Q cycle

The Q cycle (named for CoQ10) describes a series of reactions that describe how the sequential oxidation and reduction of the lipophilic electron carrier, Coenzyme Q10 (CoQ10), between the ubiquinol and ubiquinone forms, can result in the net movement of protons across a lipid bilayer (in the case of the mitochondria, the inner ).

The Q cycle was first proposed by Peter D. Mitchell, though a modified version of Mitchell's original scheme is now accepted as the mechanism by which Complex III moves protons (i.e. how complex III contributes to the biochemical generation of the proton or pH, gradient, which is used for the biochemical generation of ATP).

To summarize, the first reaction of Q cycle is:

Then the second reaction of the cycle involves the reduction of the transient semiquinone by another electron to give CoQH₂:

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

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

Ubiquinol (QH₂) binds to the Q_o site of complex III via hydrogen bonding to His182 of the Rieske iron-sulfur protein and Glu272 of . Ubiquinone (Q), in turn, binds the Q_i site of complex III. Ubiquinol is divergently oxidized (gives up one electron each) to the Rieske iron-sulfur '(FeS) protein' and to the b_Lheme. This oxidation reaction produces a transient semiquinone before complete oxidation to ubiquinone, which then leaves the Q_o site of complex III.

Having acquired one electron from ubiquinol, the 'FeS protein' is freed from its electron donor and is able to migrate to the Cytochrome c₁ subunit. 'FeS protein' then donates its electron to Cytochrome c₁, reducing its bound heme group. The electron is from there transferred to an oxidized molecule of externally bound to complex III, which then dissociates from the complex. In addition, the reoxidation of the 'FeS protein' releases the proton bound to His181 into the intermembrane space.

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