Figure 1.

Structural features of the KcsA channel and K⁺ coordination structure in the pore. (A and B) Membrane-omitted side and top views of the KcsA K⁺ channel (PDB ID 1K4C). Each monomer is a two–transmembrane segment peptide position around the pore at the axis of fourfold symmetry forming the K⁺ selective pore (green spheres). (C) High-resolution electronic density map showing the two diagonal subunits and the orientation of the carbonyl oxygen atoms to coordinate K⁺ ions. The numbers correspond to the four binding sites determined by the sequence TVGYG. (D) Antiprism and cubic cages forming the selectivity filter binding sites, the distances d₁–d₅ and heights h₁–h₄ correspond to the inter-oxygen separations described in Table 1 for several K⁺ channel structures. A and B were inspired by Doyle et al. (1998), C was modified from Zhou et al. (2001) with permission from Macmillan Publishers Ltd., and D was inspired by Chen et al. (2014).