Q-cycle refers to the sequential oxidation and reduction of the electron carrier Coenzyme Q (CoQ or ubiquinone) in mitochondria or plastoquinones in the photosynthetic system. Originally, the concept of the Q-cycle was proposed by Peter D Mitchell. Following several modifications, the Q-cycle is established, describing how CIII translocates hydrogen ions against the protonmotive force. The reduced CoQ (ubiquinol QH2) binds to the Qo site of CIII, while the oxidized CoQ (ubiquinone Q) to the Qi site of CIII. First, QH2 reduces the iron-sulfur protein and feeds cytochrome c1 with one electron. The other electron is transferred to the bL heme and reduces the bH heme, which transfers the electron to ubiquinone at the Qi-site which is reduced to a semiquinone. A second QH2 is required to fully reduce semiquinone to ubiquinol. At the end of the Q-cycle, four protons leave the mt-matrix and enter the intermembrane space, and the reduced cytochrome c transfers electrons to CIV. The ubiquinol generated at the Qi-site can be reused by binding to the Qo-site of CIII.
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