Figure 6. Permeation model, parameters and permeation features for the wild-type and mutant channels.

(a) Left panel: The eleven-state permeation diagram for K⁺ (green circle) and water (red circle) permeation through a Kir2.1 channel (left side, intracellular). Each green channel cartoon represents a K⁺-water-occupied state in the selectivity filter and cytoplasmic pore. Arrows denote states transitions, which accompany shift movements of K⁺-water columns. Blue and magenta arrows indicate transitions for the efflux and influx, respectively. Curved lines represent K⁺ entering from either side to the cavity or the selectivity filter, while curved arrows represent K⁺ exiting from them. k1 through k10 represent the rate constants for the state transitions. Right panel: Cyclic paths on the permeation diagram. Each cycle was drawn by connecting states, and by completing a cycle, a net transport of K⁺ and water occurred. In each cycle, the ratio of the coupled movements of water (red number) and K⁺ (green number) molecules is indicated. For example, cycle a exhibits a coupling ratio of 1:1 and cycle c, 2:1. (b) The affected path and the degree of modification of rate constants in the mutant channel. Solid arrows indicate transition paths with increased rate constants, whereas broken arrows indicate those with decreased rate constants.