Figure 1. Electron density at the QI site in native chicken crystals. The refined model of the complex, superimposed on a 2Fo-Fc map, where Fc and Fc are calculated from the refined model with ubiquinone and all atoms within 3.5 A of the quinone omitted to avoid phase bias, contoured at 0.9 s. The superimposed model is the refined structure of the complex, including ubiquinone with its headgroup in the center of the figure. Panels A and B show two different views related by a 90' rotation about a vertical axis.
Figure 2. A plausible model for H+ and e- transfer details for the reactions at the Qi site. The protons consumed in reduction of the quinol come from the H-bonding residues D229 and H202, which are reprotonated after dissociation of the quinol.
Figure 3. A plausible model for H+ and e- transfer details for the reactions at the Qo site. Hydroquinone binds to deprotonated E272 of cytochrome b and binds with release of a proton from H161 of the Reiske protein. After electron transfer the oxidized quinone unbinds leaving E272 and H161 protonated.