FIGURE 20.

A model of the location of amino acids whose mutation altered ${\text{NH}}_4^{+}$ affinity (red oval), SCH28080 affinity (green oval), or changed inhibition from being purely competitive to either mixed or noncompetitive. The red ovals delineate the amino acids in the ion binding domain, the green ovals delineate amino acids whose mutation decrease APA inhibitor affinity, and the yellow stars delineate those amino acids whose mutation changes inhibition from purely competitive to either mixed or noncompetitive. These amino acids affect the ability of K⁺ to enter the ion binding site in the presence of SCH28080 hence changing competitive inhibition. The background stick molecule represents a scaled down version of the pump elements.