Figure 6.

A ratchet mechanism inspired by a paper on the bacterial flagellar motor^[52a] is formally similar to a mechanical escapement (upper right hand corner). Similar pictures have been given for many other molecular machines, including myosin moving on actin.^[51] Only the solid arrows are shown for the mechanism in the figure by Xing et al.^[52] where k_on and k_off are described as rates for composite conformational transition and proton association from the periplasm or dissociation to the cytoplasm, respectively. The figure is a trompe l’oeil that leads the naive reader to the conclusion that the slope of the potential dictates the direction of motion and other thermodynamic properties. In fact, the slope does not dictate the direction of rotation—the direction of motion is determined by selection between the pathway to the right shown by the solid arrows and the pathway to the left indicated by the dotted arrows, a selection dictated by the θ dependence of the specificities for binding/release of proton to the cytoplasm/periplasm.^[42,48]