The Q cycle can be broken down into two main phases:
Phase 1: The oxidation of ubiquinol (QH2) at the Qo site of the cytochrome bc1 complex. This reaction releases two electrons; one electron is transferred to cytochrome c via the Rieske iron-sulfur protein and cytochrome c1, while the other electron is transferred to the Qi site where it reduces ubiquinone (Q) to ubiquinol (QH2). Two protons are released into the intermembrane space. Phase 2: The reduction of another molecule of ubiquinone (Q) at the Qi site, which picks up electrons and protons from the matrix to form QH2. This completes the cycle, ensuring a continuous flow of electrons and the maintenance of the proton gradient.
