Q cycle

Combining the two equations, we have the overall reaction of Q cycle:

To summarize, the first reaction of Q cycle is:

Mechanisms are based on conformational changes of the protein structure or on the Q cycle.

This reaction occurs through the Q cycle as in Complex III.

A second Q cycle is necessary, with the second electron transfer from cytochrome b reducing the semiquinone to ubiquinol.

Mitochondrial Complex III uses this second type of proton pump, which is mediated by a quinone (the Q cycle).

Particularly important is the reduction of coenzyme Q in complex III, as a highly reactive ubisemiquinone free radical is formed as an intermediate in the Q cycle.

The Q cycle is a process that occurs in cytochrome b, a component of Complex III in the electron transport chain, and that converts ubiquinol to ubiquinone in a cyclic fashion.

If, instead of the Q cycle, one molecule of QH2 were used to directly reduce two molecules of cytochrome c, the efficiency would be halved, with only one proton transferred per cytochrome c reduced.

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

Since heme x does not appear to be required for the Q cycle and is not found in Complex III, it has been proposed that it is used for cyclic photophosphorylation by the following mechanism:

Operation of the modified Q cycle in Complex III results in the reduction of Cytochrome c, oxidation of ubiquinol to ubiquinone, and the transfer of four protons into the intermembrane space, per two-cycle process.

The ultimate products of the Q cycle are four protons entering the intermembrane space, two protons taken up from the matrix and the reduction of two molecules of cytochrome c. The reduced cytochrome c is eventually reoxidized by complex IV.

As only one of the electrons can be transferred from the QH donor to a cytochrome c acceptor at a time, the reaction mechanism of complex III is more elaborate than those of the other respiratory complexes, and occurs in two steps called the Q cycle.

A page on Proteopedia, Complex III of Electron Transport Chain, contains rotatable 3-D structures of Complex III which may be used to study the peptide structures of Complex III and the mechanism of the Q cycle.