Cytochrome BC1 Complex - Coenzyme Q : Cytochrome C-Oxidoreductase
In vertebrates, Cytochrome bc1 complex is a multisubunit transmembrane protein encoded by both the mitochondrial (cytochrome b) and the nuclear genomes (all other subunits). The bc1 complex contains 11 subunits, 3 respiratory subunits (cytochrome B, cytochrome C1, Rieske protein), 2 core proteins and 6 low-molecular weight proteins.
The cytochrome bc1 complex contributes to the proton gradient by an asymmetric absorption/release of protons (electron transfer chain (ETC) complex III). Complex III is present in the mitochondria of all animals and all aerobic eukaryotes and the inner membranes of most eubacteria.
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, 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.