The Na+/K+ ATPase pump exports 3 Na+ for every 2 Ka+ imported. This process is ATP dependent, but I have not been able to find how many ATPs are required in each translocation. What is the stoichiometric ATP cost of this reaction?
Answer
Sodium/potassium-exchanging ATPase (EC 3.6.3.9):
An enzyme from the plasma membrane of animal cells that catalyzes the efflux of three Na(+) and influx of two K(+) per ATP hydrolyzed.
You can read about the mechanism on Wikipedia:
The pump, after binding ATP, binds 3 intracellular Na+ ions.
ATP is hydrolyzed, leading to phosphorylation of the pump at a highly conserved aspartate residue and subsequent release of ADP.
A conformational change in the pump exposes the Na+ ions to the outside. The phosphorylated form of the pump has a low affinity for Na+ ions, so they are released.
The pump binds 2 extracellular K+ ions. This causes the dephosphorylation of the pump, reverting it to its previous conformational state, transporting the K+ ions into the cell.
The unphosphorylated form of the pump has a higher affinity for Na+ ions than K+ ions, so the two bound K+ ions are released. ATP binds, and the process starts again.
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