Coenzyme Q - cytochrome c reductase

The Coenzyme Q - cytochrome c reductase complex, sometimes called the cytochrome bc₁ complex, and at other times Complex III, is the third complex in the electron transfer chain (PDB 1KYO (http://www.rcsb.org/pdb/cgi/explore.cgi?pid=97471034356082&page=0&pdbId=1KYO), EC 1.10.2.2). It is a transmembrane lipoprotein, and it catalyzes the reduction of cytochrome c by accepting reducing equivalents from Coenzyme Q (CoQ):

CoQH₂+ 2 Fe⁺³-cytochrome c → CoQ + 2 Fe⁺²-cytochrome c

In the process, protons are translocated across the mitochondrial membrane. Therefore, the bc1 complex is a proton pump.

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 as many as eleven subunits can be found in higher animals. The major prosthetic groups in the complex are a pair of cytochromes, the b cytochrome and the c₁ cytochrome, and a two iron, two sulfur iron-sulfur cluster.

More information can be found on the Cytochrome bc₁ complex page (http://metallo.scripps.edu/PROMISE/CYTBC1).