Insight into the K channel's selectivity from binding of K, Na and water to -methylacetamide.

In potassium channels that conduct K selectively over Na, which sites are occupied by K or water and the mechanism of selectivity are unresolved questions. The combination of the energetics and the constraints imposed by the protein structure yield the selective permeation and occupancy. To gain insight into the combination of structure and energetics, we performed density functional theory (DFT) calculations of multiple -methyl acetamide (NMA) ligands binding to K and Na, relative to hydrated K and Na. NMA is an analogue of the amino acid backbone and provides the carbonyl binding to the ions that occurs in most binding sites of the K channel. Unconstrained optimal structures are obtained through geometry optimization calculations of the NMA ligand binding. The complexes formed by 8 NMA binding to the cations have the O atoms positioned in nearly identical locations as the O atoms in the selectivity filter. The transfer free energies between bulk water and K or Na bound to 8 NMA are almost identical, implying there is no selectivity by a single site. For water optimized with 8 NMA, binding is weak and O atoms are not positioned as in the K channel selectivity filter, suggesting that the ions are much more favored than water. Optimal structures of 8 NMA binding with two cations (K or Na) are stable and have lower binding free energy than the optimal structures with just one cation. However, in the Na case, the optimal structure deforms and does not match the K channel; that is, two bound Na are destabilizing. In contrast, the two K structure is stabilized and the selectivity free energy favors K. Overall, this study shows that binding site occupancy and the mechanism for K selectivity involves multiple K binding in multiple neighboring layers or sites of the K channel selectivity filter.