The Enzyme Database

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EC 7.1.1.8     
Accepted name: quinol—cytochrome-c reductase
Reaction: quinol + 2 ferricytochrome c = quinone + 2 ferrocytochrome c + 2 H+[side 2]
Other name(s): ubiquinol—cytochrome-c reductase; coenzyme Q-cytochrome c reductase; dihydrocoenzyme Q-cytochrome c reductase; reduced ubiquinone-cytochrome c reductase; complex III (mitochondrial electron transport); ubiquinone-cytochrome c reductase; ubiquinol-cytochrome c oxidoreductase; reduced coenzyme Q-cytochrome c reductase; ubiquinone-cytochrome c oxidoreductase; reduced ubiquinone-cytochrome c oxidoreductase; mitochondrial electron transport complex III; ubiquinol-cytochrome c-2 oxidoreductase; ubiquinone-cytochrome b-c1 oxidoreductase; ubiquinol-cytochrome c2 reductase; ubiquinol-cytochrome c1 oxidoreductase; CoQH2-cytochrome c oxidoreductase; ubihydroquinol:cytochrome c oxidoreductase; coenzyme QH2-cytochrome c reductase; QH2:cytochrome c oxidoreductase; ubiquinol:ferricytochrome-c oxidoreductase
Systematic name: quinol:ferricytochrome-c oxidoreductase
Comments: The enzyme, often referred to as the cytochrome bc1 complex or complex III, is the third complex in the electron transport chain. It is present in the mitochondria of all aerobic eukaryotes and in the inner membranes of most bacteria. The mammalian enzyme contains cytochromes b-562, b-566 and c1, and a 2-iron ferredoxin. Depending on the organism and physiological conditions, the enzyme extrudes either two or four protons from the cytoplasmic to the non-cytoplasmic compartment (cf. EC 1.6.99.3, NADH dehydrogenase).
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 9027-03-6
References:
1.  Marres, C.A.M. and Slater, E.C. Polypeptide composition of purified QH2:cytochrome c oxidoreductase from beef-heart mitochondria. Biochim. Biophys. Acta 462 (1977) 531–548. [DOI] [PMID: 597492]
2.  Rieske, J.S. Composition, structure, and function of complex III of the respiratory chain. Biochim. Biophys. Acta 456 (1976) 195–247. [PMID: 788795]
3.  Wikström, M., Krab, K. and Saraste, M. Proton-translocating cytochrome complexes. Annu. Rev. Biochem. 50 (1981) 623–655. [DOI] [PMID: 6267990]
4.  Sone, N., Tsuchiya, N., Inoue, M. and Noguchi, S. Bacillus stearothermophilus qcr operon encoding rieske FeS protein, cytochrome b6, and a novel-type cytochrome c1 of quinol-cytochrome c reductase. J. Biol. Chem. 271 (1996) 12457–12462. [DOI] [PMID: 8647852]
5.  Yu, J. and Le Brun, N.E. Studies of the cytochrome subunits of menaquinone:cytochrome c reductase (bc complex) of Bacillus subtilis. Evidence for the covalent attachment of heme to the cytochrome b subunit. J. Biol. Chem. 273 (1998) 8860–8866. [DOI] [PMID: 9535866]
6.  Elbehti, A., Nitschke, W., Tron, P., Michel, C. and Lemesle-Meunier, D. Redox components of cytochrome bc-type enzymes in acidophilic prokaryotes. I. Characterization of the cytochrome bc1-type complex of the acidophilic ferrous ion-oxidizing bacterium Thiobacillus ferrooxidans. J. Biol. Chem. 274 (1999) 16760–16765. [DOI] [PMID: 10358017]
[EC 7.1.1.8 created 1978 as EC 1.10.2.2, modified 2013, transferred 2018 to EC 7.1.1.8]
 
 
EC 7.1.1.12     
Accepted name: succinate dehydrogenase (electrogenic, proton-motive force generating)
Reaction: succinate + menaquinone + 2 H+[side 1] = fumarate + menaquinol + 2 H+[side 2]
Systematic name: succinate:quinone oxidoreductase (electrogenic, proton-motive force generating)
Comments: The enzyme is very similar to EC 1.3.5.1, succinate dehydrogenase, but differs by containing two heme molecules (located in the membrane anchor component) in addition to FAD and three iron-sulfur clusters. Unlike EC 1.3.5.1, this enzyme catalyses an electrogenic reaction, enabled by electron-bifurcation via the heme molecules. In the direction of succinate oxidation by menaquinone, which is endergonic, the reaction is driven by the transmembrane electrochemical proton potential. In the direction of fumarate reduction, the electrogenic electron transfer reaction is compensated by transmembrane proton transfer pathway known as the E-pathway, which results in overall electroneutrality.
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc
References:
1.  Lancaster, C.R. Wolinella succinogenes quinol:fumarate reductase-2.2-A resolution crystal structure and the E-pathway hypothesis of coupled transmembrane proton and electron transfer. Biochim. Biophys. Acta 1565 (2002) 215–231. [DOI] [PMID: 12409197]
2.  Madej, M.G., Nasiri, H.R., Hilgendorff, N.S., Schwalbe, H., Unden, G. and Lancaster, C.R. Experimental evidence for proton motive force-dependent catalysis by the diheme-containing succinate:menaquinone oxidoreductase from the Gram-positive bacterium Bacillus licheniformis. Biochemistry 45 (2006) 15049–15055. [DOI] [PMID: 17154542]
3.  Lancaster, C.R., Herzog, E., Juhnke, H.D., Madej, M.G., Muller, F.G., Paul, R. and Schleidt, P.G. Electroneutral and electrogenic catalysis by dihaem-containing succinate:quinone oxidoreductases. Biochem Soc Trans. 36 (2008) 996–1000. [DOI] [PMID: 18793177]
4.  Lancaster, C.R. The di-heme family of respiratory complex II enzymes. Biochim. Biophys. Acta 1827 (2013) 679–687. [DOI] [PMID: 23466335]
5.  Guan, H.H., Hsieh, Y.C., Lin, P.J., Huang, Y.C., Yoshimura, M., Chen, L.Y., Chen, S.K., Chuankhayan, P., Lin, C.C., Chen, N.C., Nakagawa, A., Chan, S.I. and Chen, C.J. Structural insights into the electron/proton transfer pathways in the quinol:fumarate reductase from Desulfovibrio gigas. Sci. Rep. 8:14935 (2018). [DOI] [PMID: 30297797]
[EC 7.1.1.12 created 2022]
 
 


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