This enzyme belongs to the family of oxidoreductases, specifically those acting on diphenols and related substances as donor with a cytochrome as acceptor. The systematic name of this enzyme class is ubiquinol:ferricytochrome-c oxidoreductase. Other names in common use include 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, and QH2:cytochrome c oxidoreductase. This enzyme participates in oxidative phosphorylation. It has 4 cofactors: cytochrome c1, Cytochrome b-562, Cytochrome b-566, and 2-Iron ferredoxin.
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 . 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 (2Fe•2S).
concerted As of late 2007, 35 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , and .
The reaction mechanism for complex III (Cytochrome bc1 , Coenzyme Q: Cytochrome C Oxidoreductase) is named the Q cycle or the ubiquinone cycle. 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 BL Heme.
4. Cytocrome c1 gives its then transfers its electron to a water soluble cytochrome c (not to be confused with cytochrome c1 which is membrane bound)and the BL Heme transfers its electron to a nearby ubiquinone turning the ubiquinone into a ubisemiquinone.
5. Cytochrome c diffuses and the fully oxidized ubiquinol 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 gives its 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 to ubiquinone and ubiquinol is released.