Structure and dimensions of the MthK pore. Two of the four pore-forming subunits from the crystal structure of the MthK pore (PDB accession no. 3LDC) are shown side by side, to illustrate the transmembrane ion conduction pathway. Distances represented in the z-axis scale in subsequent computational experiments (corresponding to Figs. 5, 6, 7, 8, and 9) are shown at the left. K⁺ ions in the SF are represented by purple spheres, and selected main chain and side chain atoms are shown as indicated (red, oxygen; blue, nitrogen; gray, carbon).