Summary: UcrQ family
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Coenzyme Q – cytochrome c reductase Edit Wikipedia article
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
The coenzyme Q : cytochrome c – 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.
Ubiquinol—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 (2Fe•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 → 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 c1 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 HOO• rather than as membrane impermeable O−.
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 Björnstad 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.
- PMID 12885240. doi:10.1021/bi0341814.; 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): 9067–80.
- 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): 64–71. PMID 9651245. doi:10.1126/science.281.5373.64.
- 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): 677–84. PMID 9565029. doi:10.1038/33612.
- 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): 11168–76. PMID 22690928. doi:10.1021/ja3001908.
- Yang XH, Trumpower BL (1986). "Purification of a three-subunit ubiquinol-cytochrome c oxidoreductase complex from Paracoccus denitrificans". J Biol Chem. 261: 12282–9. 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: 21–45. ISBN 978-0-12-182786-1. PMID 15047094. doi:10.1016/S0076-6879(04)82002-0.
- Crofts AR (2004). "The cytochrome bc1 complex: function in the context of structure". Annu. Rev. Physiol. 66: 689–733. PMID 14977419. doi:10.1146/annurev.physiol.66.032102.150251.
- Ferguson SJ, Nicholls D, Ferguson S (2002). Bioenergetics (3rd ed.). San Diego: Academic. pp. 114–117. ISBN 0-12-518121-3.
- 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): 645–6. 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): 477–503. PMID 17640558. doi:10.1016/j.freeradbiomed.2007.03.034.
- 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): 169–202. PMID 8870073. doi:10.1017/s0033583500005795.
- Muller F (2000). "The nature and mechanism of superoxide production by the electron transport chain: Its relevance to aging". AGE. 23 (4): 227–253. PMC . PMID 23604868. doi:10.1007/s11357-000-0022-9.
- 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): 49064–73. PMID 15317809. doi:10.1074/jbc.M407715200.
- 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): 411–6. PMC . PMID 11139407. doi:10.1042/0264-6021:3530411.
- DiMauro S (November 2006). "Mitochondrial myopathies". Curr Opin Rheumatol. 18 (6): 636–41. PMID 17053512. doi:10.1097/01.bor.0000245729.17759.f2.
- DiMauro S (June 2007). "Mitochondrial DNA medicine". Biosci. Rep. 27 (1–3): 5–9. PMID 17484047. doi:10.1007/s10540-007-9032-5.
- 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): 388–92. PMID 11891837. doi:10.1002/ana.10151.
- 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): 540–3. PMID 11601507. doi:10.1002/ana.1224.
- 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): 12951–8. PMID 14718526. doi:10.1074/jbc.M313866200.
- Marres CM, Slater EC (1977). "Polypeptide composition of purified QH2:cytochrome c oxidoreductase from beef-heart mitochondria". Biochim. Biophys. Acta. 462 (3): 531–548. PMID 597492. doi:10.1016/0005-2728(77)90099-8.
- Rieske JS (1976). "Composition, structure, and function of complex III of the respiratory chain". Biochim. Biophys. Acta. 456 (2): 195–247. PMID 788795. doi:10.1016/0304-4173(76)90012-4.
- Wikstrom M, Krab K, Saraste M (1981). "Proton-translocating cytochrome complexes". Annu. Rev. Biochem. 50: 623–655. PMID 6267990. doi:10.1146/annurev.bi.50.070181.003203.
- cytochrome bc1 complex site (Edward A. Berry) at lbl.gov
- cytochrome bc1 complex site (Antony R. Crofts) at uiuc.edu
- PROMISE Database: cytochrome bc1 complex at scripps.edu
- Interactive Molecular Model of Complex III (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.
UcrQ family Provide feedback
The ubiquinol-cytochrome C reductase complex (cytochrome bc1 complex) is a respiratory multienzyme complex . This family represents the 9.5 kDa subunit 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
Internal database links
|Similarity to PfamA using HHSearch:||Cyt_b-c1_8|
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR004205
The ubiquinol-cytochrome C reductase complex (cytochrome bc1 complex) is a respiratory multi-enzyme complex [PUBMED:9651245], which recognises a mitochondrial targeting presequence. The bc1 complex contains 11 subunits: 3 respiratory subunits (cytochrome b, cytochrome c1 and Rieske protein), 2 core proteins and 6 low molecular weight proteins. This family represents the 9.5 kDa subunit of the complex, known as subunit 8 or protein QP-C [PUBMED:12709789]. This subunit together with cytochrome B binds to ubiquinone.
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)|
Below is a listing of the unique domain organisations or architectures in which this domain is found. More...
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1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
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This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.
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Curation and family details
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|Seed source:||PSI-blast P13271|
|Number in seed:||112|
|Number in full:||638|
|Average length of the domain:||75.90 aa|
|Average identity of full alignment:||34 %|
|Average coverage of the sequence by the domain:||57.80 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 26740544 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||15|
|Download:||download the raw HMM for this family|
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This visualisation provides a simple graphical representation of the distribution of this family across species. You can find the original interactive tree in the More....
This chart is a modified "sunburst" visualisation of the species tree for this family. It shows each node in the tree as a separate arc, arranged radially with the superkingdoms at the centre and the species arrayed around the outermost ring.
How the sunburst is generated
The tree is built by considering the taxonomic lineage of each sequence that has a match to this family. For each node in the resulting tree, we draw an arc in the sunburst. The radius of the arc, its distance from the root node at the centre of the sunburst, shows the taxonomic level ("superkingdom", "kingdom", etc). The length of the arc represents either the number of sequences represented at a given level, or the number of species that are found beneath the node in the tree. The weighting scheme can be changed using the sunburst controls.
In order to reduce the complexity of the representation, we reduce the number of taxonomic levels that we show. We consider only the following eight major taxonomic levels:
Colouring and labels
Segments of the tree are coloured approximately according to their superkingdom. For example, archeal branches are coloured with shades of orange, eukaryotes in shades of purple, etc. The colour assignments are shown under the sunburst controls. Where space allows, the name of the taxonomic level will be written on the arc itself.
As you move your mouse across the sunburst, the current node will be highlighted. In the top section of the controls panel we show a summary of the lineage of the currently highlighed node. If you pause over an arc, a tooltip will be shown, giving the name of the taxonomic level in the title and a summary of the number of sequences and species below that node in the tree.
Anomalies in the taxonomy tree
There are some situations that the sunburst tree cannot easily handle and for which we have work-arounds in place.
Missing taxonomic levels
Some species in the taxonomic tree may not have one or more of the main eight levels that we display. For example, Bos taurus is not assigned an order in the NCBI taxonomic tree. In such cases we mark the omitted level with, for example, "No order", in both the tooltip and the lineage summary.
Unmapped species names
The tree is built by looking at each sequence in the full alignment for the family. We take the name of the species given by UniProt and try to map that to the full taxonomic tree from NCBI. In some cases, the name chosen by UniProt does not map to any node in the NCBI tree, perhaps because the chosen name is listed as a synonym or a misspelling in the NCBI taxonomy.
So that these nodes are not simply omitted from the sunburst tree, we group them together in a separate branch (or segment of the sunburst tree). Since we cannot determine the lineage for these unmapped species, we show all levels between the superkingdom and the species as "uncategorised".
Since we reduce the species tree to only the eight main taxonomic levels, sequences that are mapped to the sub-species level in the tree would not normally be shown. Rather than leave out these species, we map them instead to their parent species. So, for example, for sequences belonging to one of the Vibrio cholerae sub-species in the NCBI taxonomy, we show them instead as belonging to the species Vibrio cholerae.
Too many species/sequences
For large species trees, you may see blank regions in the outer layers of the sunburst. These occur when there are large numbers of arcs to be drawn in a small space. If an arc is less than approximately one pixel wide, it will not be drawn and the space will be left blank. You may still be able to get some information about the species in that region by moving your mouse across the area, but since each arc will be very small, it will be difficult to accurately locate a particular species.
The tree shows the occurrence of this domain across different species. More...
We show the species tree in one of two ways. For smaller trees we try to show an interactive representation, which allows you to select specific nodes in the tree and view them as an alignment or as a set of Pfam domain graphics.
Unfortunately we have found that there are problems viewing the interactive tree when the it becomes larger than a certain limit. Furthermore, we have found that Internet Explorer can become unresponsive when viewing some trees, regardless of their size. We therefore show a text representation of the species tree when the size is above a certain limit or if you are using Internet Explorer to view the site.
If you are using IE you can still load the interactive tree by clicking the "Generate interactive tree" button, but please be aware of the potential problems that the interactive species tree can cause.
For all of the domain matches in a full alignment, we count the number that are found on all sequences in the alignment. This total is shown in the purple box.
We also count the number of unique sequences on which each domain is found, which is shown in green. Note that a domain may appear multiple times on the same sequence, leading to the difference between these two numbers.
Finally, we group sequences from the same organism according to the NCBI code that is assigned by UniProt, allowing us to count the number of distinct sequences on which the domain is found. This value is shown in the pink boxes.
We use the NCBI species tree to group organisms according to their taxonomy and this forms the structure of the displayed tree. Note that in some cases the trees are too large (have too many nodes) to allow us to build an interactive tree, but in most cases you can still view the tree in a plain text, non-interactive representation. Those species which are represented in the seed alignment for this domain are highlighted.
You can use the tree controls to manipulate how the interactive tree is displayed:
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There are 10 interactions for this family. More...
We determine these interactions using iPfam, which considers the interactions between residues in three-dimensional protein structures and maps those interactions back to Pfam families. You can find more information about the iPfam algorithm in the journal article that accompanies the website.
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 UcrQ domain has been found. There are 72 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 seqence.
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