Figure 7. The ion selectivity filter of Na_VAb.

A. Top view of the ion selectivity filter. Symmetry-related molecules are colored white and yellow; P-helix residues are colored green. Hydrogen bonds between Thr175 and Trp179 are indicated by grey dashes. Electron-densities from F_o-F_c omit maps are contoured at 4.0 σ (blue and grey) and subtle differences can be appreciated (small arrows) (Payandeh et al., 2011). B. Side view of the selectivity filter. Glu177 (purple) interactions with Gln172, Ser178 and the backbone of Ser180 are shown in the far subunit. F_o-F_c omit map, 4.75 σ (blue); putative cations or water molecules (red spheres, Ion_EX). Electron-density around Leu176 (grey; F_o-F_c omit map at 1.75 σ) and a putative water molecule is shown (grey sphere). Na⁺-coordination sites: Site_HFS, Site_CEN and Site_IN. (Payandeh et al., 2011)