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22-AUGUST-2008 06:13:22 - Coenzyme Q - cytochrome c reductase Redirected from Complex III schematic illustration of complex III reactions schematic illustration of complex III reactions 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 1.10.2.2, 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 1 Structure 2 Reaction 3 Reaction Mechanism 4 Inhibitors of complex III 5 Oxygen free radicals 6 References 7 See also 8 Additional images 9 External links Structure Structure of complex III 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 eleven subunits are found in higher animals 1. 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 2Fe2S. Structures of complex III: PDB 1KYO, PDB 1L0L 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,23 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. Reaction Mechanism The Q cycle The Q cycle The reaction mechanism for complex III Cytochrome bc1 , Coenzyme Q: Cytochrome C Oxidoreductase is named the Q cycle or the ubiquinone cycle as mentioned above. In this cycle four protons get released into the P or Positive side inter membrane space but only two protons get taken up from the N or Negative side matrix, see animation to the right. As a result a proton gradient is formed across the membrane. Also, two ubiquinols get oxidized to ubiquinones and one ubiquinone gets reduced to ubiquinol! All this is accomplished by the transfer of two electrons from two ubiquinols to two cytochrome c's as well as two electrons from the same two ubiquinols to a ubiquinone. The reaction goes as follows. 1. Ubiquinol binds to cytochrome b. 2. The 2Fe/2S center and BL Heme each pull an electron off the bound ubiquinone, and two hydrogens are released into the intermembrane space. 3. The 2Fe/2S center transfers its electron to cytochrome c1 and the BL Heme transfers its electron to the BH Heme. 4. Cytocrome c1 transfers its electron to a water soluble cytochrome c not to be confused with cytochrome c1 which is membrane bound, and the BH Heme transfers its electron to a nearby ubiquinone turning the ubiquinone into a ubisemiquinone. 5. Cytochrome c diffuses and the fully oxidized ubiquinone is released. 6. Another ubiquinol binds to cytochrome b. 7. The 2Fe/2S center and BL Heme each pull an electron off the bound ubiquinone and two hydrogens are released into the intermembrane space. 8. The 2Fe/2S center transfers its electron to cytochrome c1 and the BL Heme transfers its electron to the BH Heme. 9. Cytocrome c1 then transfers its electron to a water soluble cytochrome c, and the BH Heme transfers its electron as well as two hydrogens from the matrix to the nearby ubisemiquinone turning the ubisemiquinone into a ubiquinol. 10.The fully oxidized ubiquinone and ubiquinol are released.4 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 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, 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 5. Superoxide produced at the Qo site can be released both into the mitochondrial matrix 67 and intermembrane space from where it can reach the cytosol 89. This could be explained by the fact that Complex III might produce superoxide as membrane permeable HO2 rather than as membrane impermeable O2- 10. References ^ Iwata S., Lee J.W., Okada K., Lee J.K., Iwata M., Rasmussen B., Link T.A., Ramaswamy S., Jap B.K. 1998 Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex. Science 281: 64-71 ^ 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 ^ Crofts, A.R. 2004. The cytochrome bc1 complex: function in the context of structure. Annu Rev Physiol. 66, 689-733. PMID 14977419 ^ Nicholls, David and Stuart Ferguson. Bioenergetics3. Acociate Press: San Diego, California 2002. pg 114-117 ^ Skulachev, V. P. 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, 169-202 ^ Muller, F. 2000 The Nature and Mechanism of Superoxide production by the Electron Transport Chain: Its relevance to aging. J. Amer. Aging Assoc. 23, 227-253 ^ Muller, F. L., Liu, Y. and Van Remmen, H. 2004 Complex III Releases superoxide to both sides of the inner mitochondrial membrane. J. Biol. Chem. 279, 49064-49073 ^ Han, D., Williams, E. and Cadenas, E. 2001 Mitochondrial respiratory chain-dependent generation of superoxide anion and its release into the intermembrane space. Biochem. J. 353, 411-416. ^ Muller, F. 2000 The Nature and Mechanism of Superoxide production by the Electron Transport Chain: Its relevance to aging. J. Amer. Aging Assoc. 23, 227-253 ^ Muller, F. L., Liu, Y. and Van Remmen, H. 2004 Complex III Releases superoxide to both sides of the inner mitochondrial membrane. J. Biol. Chem. 279, 49064-49073 See also Cellular respiration Photosynthetic reaction centre Complex 3 Additional images ETC ETC External links 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 MeSH Coenzyme+Q-Cytochrome-c+Reductase v d e Diphenol family oxidoreductases EC 1.10 Coenzyme Q - cytochrome c reductase - Catechol oxidase - Laccase - Alternative oxidase v d e Mitochondrial electron transport chain/oxidative phosphorylation Primary Complex I/NADH dehydrogenase - Complex II/Succinate dehydrogenase - Coenzyme Q - Complex III/Coenzyme Q - cytochrome c reductase - Cytochrome c - Complex IV/Cytochrome c oxidase Other Alternative oxidase - Electron-transferring-flavoprotein dehydrogenase v d e Ion pump: proton pumps ETC ETC Complex I - ETC Complex III - ETC Complex IV Other Bacteriorhodopsin - Cytochrome b6f complex - Inorganic pyrophosphatase - V-ATPase v d e Mitochondrial enzymes and transporters Outer membrane fatty acid degradation Carnitine palmitoyltransferase I, Long fatty acyl CoA synthetase tryptophan metabolism Kynureninase monoamine neurotransmitter metabolism Monoamine oxidase Intermembrane space Adenylate kinase - Creatine kinase Inner membrane oxidative phosphorylation Coenzyme Q - cytochrome c reductase, Cytochrome c, NADH dehydrogenase, Succinate dehydrogenase pyrimidine metabolism Dihydroorotate dehydrogenase mitochondrial shuttle Malate-aspartate shuttle, Glycerol phosphate shuttle other Glutamate aspartate transporter, Glycerol-3-phosphate dehydrogenase, ATP synthase, Carnitine palmitoyltransferase II Matrix citric acid cycle Citrate synthase, Aconitase, Isocitrate dehydrogenase, Oxoglutarate dehydrogenase, Succinyl coenzyme A synthetase, Fumarase, Malate dehydrogenase anaplerotic reactions Aspartate transaminase, Glutamate dehydrogenase, Pyruvate dehydrogenase complex urea cycle Carbamoyl phosphate synthetase I, Ornithine transcarbamylase, N-Acetylglutamate synthase alcohol metabolism ALDH2 Other/to be sorted Cholesterol side-chain cleavage enzyme Mitochondrial DNA Complex I 7 units - Complex III 1 unit - Complex IV 3 units - ATP synthase 2 units Retrieved from http://en..org/wiki/Coenzyme_Q_-_cytochrome_c_reductase Categories: Cellular respiration | EC 1.10.2 | Iron-sulfur proteins | Integral membrane proteins Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Deutsch Français Italiano Polski This page was last modified on 18 August 2008, at 05:14. of the GNU Free Documentation License. ® , Inc., a U.S.
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