Mechanism of the Na+/K+ Pump
Note that initially Na+ ions (yellow) are inside the cell and K+ ions (blue) are outside the cell. The cell's plasma membrane is indicated as purple. The transmembrane transport protein (red) is open to the inside and there is a binding site for Na+. The binding site for K+ is blocked when the protein is open to the inside. Once the Na+ binds to its binding site, the transport protein can react with ATP and the transport protein is 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 Na+ leaves its binding site and the binding site changes so that it can no longer bind Na+. At the same time, the K+ ion binding site opens so that K+ is now free to bind. The binding of the K+ causes the covalently attached phosphate to fall off the protein. The removal of the phosphate causes another protein conformational change such that the ion binding sites are open to the inside again. The K+ ion leaves its binding site and enters the cell. Once the K+ leaves the binding site, the K+ site closes and the Na+ site opens again. 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. Click on the animation to see a larger version of the animation.