Probing "ambivalent" snug-fit sites in the KcsA potassium channel using three-dimensional reference interaction site model (3D-RISM) theory

The potassium channel is highly selective for K⁺ over Na⁺, and the mechanism underlying this selectivity remains unclear. We show the three-dimensional distribution functions (3D-DFs) of small cations (Li⁺, Na⁺, and K⁺) and the free energy profile of ions inside the open selectivity filter (SF) of the KcsA channel. Our previous results [S. Phongphanphanee, N. Yoshida, S. Oiki, F. Hirata. Abstract Book of 5th International Symposium on Molecular Science of Fluctuations toward Biological Functions, P062 (2012)] indicate that the 3D-DF for K⁺ exhibits distinct peaks at the sites formed by the eight carbonyl oxygen atoms belonging to the surrounding peptide-backbone and residues (the cage site). Li⁺ has sharp distributions in the 3D-DF at the center of a quadruplex composed of four carbonyl oxygen atoms (the plane site). Na⁺ has a rather diffuse distribution throughout the SF region with peaks both in the plane and in cage sites. The results provide microscopic evidence of the phenomenological findings that Li⁺ and Na⁺ are not excluded from the SF region and that the binding affinity alone does not cause the ion selectivity of KcsA. In the present study, with an ion placed explicitly along the pore axis, the free energy profiles of the ions inside the SF were calculated; from these profiles we suggest a new mechanism for selective K⁺ permeation. According to the model, a K⁺ ion must overcome a free energy barrier that is approximately half that of Na⁺ to exit from either of the SF mouths due to the existence of an intermediate local minimum along the route for climbing the barriers.