Evidence for a mobile semiquinone in the redox cycle of the mammalian cytochrome bc1 complex

Experimental evidence is presented to demonstrate that cytochromes b of the mammalian cytochrome bc1 complex may be rapidly oxidised by a pulse of oxidising equivalents which react with cytochrome c1, even when all cytochrome b is fully reduced before the pulse. The oxidation is sensitive both to antimycin and to myxothiazol. Such behaviour is inconsistent with models in which only the fully oxidised ubiquinone may move between the centres 'o' and 'i' of the complex. It is proposed that the charged semiquinone (Q-) may move between these centres, which may constitute separate reaction domains of a single ubiquinone-binding site. The bearing of this on the mechanism of electron, proton and charge transfer in the complex is discussed.