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Na+/K+-ATPase (also known as the Na+/K+ pump or sodium-potassium pump) is an enzyme (EC 18.104.22.168) located in the plasma membrane (specifically an electrogenic transmembrane ATPase). It is found in the plasma membrane of virtually every human cell and is common to all cellular life. It helps maintain cell potential and regulate cellular volume.
In order to maintain the cell potential, cells must keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular). Outside cells (extracellular), there are high concentrations of sodium and low concentrations of potassium, so diffusion occurs through ion channels in the plasma membrane. In order to keep the appropriate concentrations, the sodium-potassium pump pumps sodium out and potassium in through active transport.
The mechanism is:
- The pump, with bound 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!
As the plasma membrane is far less permeable to sodium than it is to potassium ions, an electric potential (negative intracellularly) is the eventual result.
The electrical and concentration gradient established by the sodium-potassium ATPase supports not only the cell resting potential but the action potentials of nerves and muscles. Export of sodium from the cell provides the driving force for several facilitated transporters, which import glucose, amino acids and other nutrients into the cell. Translocation of sodium from one side of an epithelium to the other side creates an osmotic gradient that drives the absorption of water.
Another important task of the Na+-K+ pump is to provide a Na+ gradient that is used by certain carrier processes. In the gut, for example, sodium is transported out of the resorbing cell on the blood side via the Na+-K+ pump, whereas, on the resorbing side, the Na+-Glucose symporter uses the created Na+ gradient as a source of energy to import both Na+ and Glucose, which is far more efficient than simple diffusion. Similar processes are located in the renal tubular system.
The Na+-K+ pump found in the membrane of heart cells is an important target of cardiac glycosides (for example digoxin and ouabain), inotropic drugs used to improve heart performance by increasing its force of contraction.
Contraction of any muscle is dependent on a 100- to 10,000-times higher-than-resting intracellular Ca2+ concentration, which, as soon as it is put back again on its normal level by a carrier enzyme in the plasma membrane, and a calcium pump in sarcoplasmic reticulum, muscle relaxes.
Since this carrier enzyme (Na+-Ca2+ translocator) uses the Na gradient generated by the Na+-K+ pump to remove Ca2+ from the intracellular space, slowing down the Na+-K+ pump results in a permanently-higher Ca2+ level in the muscle, which will eventually lead to stronger contractions.
- Alpha: ATP1A1, ATP1A2, ATP1A3, ATP1A4. #1 predominates in kidney. #2 is also known as "alpha(+)"
- Beta: ATP1B1, ATP1B2, ATP1B3, ATP1B4
- ↑ Skou J (1957). The influence of some cations on an adenosine triphosphatase from peripheral nerves.. Biochim Biophys Acta 23 (2): 394-401. PMID 13412736.
- ↑ http://nobelprize.org/chemistry/laureates/1997/index.html
A pdf copy of the paper (reference 1) appears on http://jasn.asnjournals.org/cgi/reprint/9/11/2170.pdf
Acid anhydride hydrolases: ATPases (EC 3.6.3-3.6.4)
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