Cytochrome c reductase

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Cytochrome c reductase
Cytochrome c reductase
Crystal structure
External IDs
Enzyme classification
EC, category oxidoreductase
Response type Redox reaction
Substrate reduced coenzyme Q + oxidized cytochrome c
Products oxidized coenzyme Q + reduced cytochrome c

Schematic representation of the cytochrome c reductase with the reactions.
Schematic representation of the Q cycle.

The enzyme cytochrome c reductase , more precisely coenzyme Q: cytochrome c oxidoreductase (systematic name), cytochrome bc 1 complex or complex III of the mitochondrial respiratory chain , is a protein complex . The enzyme functions as an oxidoreductase ; in a coupled reaction ( Q cycle ) it catalyzes the oxidation of coenzyme Q with the reduction of cytochrome c and the translocation of protons from the matrix space (M) into the intermembrane space (IM). Mutations in the MT-CYB - gene are rare hereditary diseases such as myopathy and optic neuropathy type liver (LHON) cause.


Mitochondrial cytochrome c reductase is a transmembrane protein consisting of eleven subunits. The minimally functional complex consists of only three essential catalytic subunits with four cofactors :

  • Cytochrome b with two heme b ( b L and b H )
  • Cytochrome c 1 with a heme c
  • Rieske (ISP iron-sulfur protein engl. Iron sulfur protein) with a [2Fe · 2S] center


The function of cytochrome c reductase consists in that

  • Transfer of electrons from coenzyme Q (two-electron carrier) to cytochrome c (one-electron carrier)
  • Transport of protons by means of coenzyme Q (two-proton carrier) from the electronegative side of the membrane (matrix space, M) to the electropositive side of the membrane (intermembrane space, IM)

The electron transfer and proton translocation are the cytochrome c reductase via the so-called Q-cycle (Q for engl. Coupled Quinone).

The enzyme has two binding or reaction centers (Q o and Q i ) for the substrate coenzyme Q:

  • Q o or Q P center (o English output, P English positive site): This is where reduced coenzyme Q (ubihydroquinone, QH 2 ) binds and is oxidized to ubiquinone (Q).
  • Q i - Q or N -Zentrum (i . English input, N . Engl negative site): Here binds oxidized coenzyme Q (ubiquinone, Q) and becomes Ubihydrochinon (QH 2 reduced).

The following reactions take place at these two reaction centers:

A ubihydroquinone binds to the Q o center. Ubihydroquinone (QH 2 ) transfers an electron via the FeS center and heme c 1 to the heme group of a bound cytochrome c . The resulting ubisemiquinon (Q · - ) at the Q o center transfers another electron via heme b L to heme b H and releases two protons to the positively charged membrane side (P). The resulting ubiquinone (Q) now leaves the binding site. This half cycle is now run through a second time.

Q o center: 2 × (QH 2 → Q + 2 e - + 2 H + P )

An ubiquinone binds to the Q i center. Ubiquinone (Q) accepts an electron via heme b L , and heme b H on. The resulting ubisemiquinon (Q · - ) takes on another electron via heme b L and heme b H and two protons from the negatively charged membrane side (N). The resulting ubihydroquinone (QH 2 ) now leaves the binding site. This means that one ubiquinone is regenerated per cycle.

Q i center: Q + 2 e - + 2 H + N → QH 2

In the course of a complete Q cycle, on the one hand two ubihydroquinones are oxidized to ubiquinone at the Q o center, on the other hand one ubihydroquinone is recovered at the Q i center. Two cytochromes c (Cyt c ) are reduced. Furthermore, two protons are withdrawn from the negatively charged side of the membrane (N) and four protons are released to the positively charged side of the membrane (P).

Sum: QH 2 + 2 Cyt c ox + 2 H + N → Q + 2 Cyt c red + 4 H + P


Alternative names


  • Mitchell, P. (1961): Coupling of phosphorylation to electron and hydrogen transfer by a chemiosmotic type of mechanism . In: Nature . Vol. 191, pp. 144-148. PMID 13771349
  • Mitchell, P. (1975): Protonmotive redox mechanism of the cytochrome bc1 complex in the respiratory chain: protonmotive ubiquinone cycle . In: FEBS Lett . Vol. 56, pp. 1-6. PMID 239860

See also

Other complexes of the respiratory chain:

Web links