COENZYME Q - CYTOCHROME C REDUCTASE
- + Add to Faves
- + Translate
- + Share
العربية
ä¸å›½
Français
Deutsch
Ελληνική
हिनà¥à¤¦à¥€
Italiano
日本語
Português
РуÑÑкий
Español
(Redirected from Cytochrome b)
The 'coenzyme Q : cytochrome ''c'' — oxidoreductase', sometimes called the 'cytochrome ''bc''1 complex', and at other times 'complex III', is the third complex in the electron transport chain (), playing a critical role in biochemical generation of ATP (oxidative phosphorylation). Complex III is a multisubunit transmembrane lipoprotein 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.
It catalyzes the reduction of cytochrome ''c'' by
oxidation of coenzyme Q (CoQ) and the concomitant pumping of 4 protons from the mitochondrial matrix to the intermembrane space:
: QH2 + 2 cytochrome ''c'' (FeIII) + 2 H+in → Q + 2 cytochrome ''c'' (FeII) + 4 H+out
In the process called Q cycle,[1][2] two protons are consumed from the matrix (M), four protons are released into the inter membrane space (IM) and two electrons are passed to cytochrome ''c''.
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 as many as eleven subunits can be found in higher animals. Three subunits are found to have prosthetic groups. The cytochrome ''b'' subunit has two ''b''-type hemes (''b''L and ''b''H), the cytochrome ''c'' sununit has one ''c''-type heme (''c''1), and the Rieske Iron Sulfur Protein subunit (ISP) has a two iron, two sulfur iron-sulfur cluster (2Fe•2S).
Structures 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 ''b''H 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 pockets within the Qo site.
★
★ Myxothiazol binds very close to cytochrome bL (hence termed a "proximal" inhibitor).
★
★ Stigmatellin binds near the Rieske Iron sulfur protein, with which it strongly interacts.
Some have been commercialized as fungicides (the strobilurin derivates) and as anti-malaria agents (atovaquone).
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, including 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.
1. Kramer, D. M.; Roberts, A. G.; Muller, F.; Cape, J.; Bowman, M. K. Q-cycle bypass reactions at the Qo site of the cytochrome bc1 (and related) complexes. Methods Enzymol. 382:21-45; 2004. PMID 15047094
2. Crofts, A.R. (2004). The cytochrome bc1 complex: function in the context of structure. Annu Rev Physiol. 66, 689-733. PMID 14977419
★ Cellular respiration
★ Photosynthetic reaction centre
★ cytochrome ''bc''1 complex site (Edward A. Berry) at lbl.gov
★ cytochrome ''bc''1 complex site (Antony R. Crofts) at uiuc.edu
★ PROMISE Database: cytochrome ''bc''1 complex at scripps.edu
★ Interactive Molecular Model of Complex III (Requires MDL Chime)
★ - Calculated positions of bc1 and related complexes in membranes
★
schematic illustration of complex III reactions
The 'coenzyme Q : cytochrome ''c'' — oxidoreductase', sometimes called the 'cytochrome ''bc''1 complex', and at other times 'complex III', is the third complex in the electron transport chain (), playing a critical role in biochemical generation of ATP (oxidative phosphorylation). Complex III is a multisubunit transmembrane lipoprotein 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.
| Contents |
| Reaction |
| Structure |
| Inhibitors of complex III |
| Oxygen free radicals |
| References |
| See also |
| Additional images |
| External links |
Reaction
It catalyzes the reduction of cytochrome ''c'' by
oxidation of coenzyme Q (CoQ) and the concomitant pumping of 4 protons from the mitochondrial matrix to the intermembrane space:
: QH2 + 2 cytochrome ''c'' (FeIII) + 2 H+in → Q + 2 cytochrome ''c'' (FeII) + 4 H+out
In the process called Q cycle,[1][2] two protons are consumed from the matrix (M), four protons are released into the inter membrane space (IM) and two electrons are passed to cytochrome ''c''.
Structure
structure of complex III
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 as many as eleven subunits can be found in higher animals. Three subunits are found to have prosthetic groups. The cytochrome ''b'' subunit has two ''b''-type hemes (''b''L and ''b''H), the cytochrome ''c'' sununit has one ''c''-type heme (''c''1), and the Rieske Iron Sulfur Protein subunit (ISP) has a two iron, two sulfur iron-sulfur cluster (2Fe•2S).
Structures of complex III: ,
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 ''b''H 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 pockets within the Qo site.
★
★ Myxothiazol binds very close to cytochrome bL (hence termed a "proximal" inhibitor).
★
★ Stigmatellin binds near the Rieske Iron sulfur protein, with which it strongly interacts.
Some have been commercialized as fungicides (the strobilurin derivates) and as anti-malaria agents (atovaquone).
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, including 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.
References
1. Kramer, D. M.; Roberts, A. G.; Muller, F.; Cape, J.; Bowman, M. K. Q-cycle bypass reactions at the Qo site of the cytochrome bc1 (and related) complexes. Methods Enzymol. 382:21-45; 2004. PMID 15047094
2. Crofts, A.R. (2004). The cytochrome bc1 complex: function in the context of structure. Annu Rev Physiol. 66, 689-733. PMID 14977419
See also
★ Cellular respiration
★ Photosynthetic reaction centre
Additional images
External links
★ cytochrome ''bc''1 complex site (Edward A. Berry) at lbl.gov
★ cytochrome ''bc''1 complex site (Antony R. Crofts) at uiuc.edu
★ PROMISE Database: cytochrome ''bc''1 complex at scripps.edu
★ Interactive Molecular Model of Complex III (Requires MDL Chime)
★ - Calculated positions of bc1 and related complexes in membranes
★
This article provided by Wikipedia. To edit the contents of this article, click here for original source.
psst.. try this: add to faves
