The MgATP complex analogue cobalt-tetrammine-ATP [CO(NH~)~ATP] inactivates (Na' + K ')-ATPase at Despite the inactivation of (Na' + K+)-ATPase by C O ( N H~)~A T P from the low-affinity ATP binding site, there is no change in the capacity of the high-affinity ATP binding site (Kd = 0.9 pM) nor of its capability to phosphorylate the enzyme Na+-dependently. Since (Na' + K+)-ATPase is phosphorylated Na+-dependently from the high-affinity ATP binding site although the catalytic cycle is arrested in the E2 conformational state by specific modification of the low-affinity ATP binding site, it is concluded that both ATP binding sites coexist at the same time in the working sodium pump. This demonstration of interacting catalytic subunits in the El and E2 conformational states excludes the proposal that a single catalytic subunit catalyzes (Na' + K +)-transport.Active transport of sodium and potassium through cellular membranes catalyzed by (Na' + K+)-ATPase is accompanied by conformational changes of the enzyme protein [I -41. High-and low-affinity ATP binding sites and a nonhyperbolic ATP saturation curve of the catalytic activity seem to be an expression this [5 -1 I]. While binding of ATP to the high-affinity ATP binding site appears to be essential for the Correspondence to G.
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