Further information on the Cytochrome b-c complex site, and from The Crofts lab at the University of Illinois at Urbana-Champaign.

The Crofts model for quinol oxidation at the Qo site of the cytochrome bc1 complex: bifurcated electron transfer. The cytochome bc1 complex contains an iron-sulphur protein, a cytochrome b, and a cytochrome c1 (the latter not shown). At the Qo site of the complex is the iron-sulphur centre itself, and the two haems of the b-type cytochrome. On entering it's binding site, the quinol passes an electron to the iron-sulphur centre and releases a proton. The semiquinone then moves in space, from the iron-sulphur protein to the low potential b-heme, passing an electron to the haem and releasing a second proton. Electron transfer occurs from the low-potential b-haem to the high potential b-haem, and the quinone leaves its binding site.

Subequently, the mobile head of iron-sulphur protein rotates, carrying the iron-sulphur centre away from the Qo site and towards the haem of cytochrome c1, to which it donates the electron. The overall result is that single electron has moved from the quinone pool to the cytochrome c1, but two protons have been released into the outer, aqueous phase.

The figure is adapted from graphics kindly provided by A. R. Crofts (Crofts et al. 1998).