Thermodynamic and kinetic considerations of Q-cycle mechanisms and the oxidant-induced reduction of cytochromesb

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Abstract
In coenzyme Q-cycles, it is proposed that one electron from the quinol reduces the Rieske iron sulfur center (E m∼280 mV) and the remaining electron on the semiquinone reduces cytochromeb T (E m∼−60 mV). TheE mfor the two-electron oxidation of the quinol is ∼60 mV and therefore the reduction of cytochromeb T by quinol is not favorable. As the stability constant for the dismutation of the semiquinone decreases, the calculatedE mfor the Q/QH• couple is lowered to values below theE mof cytochromeb T. Contemporary coenzyme Q-cycles are based on the belief that the lower value for theE mof the Q/QH• couple compared to theE mfor cytochromeb T means that the semiquinone is a spontaneous reducing agent for theb-cytochrome. The analysis in the paper shows that this is not necessarily so and that neither binding sites nor ionization of the semiquinoneper se alters this situation. For a Q-cycle mechanism to function,ad hoc provisions must be made to drive the otherwise unfavorable reduction of cytochromeb T by the semiquinone or for the simultaneous transfer of both electrons to cytochromeb T and cytochromec 1 (or the iron sulfur protein). Q-cycle mechanisms with these additional provisions can explain the observation thus far accumulated. A linear path which is functionally altered by conformational changes may also explain the data.