Ion channel selectivity through stepwise changes in binding affinity.

Voltage-gated Ca2+ channels select Ca2+ over competing, more abundant ions by means of a high affinity binding site in the pore. The maximum off rate from this site is approximately 1,000x slower than observed Ca2+ current. Various theories that explain how high Ca2+ current can pass through such a sticky pore all assume that flux occurs from a condition in which the pore's affinity for Ca2+ transiently decreases because of ion interactions. Here, we use rate theory calculations to demonstrate a different mechanism that requires no transient changes in affinity to quantitatively reproduce observed Ca2+ channel behavior. The model pore has a single high affinity Ca2+ binding site flanked by a low affinity site on either side; ions permeate in single file without repulsive interactions. The low affinity sites provide steps of potential energy that speed the exit of a Ca2+ ion off the selectivity site, just as potential energy steps accelerate other chemical reactions. The steps could be provided by weak binding in the nonselective vestibules that appear to be a general feature of ion channels, by specific protein structures in a long pore, or by stepwise rehydration of a permeating ion. The previous ion-interaction models and this stepwise permeation model demonstrate two general mechanisms, which might well work together, to simultaneously generate high flux and high selectivity in single file pores.