Q-cycle

high-resolution terminology - matching measurements at high-resolution

Q-cycle

Description

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 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 c₁ 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.