Fig. 8

A proposed model for potassium-selectivity in CPA1s. Side view (a, b), top view (c, d), and schematic two-dimensional projection (e, f) of the binding site of the Na⁺-selective PaNhaP (PDB id 4CZA). a, c, and e are the wild-type structure and b, d, and f are a model structure of a proposed PaNhaP mutant, which we suggest to be K⁺-selective. Residues that participate in ion coordination are shown as sticks. The positions where we suggest mutations are in bold. A thallium ion (bronze) and a water molecule (red) are shown in the binding site. e vs. f The suggested additional coordinating interaction between the protein and the ion and the slight increase in the pore diameter resulting from the A128S (light-blue) and P131A (light-yellow) mutations, respectively, are shown. The A128S substitution is expected to provide another polar interaction with the ion, consistent with the expected larger coordination number of potassium ions compared to sodium. The P131A substitution is expected to slightly increase the cavity to accommodate the larger potassium ions a vs. b