K+/Na+ Selectivity in Toy Cation Binding Site Models Is Determined by the 'Host'

The macroscopic ion-selective behavior of K+ channels is mediated by a multitude of physiological factors. However, considering the carbonyl-lined binding site of a conductive K+ channel as a canonical eightfold coordinated construct can be useful in understanding the principles that correlate the channel's structure with its function. We probe the effects of structure and chemical composition on the K+/Na+ selectivity provided by a variety of simplified droplet-like ion binding site models. We find that when carbonyl- and water-based models capture the qualitative structural features of the K+ channel binding site, a selective preference for K+ emerges. Thus our findings suggest that the preference for K+ over Na+ exhibited by such models is principally built-in, and is not due to a unique K+-selective property of carbonyl functional groups. This suggestion is confirmed by a general thermodynamic assessment, which provides a basis for using simplified models to study the design principles underlying the molecular evolution of K+ channels.