A defective Na+/K+ ATPase pump is isolated from a plasma membrane and studied. Careful measurements in the laboratory determine that the pump is able to hydrolyze ATP and transports 3 Na+ ions across the plasma membrane. The pump is also able to bind 2 K+ ions with high affinity. However, the pump is unable to shuttle K+ ions across the plasma membrane. Based on these observations, what is the most likely cause of the defect in this pump?

Na+/K+ ATPase pump cycles between two different forms: the phosphorylated form of the Na+/K+ ATPase has a high affinity for K+ ions and low affinity for Na+ ions. The release of phosphate from ATP and phosphorylation of Na+/K+ ATPase makes the pump to release the bound 3 Na+ ions outside the cell and to bind to the 2 K+ ions from the surroundings. As the Na+/K+ ATPase is dephosphorylated, its affinity for K+ ions is reduced leading to the release of K+ inside the cell.

A defective Na+/K+ ATPase that is able to pick and bind the K+ ions but is not able to shuttle them across the membrane should be defective to achieve its dephosphorylated form. Without releasing phosphate, the affinity of the pump for K+ ions is not reduced and the pump would not be able to release them to the opposite side of the membrane.