: The reaction catalyzed by
EC-7.1.1.6 is achieved by the Q-cycle. The Q-cycle can be divided into two halves. In the first half, a molecule of
Plastoquinols plastoquinol transfers its electrons via bifurcation to two acceptors: the first electron is transferred to
Plastocyanin-Reduced, releasing a proton to the thylakoid lumen and forming a radical
Semiplastoquinones semiplastoquinone. The semiquinone radical donates the second electron to cytochrome b6, where it is transferred to a different
PLASTOQUINONE plastoquinone molecule, forming another
semiplastoquinone radical. This sequence of events can be summarized by the following reaction:
RXN-22952 In the second half of the cycle, another
Plastoquinols plastoquinol is processed in a similar manner. It too transfers its electrons via bifurcation to
Plastocyanin-Reduced, releasing a second proton to the thylakoid lumen and forming a radical
Semiplastoquinones semiplastoquinone. This radical transfers the second electron to cytochrome b6. However, at this point the second cycle differs from the first one, since the second electron reaches the semiquinone that formed during the first half cycle, resulting in its reduction to
Plastoquinols plastoquinolin a reaction that extracts two protons from the cytoplasmic side of the thylakoid membrane. This half cycle could be summarized as follows:
TRANS-RXN-509
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