Mechanism of the Na+/K+ Pump
Note that initially Na+ ions (orange) are inside the cell and K+ ions (blue) are outside the cell. The cell's plasma membrane is indicated as black. The transmembrane transport protein (yellow) is open to the inside. Once three Na+ ions enter the transport protein and bind to their binding sites, the transport protein can react with ATP and the transport protein becomes phosphorylated. The phosphorylated transport protein has an inorganic phosphate ion covalently attached to the protein. The phosphorylation of the transport protein causes a conformational change in the transport protein such that the ion binding sites are now open to the outside of the cell. The three Na+ ions leave their binding sites and the binding sites change so that they are less likely to bind Na+. At the same time, the K+ ion binding sites become exposed so that K+ ions are free to bind. The binding of two K+ ions causes the hydrolysis of the covalently attached phosphate such that the phosphate is removed from the transport protein. The removal of the phosphate causes another protein conformational change such that the ion binding sites are again open to the inside of the cell. The K+ ions leave their binding sites and enter the cell. The transport protein is now ready for another transport cycle. The Na/K pump is an active transport system. The energy for the active transport comes from the ATP molecule which is used to phosphorylate the transporter. Thus, to function, this pump requires a continuous source of ATP. Note that for every ATP molecule used, three Na+ ions are pumped out of the cell and 2 K+ ions are pumped into the cell.
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