Na
+,K
+-ATPase actively extrudes three cytoplasmic Na
+ ions in exchange for two extracellular K
+ ions for each ATP hydrolyzed. The atomic structure with bound Na
+ identifies three Na
+ sites, named I, II, and III. It has
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Na
+,K
+-ATPase actively extrudes three cytoplasmic Na
+ ions in exchange for two extracellular K
+ ions for each ATP hydrolyzed. The atomic structure with bound Na
+ identifies three Na
+ sites, named I, II, and III. It has been proposed that site III is the first to be occupied and site II last, when Na
+ binds from the cytoplasmic side. It is usually assumed that the occupation of all three Na
+ sites is obligatory for the activation of phosphoryl transfer from ATP. To obtain more insight into the individual roles of the ion-binding sites, we have analyzed a series of seven mutants with substitution of the critical ion-binding residue Ser777, which is a shared ligand between Na
+ sites I and III. Surprisingly, mutants with large and bulky substituents expected to prevent or profoundly disturb Na
+ access to sites I and III retain the ability to form a phosphoenzyme from ATP, even with increased apparent Na
+ affinity. This indicates that Na
+ binding solely at site II is sufficient to promote phosphorylation. These mutations appear to lock the membrane sector into an E
1-like configuration, allowing Na
+ but not K
+ to bind at site II, while the cytoplasmic sector undergoes conformational changes uncoupled from the membrane sector.
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