Fig. 6.
Q-cycle within Complex III. Ubiquinol (QH2) is oxidized at the Qo site (shown in green), passing electrons to either cytochrome c (cyt c) through the [2Fe–2S] cluster within the ISP and cytochrome cl (cyt c1), or to the Qisite (shown in blue) through cyt bL and bH. As shown on the left side of the complex, ubiquinone (Q) is reduced to ubisemiquinone (SQ) at the Qi site after a single round of the Q-cycle, while a second round of the cycle reduces SQ to QH2, as shown on the right side of the complex. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)