FIGURE 7.

(A) Energy of a K⁺ ion (♦) and the same ion with the first hydration shell comprising eight water molecules (□) pulled via the Shaker locked-open double mutant Val-474-Cys/Val-476-Cys blocked by two Cd²⁺ ions. The pulling was accomplished by constraining K⁺ to the plane, which was translated normally to the pore axis. The abscissa shows the distance (d) between the plane and the K⁺ ion bound to Thr residues in the selectivity-filter sequence TVGYG. The energy values are partitioned from the structures MC-minimized at each position of the K⁺-constraining plane. The energies are given relative to the point d = 11 Å. Note that at position d = 10 Å, K⁺ contributes ∼50% to the energy barrier, and eight water molecules contribute the remaining ∼50%. (B) MC-minimized structure with hydrated K⁺ constrained at the level of d = 10 Å. In addition to electrostatic repulsion between the closely spaced cations, the system is destabilized by the unfavorable orientation of water molecules, some of which cannot avoid exposure of their hydrogen atoms to Cd²⁺ ions.