Figure 2.

Free-energy map for the configuration of ions K⁺/K⁺/K⁺/Na⁺ (A), K⁺/K⁺/Na⁺/K⁺ (B), K⁺/Na⁺/K⁺/Na⁺ (C), and Na⁺/K⁺/K⁺/Na⁺ (D). Two-dimensional projections of the calculated 4-dimensional energy map are shown. Contour lines are drawn every 2 kcal/mol. Labels along the x/y axes refer to the average positions along the permeation pathway of the carbonyl oxygen atoms of residues Thr⁶³ (T63), Val⁶⁴ (V64), Gly⁶⁵ (G65), Asp⁶⁶ (D66), and of the side-chain oxygen atoms of residues Thr⁶³ (T63-OG). These oxygen atoms delimit the K⁺-binding sites S1–S4. Roman numerals are used to identify the different energy minima in the 2-dimensional projections.