Fig. 8.

Schematic representation of Na⁺ coordination at the Na2 site. (Upper) Na⁺ coordination at the Na2 site of WT NIS, when the Na2 (blue box); Na2 and Na1 (red box); and Na2, Na1, and I⁻ (black box) sites are occupied. (Lower) Na⁺ coordination at the Na2 site of NIS mutants, with all three ions bound. Blue circles show substituted amino acids. Residual activity is given in parentheses as % of WT NIS activity. Double red lines represent distances ≤2.5 Å; green lines, distances >2.6 and ≤3.9 Å; and violet lines, distances ≥4 Å. Ep and E⁻, respectively, refer to the protonated and nonprotonated forms of glutamate.