Figure 2.

Schematic view of the electron transfer pathways predicted by the ion-pump (Left) and Q-cycle (Right) models of proton translocation in cytochrome bo₃. Dashed arrows denote electron transfers. The numbers in the ion-pump model denote the first and second electrons transferred during the oxidation of quinol. In the ion-pump model, Q_B is assumed to be bound at all times during the reaction cycle (figure adapted from ref. 16).