Figure 7.

a) Illustration of a catalytically driven shift ratchet. The clear implication is that the slope of the potential dictates the direction of motion. For a system in which the flipping between the two potentials is accomplished externally by, for example, an applied electric field, this is in fact the case. b) However, when the flipping between the two potentials is mediated by the binding of substrate and release of product in a catalytic process, the rate constants are constrained by microscopic reversibility and we see that the direction of motion is governed not by the slopes of the potentials but by the θ dependence of the chemical specificities.^[28]