Combined effect of pore radius and protein dielectric coefficient on the selectivity of a calcium channel.

Calcium-selective ion channels often contain a selectivity filter made of similar amino acids, rich in carboxlylates, although the Ca2+ affinities of these channels range from micromolar to millimolar. To understand the physical mechanism for this range of affinities, we use grand canonical Monte Carlo simulations to study the competition of Na+ and Ca2+ in the selectivity filter of a reduced model of a Ca channel. We show that Ca2+ affinity is increased dramatically when both the volume and dielectric coefficient of the protein are reduced.