Thermodynamic and kinetic considerations of Q-cycle mechanisms and the oxidant-induced reduction of cytochromes b.

ABSTRACT

In coenzyme Q-cycles, it is proposed that one electron from the quinol reduces the Rieske iron sulfur center (Em approximately 280 mV) and the remaining electron on the semiquinone reduces cytochrome br (Em approximately -60 mV). The Em for the two-electron oxidation of the quinol is approximately 60 mV and therefore the reduction of cytochrome bT by quinol is not favorable. As the stability constant for the dismutation of the semiquinone decreases, the calculated Em for the Q/QH. couple is lowered to values below the Em of cytochrome bT. Contemporary coenzyme Q-cycles are based on the belief that the lower value for the Em of the Q/QH. couple compared to the Em for cytochrome bT means that the semiquinone is a spontaneous reducing agent for the b-cytochrome. The analysis in the paper shows that this is not necessarily so and that neither binding sites nor ionization of the semiquinone per se alters this situation. For a Q-cycle mechanism to function, ad hoc provisions must be made to drive the otherwise unfavorable reduction of cytochrome bT by the semiquinone or for the simultaneous transfer of both electrons to cytochrome bT and cytochrome c1 (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.