intermediates.
- 2 x QH2 oxidised to Q
- 1 x Q reduced to QH2
- 2 x Cyt c reduced
- 4 x H+ released into intermembrane space
- 2 x H+ picked up from matrix
The reaction proceeds according to the following steps:
- Cytochrome b binds a ubiquinol and a ubiquinone.
- The 2Fe/2S center and BL heme each pull an electron off the bound ubiquinol, releasing two hydrogens into the intermembrane space.
- One electron is transferred to cytochrome c1 from the 2Fe/2S centre, whilst another is transferred from the BL heme to the BH Heme.
- Cytochrome c1 transfers its electron to cytochrome c (not to be confused with cytochrome c1), and the BH Heme transfers its electron to a nearby ubiquinone, resulting in the formation of a ubisemiquinone.
- Cytochrome c diffuses. The first ubiquinol (now oxidised to ubiquinone) is released, whilst the semiquinone remains bound.
- A second ubiquinol is bound by cytochrome b.
- The 2Fe/2S center and BL heme each pull an electron off the bound ubiquinol, releasing two hydrogens into the intermembrane space.
- One electron is transferred to cytochrome c1 from the 2Fe/2S centre, whilst another is transferred from the BL heme to the BH Heme.
- Cytocrome c1 then transfers its electron to cytochrome c, whilst the nearby semiquinone produced from round 1 picks up a second electron from the BH heme, along with two protons from the matrix.
- The second ubiquinol (now oxidised to ubiquinone), along with the newly formed ubiquinol are released.
Inhibitors of complex III
There are three distinct groups of Complex III inhibitors.
- Antimycin A binds to the Qi site and inhibits the transfer of electrons in Complex III from heme bH to oxidized Q (Qi site inhibitor).
- Myxothiazol and stigmatellin binds to the Qo site and inhibits the transfer of electrons from reduced QH2 to the Rieske Iron sulfur protein. Myxothiazol and stigmatellin bind to distinct but overlapping pockets within the Qo site.
- Myxothiazol binds nearer to cytochrome bL (hence termed a "proximal" inhibitor).
- Stigmatellin binds farther from heme bL and nearer the Rieske Iron sulfur protein, with which it strongly interacts.
Oxygen free radicals
A small fraction of electrons leave the electron transport chain before reaching complex IV. Premature electron leakage to oxygen results in the formation of superoxide. The relevance of this otherwise minor side reaction is that superoxide and other reactive oxygen species are highly toxic and are thought to play a role in several pathologies, as well as aging (the free radical theory of aging). Electron leakage occurs mainly at the Qo site and is stimulated by antimycin A. Antimycin A locks the b hemes in the reduced state by preventing their re-oxidation at the Qi site, which, in turn, causes the steady-state concentrations of the Qo semiquinone to rise, the latter species reacting with oxygen to form superoxide. The effect of high membrane potential is thought to have a similar effect. Superoxide produced at the Qo site can be released both into the mitochondrial matrix and into the intermembrane space, where it can then reach the cytosol. This could be explained by the fact that Complex III might produce superoxide as membrane permeable HOOâ€¢ rather than as membrane impermeable Oâˆ’.
Human gene names
CYCS: cytochrome c
UQCRFS1: Rieske iron sulfur protein
UQCRB: Ubiquinone binding protein, mutation linked with mitochondrial complex III deficiency nuclear type 3
UQCRH: hinge protein
UQCRC2: Core 2, mutations linked to mitochondrial complex III deficiency, nuclear type 5
UQCRC1: Core 1
UQCR: 6.4KD subunit
UQCR10: 7.2KD subunit
TTC19: Newly identified subunit, mutations linked to complex III deficiency nuclear type 2.
Mutations in complex III genes in human disease
Mutations in complex III-related genes typically manifest as exercise intolerance. Other mutations have been reported to cause septo-optic dysplasia and multisystem disorders. However, mutations in BCS1L, a gene responsible for proper maturation of complex III, can result in BjÃ¶rnstad syndrome and the GRACILE syndrome, which in neonates are lethal conditions that have multisystem and neurologic manifestations typifying severe mitochondrial disorders. The pathogenicity of several mutations has been verified in model systems such as yeast.
The extent to which these various pathologies are due to bioenergetic deficits or overproduction of superoxide is presently unknown.
- doi:10.1021/bi0341814. PMID 12885240. ; Gao X, Wen X, Esser L, Quinn B, Yu L, Yu CA, Xia D (August 2003). "Structural basis for the quinone reduction in the bc1 complex: a comparative analysis of crystal structures of mitochondrial cytochrome bc1 with bound substrate and inhibitors at the Qi site". Biochemistry. 42 (30): 9067â€“80.
- Iwata S, Lee JW, Okada K, Lee JK, Iwata M, Rasmussen B, Link TA, Ramaswamy S, Jap BK (July 1998). "Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex". Science. 281 (5373): 64â€“71. doi:10.1126/science.281.5373.64. PMID 9651245.
- Zhang Z, Huang L, Shulmeister VM, Chi YI, Kim KK, Hung LW, et al. (1998). "Electron transfer by domain movement in cytochrome bc1". Nature. 392 (6677): 677â€“84. doi:10.1038/33612. PMID 9565029.
- Hao GF, Wang F, Li H, Zhu XL, Yang WC, Huang LS, et al. (2012). "Computational discovery of picomolar Q(o) site inhibitors of cytochrome bc1 complex". J Am Chem Soc. 134 (27): 11168â€“76. doi:10.1021/ja3001908. PMID 22690928.
- Yang XH, Trumpower BL (1986). "Purification of a three-subunit ubiquinol-cytochrome c oxidoreductase complex from Paracoccus denitrificans". J Biol Chem. 261 (26): 12282â€“9. PMID 3017970.
- Kramer DM, Roberts AG, Muller F, Cape J, Bowman MK (2004). Q-cycle bypass reactions at the Qo site of the cytochrome bc1 (and related) complexes. Meth. Enzymol. Methods in Enzymology. 382. pp. 21â€“45. doi:10.1016/S0076-6879(04)82002-0. ISBN 978-0-12-182786-1. PMID 15047094.
- Crofts AR (2004). "The cytochrome bc1 complex: function in the context of structure". Annu. Rev. Physiol. 66: 689â€“733. doi:10.1146/annurev.physiol.66.032102.150251. PMID 14977419.
- Ferguson SJ, Nicholls D, Ferguson S (2002). Bioenergetics (3rd ed.). San Diego: Academic. pp. 114â€“117. ISBN 978-0-12-518121-1.
- Holmes, J. H.; Sapeika, N; Zwarenstein, H (1975). "Inhibitory effect of anti-obesity drugs on NADH dehydrogenase of mouse heart homogenates". Research Communications in Chemical Pathology and Pharmacology. 11 (4): 645â€“6. PMID 241101.
- Muller, F. L.; Lustgarten, M. S.; Jang, Y.; Richardson, A. & Van Remmen, H. (2007). "Trends in oxidative aging theories". Free Radic. Biol. Med. 43 (4): 477â€“503. doi:10.1016/j.freeradbiomed.2007.03.034. PMID 17640558.
- Skulachev VP (May 1996). "Role of uncoupled and non-coupled oxidations in maintenance of safely low levels of oxygen and its one-electron reductants". Q. Rev. Biophys. 29 (2): 169â€“202. doi:10.1017/s0033583500005795. PMID 8870073.
- Muller F (2000). "The nature and mechanism of superoxide production by the electron transport chain: Its relevance to aging". AGE. 23 (4): 227â€“253. doi:10.1007/s11357-000-0022-9. PMC 3455268. PMID 23604868.
- Muller FL, Liu Y, Van Remmen H (November 2004). "Complex III releases superoxide to both sides of the inner mitochondrial membrane". J. Biol. Chem. 279 (47): 49064â€“73. doi:10.1074/jbc.M407715200. PMID 15317809.
- Han D, Williams E, Cadenas E (January 2001). "Mitochondrial respiratory chain-dependent generation of superoxide anion and its release into the intermembrane space". Biochem. J. 353 (Pt 2): 411â€“6. doi:10.1042/0264-6021:3530411. PMC 1221585. PMID 11139407.
- DiMauro S (November 2006). "Mitochondrial myopathies" (PDF). Curr Opin Rheumatol. 18 (6): 636â€“41. doi:10.1097/01.bor.0000245729.17759.f2. PMID 17053512.
- DiMauro S (June 2007). "Mitochondrial DNA medicine". Biosci. Rep. 27 (1â€“3): 5â€“9. doi:10.1007/s10540-007-9032-5. PMID 17484047.
- Schuelke M, Krude H, Finckh B, Mayatepek E, Janssen A, Schmelz M, Trefz F, Trijbels F, Smeitink J (March 2002). "Septo-optic dysplasia associated with a new mitochondrial cytochrome b mutation". Ann. Neurol. 51 (3): 388â€“92. doi:10.1002/ana.10151. PMID 11891837.
- Wibrand F, Ravn K, Schwartz M, Rosenberg T, Horn N, Vissing J (October 2001). "Multisystem disorder associated with a missense mutation in the mitochondrial cytochrome b gene". Ann. Neurol. 50 (4): 540â€“3. doi:10.1002/ana.1224. PMID 11601507.
- Fisher N, Castleden CK, Bourges I, Brasseur G, Dujardin G, Meunier B (March 2004). "Human disease-related mutations in cytochrome b studied in yeast". J. Biol. Chem. 279 (13): 12951â€“8. doi:10.1074/jbc.M313866200. PMID 14718526.
- Marres CM, Slater EC (1977). "Polypeptide composition of purified QH2:cytochrome c oxidoreductase from beef-heart mitochondria". Biochim. Biophys. Acta. 462 (3): 531â€“548. doi:10.1016/0005-2728(77)90099-8. PMID 597492.
- Rieske JS (1976). "Composition, structure, and function of complex III of the respiratory chain". Biochim. Biophys. Acta. 456 (2): 195â€“247. doi:10.1016/0304-4173(76)90012-4. PMID 788795.
- Wikstrom M, Krab K, Saraste M (1981). "Proton-translocating cytochrome complexes". Annu. Rev. Biochem. 50: 623â€“655. doi:10.1146/annurev.bi.50.070181.003203. PMID 6267990.
- cytochrome bc1 complex site (Edward A. Berry) at the Wayback Machine (archived October 9, 2006) at lbl.gov
- cytochrome bc1 complex site (Antony R. Crofts) at uiuc.edu
- PROMISE Database: cytochrome bc1 complex at Archive.today (archived August 27, 1999) at scripps.edu
- Interactive Molecular Model of Complex III at the Wayback Machine (archived January 12, 2009) (Requires MDL Chime)
- UMich Orientation of Proteins in Membranes families/superfamily-3 - Calculated positions of bc1 and related complexes in membranes
- Coenzyme+Q-Cytochrome-c+Reductase at the US National Library of Medicine Medical Subject Headings (MeSH)
This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.
Ubiquinol cytochrome reductase transmembrane region Provide feedback
Each subunit of the cytochrome bc1 complex provides a single helix (this family) to make up the transmembrane region of the complex.
Iwata S, Lee JW, Okada K, Lee JK, Iwata M, Rasmussen B, Link TA, Ramaswamy S, Jap BK; , Science 1998;281:64-71.: Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex [see comments] PUBMED:9651245 EPMC:9651245
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR004192
The ubiquinol cytochrome c reductase (cytochrome bc1) complex is a respiratory chain that generates an electrochemical potential coupled to ATP synthesis. The bc1 complex contains 11 subunits, 3 respiratory subunits (cytochrome B, cytochrome C1, Rieske protein), 2 core proteins and 6 low-molecular weight proteins [ PUBMED:9651245 ]. Each subunit of the cytochrome bc1 complex provides a single helix to make up the transmembrane region of the complex.
This entry represents the transmembrane domain of the iron-sulfur subunit Rieske.
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Molecular function||ubiquinol-cytochrome-c reductase activity (GO:0008121)|
|Biological process||oxidation-reduction process (GO:0055114)|
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This motif is found in a wide range of secreted proteins. It is named after the conserved pair of arginines that is followed by a hydrophobic stretch.
The clan contains the following 3 members:TAT_signal UCR_Fe-S_N UCR_TM
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|Seed source:||ref |
|Number in seed:||43|
|Number in full:||1705|
|Average length of the domain:||59.60 aa|
|Average identity of full alignment:||35 %|
|Average coverage of the sequence by the domain:||20.93 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 57096847 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||16|
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For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the UCR_TM domain has been found. There are 119 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein sequence.
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AlphaFold Structure Predictions
The list of proteins below match this family and have AlphaFold predicted structures. Click on the protein accession to view the predicted structure.