Coenzyme Q – cytochrome c reductase. An ubiquinol—cytochrome-c reductase is an enzyme which catalyzes the following reaction. QH2 + 2 ferricytochrome.

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Coenzyme Q – cytochrome c reductase

An ubiquinol—cytochrome-c reductase is an enzyme which catalyzes the following reaction. QH2 + 2 ferricytochrome c Q + 2 ferrocytochrome c + 2 H+ Thus, the two substrates of this enzyme are: - dihydroquinone (QH2) and -ferri- (Fe3+) cytochrom C, whereas its 3 products are - quinone (Q), -ferro- (Fe2+) cytochrome c, - and H+ It participates in Oxidative phosphorylation.

Subunits The cytochrom 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, two iron sulfur cluster (2Fe2S).

The “Q cycle” depends on mobility of coenzyme Q within the lipid bilayer. There is evidence for one-electron transfers, with an intermediate semiquinone radical.

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 ubiquinol are oxidized to ubiquinone and one ubiquinon is reduced to ubiquinol. In the complete mechanism, two electrons are transferred from ubiquinol to ubiquinone, via two cytochrome c intermediates.

Overall: 2 x QH2 oxidised to Q 1 x Q reduced to QH 2 x Cyt c1 reduced 4 x H+ released into intermembrane space 2 H+ picked up from matrix