Q cycle

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In biochemistry, the Q cycle (from Q uinone) usually refers to a sequence of redox reactions involving ubiquinone (Q) or ubihydroquinone (QH 2 ) and cytochrome c in complex III of the respiratory chain . A similar cycle occurs with plastoquinone (PQ) on the cytochrome b 6 f complex in plants, algae and cyanobacteria . This fulfills the same function as complex III for the equivalent of the respiratory chain, photosynthesis .

function

Schematic representation of the Q cycle in complex III of the respiratory chain.

Ubiquinone serves in the respiratory chain as a mobile electron carrier between complex I or complex II and complex III in the inner membrane of mitochondria . The ubiquinone is reduced to ubihydroquinone by complex I / II, taking up two electrons and two protons . The ubihydroquinone can now diffuse in the membrane to complex III in order to pass on its two captured electrons. For this purpose, ubihydroquinone binds to the binding site designated as Q o in complex III, which faces the space between the inner and outer membrane (IM). Cytochrome c, mediated by the Rieske protein containing an iron-sulfur center , and cytochrome c 1 , which are components of complex III, serve as the electron acceptor in the membrane .

However, cytochrome c can only accept one electron, so QH 2 is initially only oxidized to the free radical Ubisemiquinon (QH • ). However, this is unstable and immediately releases the second electron via the proteins cytochrome b L and cytochrome b H of the complex to another ubiquinone, which is bound to the inner, matrix-side (M) binding site Q i . This is reduced to the radical Ubisemichinon (QH • ). A newly added ubihydroquinone at the Q o position now releases its protons to the intermembrane space and is again in its oxidized form. It diffuses from the Q o position and enters the ubiquinone pool .

In a second step, another ubihydroquinone binds to the Q o site and is oxidized analogously to ubiquinone, an electron being passed on to cytochrome c and two protons being transferred into the intermembrane space. The ubisemiquinone at the Q i site is reduced to hydroquinone by binding two protons from the matrix, is replaced by oxidized quinone and can now bind to the Q o site itself .

In this cyclical process, a total of two electrons are passed between ubihydroquinone and cytochrome c, two protons are removed from the matrix and four protons are released into the intermembrane space. This creates a chemiosmotic membrane potential between the matrix and the intermembrane space in the mitochondrion.

Formation of the superoxide radical

If the flow of the second electron to the Q i site is blocked, experimentally for example by antimycin A , QH • stays longer at the Q o site. It will auto-oxidation instead, the freed electron is taken by an oxygen molecule and so the free radical superoxide (O 2 • - ) is formed. This suggests that more superoxide is released into the membrane space at complex III, which was confirmed for mitoplasts without an outer membrane. However, reactive oxygen species can also be formed on the matrix side , especially when there is little electron transfer into the Q o center. The relative production rates and conditions under physiological conditions are unclear.

history

It used to be assumed that the electrons from the membrane-bound enzyme complexes of the respiratory chain and the interposed mobile electron carriers (ubiquinone and cytochrome c) were passed on to the oxygen in a cascade manner. This image of a straight electron flow began to change in the mid-1960s, when the later Nobel Prize winner Peter D. Mitchell came to the conclusion, initially on the basis of mathematical considerations, that ubiquinone only passed on one bound electrons instead of the two originally assumed.

This theory was substantiated when the group around Hans Reichenbach (GBF Braunschweig) isolated the inhibitor myxothiazole from the myxobacterium Myxococcus fulvus in the 1970s . With this, Gebhard von Jagow (University of Munich) succeeded in blocking the respiratory chain exactly where the electrons from the ubiquinone flow in the direction of oxygen; the second way could be sealed off with the antibiotic antimycin A.

The respiratory chain branches out at the ubiquinone, whereby part of the energy of the electron that is passed on in a linear manner is sufficient to "spin back" the second electron into the ubiquinone reservoir. The flow of energy does not correspond to a "waterfall", but rather "barrages" are switched on, which enable a higher energy yield .

This "electron vortex" on ubiquinone is not a specialty of mammalian mitochondria: the flow of electrons is also divided in the chloroplasts of green plants and the respiratory chain of bacteria. This economical use of energy was established early on in evolution.

literature

See also

supporting documents

  1. ↑ Baniulis et al (2008): Structure-function of the cytochrome b6f complex . In: Photochem Photobiol 84 (6) pp. 1349-1358, PMID 19067956
  2. ↑ a b c d David F. Stowe and Amadou KS Camara: Mitochondrial Reactive Oxygen Species Production in Excitable Cells: Modulators of Mitochondrial and Cell Function (Review). Antioxidants & Redox Signaling , Volume 11, Number 6, 2009, pp. 1381-84. PMID 19187004