Q cycle

The Q - cycle (of English. Quinone ) referred to in biochemistry usually a result of redox reactions with the participation of ubiquinone (Q ) or Ubihydrochinon ( QH2 ) and cytochrome c in the complex III of the respiratory chain. A similar cycle occurs in plants, algae and cyanobacteria with plastoquinone (PQ ) on the cytochrome b6f complex. This met for the counterpart of the respiratory chain, photosynthesis, the same function as complex III.


Ubiquinone used in the respiratory chain as an electron carrier between the movable complex I or complex II and III complex in the inner membrane of mitochondria. The ubiquinone is reduced by complex I / II, receiving two electrons and two protons to Ubihydrochinon. The Ubihydrochinon is now in the membrane to complex III diffuse his two captured electrons to pass. Ubihydrochinon binds thereto to the called Qo binding site in the complex III, which faces the space between the inner and outer membrane (IM). In the membrane serves as an electron cytochrome C mediated by the containing an iron -sulfur center Rieske protein and cytochrome c1, which are components of complex III.

Cytochrome c, however, can accommodate only one electron, therefore, is first QH2 only on the free radical Ubisemichinon (QH • ) oxidized. This, however, is unstable and is the second electron from now about the proteins cytochrome bL and cytochrome bH of the complex to another ubiquinone, which is bonded to the inner side matrix (M) binding site Qi. This is reduced to the radical Ubisemichinon (QH • ). The resulting at Qo site Ubihydrochinon are now its protons to the intermembrane space from and is again present in its oxidized form. It diffuses from the Qo site and is included in the Ubichinonpool.

In a second step, another binds Ubihydrochinon at the Qo - site and is oxidized analogously to ubiquinone, again using an electron to cytochrome c is passed and two protons are transferred to the intermembrane space. The Ubisemichinon at the Qi site is here reduced under binding of two protons from the matrix to hydroquinone, is replaced by oxidized quinone and is now able to bind to the Qo site.

In this cyclical process that is, two electrons between cytochrome c and Ubihydrochinon be passed and thereby taken from two protons from the matrix and released four protons in the intermembrane space in total. Characterized a proton gradient between the matrix and the inter- membrane space is constructed in the mitochondrion.

Emergence of the radical superoxide

The flow of the second electron to Qi - point blocked experimentally as by antimycin A, QH • retain at Qo site. It takes place auto-oxidation, the liberated electron is captured by an oxygen molecule and so the free radical superoxide (O2 • - ) formed. This suggests that in more complex III superoxide is discharged into the diaphragm gap, which has been confirmed for mitoplasts without outer membrane. However, other matrix- reactive oxygen species may be formed, especially in low electron transfer in the Qo center. The relative production rates and conditions under physiological conditions are unknown.


Previously it was assumed that the electron ( c ubiquinone and cytochrome ) would cascade forwarded to the oxygen of the membrane-bound enzyme complexes of the respiratory chain and the intermediate movable Elektronenübertägern. This image of a rectilinear electron flow began to change in the mid- sixties, when the future Nobel Prize winner Peter D. Mitchell, arrived first due to computational considerations, to the conviction that is passed from ubiquinone only one instead of the originally adopted two bound electrons.

This theory was supported, as the group around Hans Reichenbach (GBF Braunschweig) in the seventies, the inhibitor myxothiazol in the myxobacterium Myxococcus fulvus isolated. They succeeded Gebhard von Jagow (University of Munich ), to block the respiratory chain exactly where the electrons from the ubiquinone continue to flow in the direction of oxygen; the second way could be sealed off with the antibiotic antimycin.

The respiratory chain that is branched at the ubiquinone, a portion of the energy of the linear electron -handoff sufficient " zurückzustrudeln " second electron in the ubiquinone reservoir. The flow of energy does not correspond to a "waterfall", but there are " barrages " turned on, allow a higher energy yield.

This " electron vortex " at the ubiquinone is not a specialty of mammalian mitochondria in the chloroplasts of green plants and of the respiratory chain of bacteria divides the flow of electrons. This economic use of energy has therefore prevailed early in evolution.