FIGURE 11-13 Operational model of the plasma-membrane Na⁺/K⁺ ATPase. Only one of the two catalytic α subunits of this P-class pump is depicted. It is not known whether just one or both subunits in a single ATPase molecule transport ions. Ion pumping by the Na⁺/K⁺ ATPase involves phosphorylation, dephosphorylation, and conformational changes similar to those in the muscle Ca²⁺ ATPase (see Figure 11-11). In this case, hydrolysis of the E2–P intermediate powers the E2 → E1 conformational change and concomitant transport of two K⁺ ions inward. Na⁺ ions are indicated by red circles; K⁺ ions, by purple squares; high-energy acyl phosphate bond, by ~P; low-energy phosphoester bond, by –P.