Question

You have isolated the Na/K-ATPase in a semi-purified membrane system. Given higher concentrations of Na' out and higher K inside, addition of ATP to the inside leads to increases of Nat olutside and Kt inside as expected. If instead, you add the ATP to the outside of the membrane, you don't see any change in the ion levels. Why? • ATP acts as an irreversible inhibitor of the Na/K-ATPase if it binds on the exterior side. The active site for ATP is on the cytosolic side of the membrane. ATP reacts with Nations in high concentrations, resulting in inactive products, ATP reacts with Nations, effectively lowering the Nat concentration outside the cells below the point where the Nat/K-ATPase can function.

Answer 1

The active site for ATP is on the cytosolic side of the membrane.

Therefore, addition of ATP to the outside of the membrane will not change the ion Na⁺/K⁺ levels.