Inactive conformation enhances binding function in physiological conditions.

Many receptors display conformational flexibility, in which the binding pocket has an open inactive conformation in the absence of ligand and a tight active conformation when bound to ligand. Here we study the bacterial adhesin FimH to address the role of the inactive conformation of the pocket for initiating binding by comparing two variants: a wild-type FimH variant that is in the inactive state when not bound to its target mannose, and an engineered activated variant that is always in the active state. Not surprisingly, activated FimH has a longer lifetime and higher affinity, and bacteria expressing activated FimH bound better in static conditions. However, bacteria expressing wild-type FimH bound better in flow. Wild-type and activated FimH demonstrated similar mechanical strength, likely because mechanical force induces the active state in wild-type FimH. However, wild-type FimH displayed a faster bond association rate than activated FimH. Moreover, the ability of different FimH variants to mediate adhesion in flow reflected the fraction of FimH in the inactive state. These results demonstrate a new model for ligand-associated conformational changes that we call the kinetic-selection model, in which ligand-binding selects the faster-binding inactive state and then induces the active state. This model predicts that in physiological conditions for cell adhesion, mechanical force will drive a nonequilibrium cycle that uses the fast binding rate of the inactive state and slow unbinding rate of the active state, for a higher effective affinity than is possible at equilibrium.