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• Q-cycle  + ('''Q-cycle''' refers to the sequential oxi … '''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 [[Mitchell P|Peter D Mitchell]]. Following several modifications, the Q-cycle is established, describing how [[CIII]] translocates hydrogen ions against the protonmotive force. The reduced CoQ ([[quinol |ubiquinol]] QH₂) binds to the Q_o site of CIII, while the oxidized CoQ ([[ubiquinone]] Q) to the Q_i site of CIII. First, QH₂ reduces the iron-sulfur protein and feeds cytochrome ''c''₁ with one electron. The other electron is transferred to the ''b''_L heme and reduces the ''b''_H heme, which transfers the electron to ubiquinone at the Q_i-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 Q_i-site can be reused by binding to the Q_o-site of CIII.chrome ''c'' transfers electrons to CIV. The ubiquinol generated at the Q_i-site can be reused by binding to the Q_o-site of CIII.)

• Q-cycle  +