FIGURE 4.

Convergence of the Shaker models from the x-ray-based starting structures to conformations with coordinating bonds Cys-476_S^γ—Cd²⁺—N^ɛ2_His-486. Cd²⁺ ions were constrained to S^γ_Cys-476 at the distance of 2.65 Å. Distance constraints Cd²⁺—N^ɛ2_His-486 were decreased with a step 0.5 Å from the starting values found in the x-ray-based structures. At each step, the energy was MC-minimized until the distances of ∼2.3 Å were achieved. In the plots of MC-minimized energy against the imposed distance Cd²⁺—N^ɛ2_His-486, the energy is shown relative to the lowest-energy structure found. The locked-open conformations are more preferable energetically than the starting KvAP- and MthK-based structures. KcsA-based conformation with coordinating bonds Cys-476_S^γ—Cd²⁺—N^ɛ2_His-486 has higher energy than the starting structure. Superposed MC-minimized structures are at the right of the respective energy plots. In the side and cytoplasmic views, only the inner helices are shown for clarity. Cys and His are shown as sticks and Cd²⁺ ions as magenta spheres. Locking open the KvAP-based structure is energetically preferable and requires minimal structural deformations.