-ATPase (sodium-potassium adenosine triphosphatase, also known as the Na+
pump or sodium–potassium pump) is an enzyme (EC 188.8.131.52) (an electrogenic transmembrane ATPase) found in the plasma membrane of all animal cells. The Na+
-ATPase enzyme is a solute pump that pumps sodium out of cells while pumping potassium into cells, both against their concentration gradients. This pumping is active (i.e. it uses energy from ATP) and is important for cell physiology. An example application is nerve conduction.
|Na+/K+ ATPase pump|
Sodium-potassium pump, E2-Pi state. Calculated hydrocarbon boundaries of the lipid bilayer are shown as blue (intracellular) and red (extracellular) planes
|PDB structures||RCSB PDB PDBe PDBsum|
It has antiporter-like activity, but since it moves both molecules against their concentration gradients it is not a true antiporter, which would require one solute to move with its gradient, not against it.
Its simple function is to pump 3 sodium ions out for every 2 potassium ions taken in and since they both have equal ionic charges, this creates an electrochemical gradient between a cell and its exterior. The protein can also let the reverse operation happen, thus harnessing the energy it has stored in the form of an electric field potential. The reverse process is, unlike the forward, spontaneous.
The sodium pump is a membrane protein that uses energy in the form of adenosine triphosphate to perform active transport of sodium ions out of the cell in exchange for potassium ions into the cell. For every adenosine triphosphate molecule that the pump uses, three sodium ions are exported, while two potassium are imported; there is hence a net export of a single positive charge per pump cycle. All mammals have four different sodium pump sub-types, or isoforms, that each has unique properties and tissue expression patterns.
This was investigated by following the passage of radioactively labeled ions across the plasma membrane of certain cells. It was found that the concentrations of sodium and potassium ions on the two sides of the membrane are interdependent, suggesting that the same carrier transports both ions. It is now known that the carrier is an ATP-ase and that it pumps three sodium ions out of the cell for every two potassium ions pumped in.
The sodium-potassium pump was discovered in the 1950s by a Danish scientist, Jens Christian Skou, who was awarded a Nobel Prize in 1997. It marked an important step forward in the understanding of how ions get into and out of cells, and it has a particular significance for excitable cells such as nervous cells, which depend on this pump for responding to stimuli and transmitting impulses.
-ATPase helps maintain resting potential, effect transport, and regulate cellular volume. It also functions as a signal transducer/integrator to regulate MAPK pathway, ROS, as well as intracellular calcium. In most animal cells, the Na+
-ATPase is responsible for about 1/5 of the cell's energy expenditure. For neurons, the Na+
-ATPase can be responsible for up to 2/3 of the cell's energy expenditure.
In order to maintain the cell membrane potential, cells keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular). The sodium-potassium pump mechanism moves 3 sodium ions out and moves 2 potassium ions in, thus, in total, removing one positive charge carrier from the intracellular space. (Please see Mechanism for details).
Export of sodium from the cell provides the driving force for several secondary active transporters membrane transport proteins, which import glucose, amino acids, and other nutrients into the cell by use of the sodium gradient.
Another important task of the Na+
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 reabsorbing cell on the blood (interstitial fluid) side via the Na+
pump, whereas, on the reabsorbing (lumenal) 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.
Controlling cell volumeEdit
Failure of the Na+
pumps can result in swelling of the cell. A cell's osmolarity is the sum of the concentrations of the various ion species and many proteins and other organic compounds inside the cell. When this is higher than the osmolarity outside of the cell, water flows into the cell through osmosis. This can cause the cell to swell up and lyse. The Na+
pump helps to maintain the right concentrations of ions. Furthermore, when the cell begins to swell, this automatically activates the Na+
Functioning as signal transducerEdit
Within the last decade[when?], many independent labs have demonstrated that, in addition to the classical ion transporting, this membrane protein can also relay extracellular ouabain-binding signalling into the cell through regulation of protein tyrosine phosphorylation. The downstream signals through ouabain-triggered protein phosphorylation events include activation of the mitogen-activated protein kinase (MAPK) signal cascades, mitochondrial reactive oxygen species (ROS) production, as well as activation of phospholipase C (PLC) and inositol triphosphate (IP3) receptor (IP3R) in different intracellular compartments.
Protein-protein interactions play a very important role in Na+
pump-mediated signal transduction. For example, Na+
pump interacts directly with Src, a non-receptor tyrosine kinase, to form a signaling receptor complex. Src kinase is inhibited by Na+
pump, while, upon ouabain binding, the Src kinase domain will be released and then activated. Based on this scenario, NaKtide, a peptide Src inhibitor derived from Na+
pump, was developed as a functional ouabain-Na+
pump-mediated signal transduction. Na+
pump also interacts with ankyrin, IP3R, PI3K, PLC-gamma and cofilin.
Controlling neuron activity statesEdit
pump has been shown to control and set the intrinsic activity mode of cerebellar Purkinje neurons, accessory olfactory bulb mitral cells and probably other neuron types. This suggests that the pump might not simply be a homeostatic, "housekeeping" molecule for ionic gradients; but could be a computation element in the cerebellum and the brain. Indeed, a mutation in the Na+
pump causes rapid onset dystonia parkinsonism, which has symptoms to indicate that it is a pathology of cerebellar computation. Furthermore, an ouabain block of Na+
pumps in the cerebellum of a live mouse results in it displaying ataxia and dystonia. Alcohol inhibits sodium-potassium pumps in the cerebellum and this is likely how it corrupts cerebellar computation and body co-ordination. The distribution of the Na+
pump on myelinated axons, in human brain, was demonstrated to be along the internodal axolemma, and not within the nodal axolemma as previously thought.
- The pump, after binding ATP, binds 3 intracellular Na+
- 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.
-ATPase is upregulated by cAMP. Thus, substances causing an increase in cAMP upregulate the Na+
-ATPase. These include the ligands of the Gs-coupled GPCRs. In contrast, substances causing a decrease in cAMP downregulate the Na+
-ATPase. These include the ligands of the Gi-coupled GPCRs.
Note: Early studies indicated the opposite effect, but these were later found to be inaccurate due to additional complicating factors.
-ATPase can be pharmacologically modified by administrating drugs exogenously.
For instance, Na+
-ATPase 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.
Muscle contraction is dependent on a 100- to 10,000-times-higher-than-resting intracellular Ca2+
concentration, which is caused by Ca2+
release from the muscle cells' sarcoplasmic reticulum. Immediately after muscle contraction, intracellular Ca2+
is quickly returned to its normal concentration by a carrier enzyme in the plasma membrane, and a calcium pump in sarcoplasmic reticulum, causing the muscle to relax.
Since this carrier enzyme (Na+
translocator) uses the Na gradient generated by the Na+
pump to remove Ca2+
from the intracellular space, slowing down the Na+
pump results in a permanently elevated Ca2+
level in the muscle, which may be the mechanism of the long-term inotropic effect of cardiac glycosides such as digoxin.
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