Figure 5. Selectivity Filter of Na_VAb.

(a) Close-up view of the extracellular entrance of Na_VAb-I217C with the high-strength-field site, Glu177, highlighted in yellow [9]. (b) Side view of the Na_VAb-I217C selectivity filter showing Glu177 (yellow) and the backbone carbonyls of Leu176 and Thr175, all of which are involved in selecting and permeating partially hydrated sodium ions. (c) Superposition of Na_VAb and a K⁺-channel (PDB code 1K4C) selectivity filter. The structural alignment, which is based on the common P-helices of the two channels, highlights the significant differences in the width of the two selectivity filters. (d) Different conformational states of the four high-strength-field site Glu177 residues captured in equilibrium molecular dynamics simulation of Na_VAb in a hydrated lipid bilayer with Na⁺ moving in and out of the pore [40]. The side chains of Glu177 point either out toward the mouth of the selectivity filter or into the lumen (0-4 indicate the number of Glu side chains dunked; Na⁺ ions not shown). (e) Axial distribution of Na⁺ atoms in the selectivity filter and central cavity of Na_VAb. Three distinguishable states are highlighted in which Na⁺ is directly bound to Glu177 (green), to both Glu177 and the backbone carbonyl of Leu176 (yellow), or to neither (brown). (d) and (e) are adapted from reference [40].