Question

Describe how the Na⁺ –K⁺ ATPase manages to cycle through its various states unidirectionally (pumping 3Na⁺ ions out of and 2K⁺ ions into the cell per cycle) and how the pump discriminates between Na⁺ and K⁺ ions (which bind to the same site!).

Answer 1

Na⁺/K⁺-ATPase (sodium-potassiumadenosine triphosphatase, also known as the Na⁺/K⁺ pump or sodium–potassium pump) is an enzyme (an electrogenic trans membrane ATPase) found in the plasma membrane of all animal cells.

The Na+/K+ -ATPase enzyme is a solute pump that pumps potassium into cells while pumping sodium out of cells, both against their concentration gradients. This pumping is active (i.e. it uses energy from ATP). For every ATP molecule that the pump uses, three sodium ions are exported and two potassium ions are imported; there is hence a net export of a single positive charge per pump cycle.

•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