Coenzyme Q - cytochrome c reductase

Encyclopedia : C : CO : COE : Coenzyme Q - cytochrome c reductase

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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 transport chain (PDB [1KYO], PDB [1L0L] 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):

QH₂ + 2 Fe⁺³—cytochrome c + 2 H⁺_in → Q + 2 Fe⁺²—cytochrome c + 4 H⁺_out

In the process called Q cycle, two protons are consumed from the matrix, four protons are released into the inter membrane space and two electrons are passed to cytochrome c.

Structure

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 prosthetic groups in the complex are a pair of heme b (b_L and b_H), one heme c (c₁), and a two iron, two sulfur iron-sulfur cluster (2Fe•2S).

Inhibitors of complex III

Antimycin A binds to the Q_i site and inhibits the transfer of electrons in Complex III from heme b_H to oxidized Q. Myxothiazol and Stigmatellin binds to the Q_o site und inhibits the transfer of electrons from reduced QH₂ to cytochrome c.

Some have been commercialized as fungicides (the strobilurin derivates) and as anti-malaria agents (atovaquone).

Oxygen free radicals

A small fraction of electrons leave the electron tranport chain before reaching complex IV. Premature electron leakage to oxygen results in the formation of superoxide. The relevance of this otheriwse minor side reaction is that superoxide and other reactive oxygen species are thought to play a role in several pathologies, including aging. Electron leakage occurs mainly at the Q_o site and is stimulated by antimycin A. Antimycin A locks the b hemes in the reduced state by preventing their re-oxidation at the Q_i site, which in turn causes the steady state concentrations of the Q_o semiquinone to rise, the latter species reacting with oxygen to form superoxide. The effect of high membrane potential is thought to have a similar effect.

See also

• Cellular respiration
• Photosynthetic reaction centre

External links

• [Cytochrome bc₁ complex page].
• [Interactive Molecular model of Coenzyme Q—cytochrome c reductase] (Requires [MDL Chime])

 

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