Q cycle

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The Q cycle is part of oxidative phosphorylation in mitochondria. The letter Q stands for Coenzyme Q, a mobile electron transporter between complex I and III, or II and III of the electron transport chain. Both at complex I or complex II, two electrons and two protons are passed to Coenzyme Q reducing it to ubiquinol. During this process, complex I pumps another 4 protons from the matrix into the intermembral space. In contrast, complex II is not directly involved in proton pumping but is part of the citric acid cycle in the matrix. The ubiquinol travels next to complex III and binds to it. Hereby, it passes one electron to cytochrome c and one electron to a ubiquinone, which is also part of complex III. By this process the ubiquinol is oxidized to ubiquinone again and detaches complex III. It can reenter phosphorylative oxidation. Cytochrome c, just having been reduced, also detaches from complex III and travels to complex IV. The two protons from ubiquinol are pumped into the intermembrane space. And, the attached ubiquinone is reduced to a semiquinone radical.

Because of the last step, the reaction is not fully completed. Here are three possibilities to complete it:

1. Another ubiquinol attaching to complex III, could pass one of its electrons to cytochrome c and the other to the semiquinone radical, thereby stabilizing it.
2. The semiquinone radical could pass its one electron to Oxygen thereby creating a ROS, a reactive and toxic oxygen species.
3. The semiquinone radical could be stabilized by the oxidizing action of ketones that enter mitochondria during starvation. (PNAS May 9, 2000 vol. 97 no. 10 5440-5444)

In any case, the semiquinone radical eventually takes up two protons from the matrix, reducing it fully to ubiquinol. The two protons from the oxidized ubiquinol are transferred into the intermembrane space. Oxidized ubiquinone, and the reduced cytochrome c and ubiquinol detach from complex III and reenter phosphorylative oxidation. The process is cyclic as ubiquinone can be reused again. Overall, two protons are consumed from the matrix, four protons are released into the intermembral space and two electrons are passed down to complex IV.