Sodium–potassium pump

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Template:Short description Template:Infobox enzyme

File:Sodium-potassium pump.svg
Flow of ions
File:Sodium Pump.svg
Alpha and beta units

The sodium–potassium pump (sodium–potassium adenosine triphosphatase, also known as Template:Chem2-ATPase, Template:Chem2 pump, or sodium–potassium ATPase) is an enzyme (an electrogenic transmembrane ATPase) found in the membrane of all animal cells. It performs several functions in cell physiology.

The Template:Chem2-ATPase enzyme 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.[1] Thus, there is a net export of a single positive charge per pump cycle. The net effect is an extracellular concentration of sodium ions which is 5 times the intracellular concentration, and an intracellular concentration of potassium ions which is 30 times the extracellular concentration.[1]

The sodium–potassium pump was discovered in 1957 by the Danish scientist Jens Christian Skou, who was awarded a Nobel Prize for his work in 1997. Its discovery marked an important step forward in the understanding of how ions get into and out of cells, and it has particular significance for excitable cells such as nerve cells, which depend on this pump to respond to stimuli and transmit impulses.

All mammals have four different sodium pump sub-types, or isoforms. Each has unique properties and tissue expression patterns.[2] This enzyme belongs to the family of P-type ATPases.

Function

The Template:Chem2-ATPase helps maintain resting potential, affects transport, and regulates cellular volume.[3] It also functions as a signal transducer/integrator to regulate the MAPK pathway, reactive oxygen species (ROS), as well as intracellular calcium. In fact, all cells expend a large fraction of the ATP they produce (typically 30% and up to 70% in nerve cells) to maintain their required cytosolic Na and K concentrations.[4] For neurons, the Template:Chem2-ATPase can be responsible for up to 3/4 of the cell's energy expenditure.[5] In many types of tissue, ATP consumption by the Template:Chem2-ATPases have been related to glycolysis. This was first discovered in red blood cells (Schrier, 1966), but has later been evidenced in renal cells,[6] smooth muscles surrounding the blood vessels,[7] and cardiac Purkinje cells.[8] Recently, glycolysis has also been shown to be of particular importance for Template:Chem2-ATPase in skeletal muscles, where inhibition of glycogen breakdown (a substrate for glycolysis) leads to reduced Template:Chem2-ATPase activity and lower force production.[9][10][11]

Resting potential

File:Sodium-potassium pump and diffusion.png
The Template:Chem2-ATPase, as well as effects of diffusion of the involved ions maintain the resting potential across the membranes.

Script error: No such module "Labelled list hatnote". 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 (see Template:Slink for details). In addition, there is a short-circuit channel (i.e. a highly K-permeable ion channel) for potassium in the membrane, thus the voltage across the plasma membrane is close to the Nernst potential of potassium.

Reversal potential

Even if both Template:Chem2 and Template:Chem2 ions have the same charge, they can still have very different equilibrium potentials for both outside and/or inside concentrations. The sodium-potassium pump moves toward a nonequilibrium state with the relative concentrations of Template:Chem2 and Template:Chem2 for both inside and outside of cell. For instance, the concentration of Template:Chem2 in cytosol is 100 mM, whereas the concentration of Template:Chem2 is 10 mM. On the other hand, in extracellular space, the usual concentration range of Template:Chem2 is about 3.5-5 mM, whereas the concentration of Template:Chem2 is about 135-145 mM.Script error: No such module "Unsubst".

Transport

Export of sodium ions from the cell provides the driving force for several secondary active transporters such as membrane transport proteins, which import glucose, amino acids and other nutrients into the cell by use of the sodium ion gradient.

Another important task of the Template:Chem2-Template:Chem2 pump is to provide a Template:Chem2 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 Template:Chem2-Template:Chem2 pump, whereas, on the reabsorbing (lumenal) side, the Template:Chem2-glucose symporter uses the created Template:Chem2 gradient as a source of energy to import both Template:Chem2 and glucose, which is far more efficient than simple diffusion. Similar processes are located in the renal tubular system.

Controlling cell volume

Failure of the Template:Chem2-Template:Chem2 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 will cause the cell to swell up and lyse. The Template:Chem2-Template:Chem2 pump helps to maintain the right concentrations of ions. Furthermore, when the cell begins to swell, this automatically activates the Template:Chem2-Template:Chem2 pump because it changes the internal concentrations of Template:Chem2-Template:Chem2 to which the pump is sensitive.[12]

Functioning as signal transducer

Within the last decadeTemplate: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. For instance, a study investigated the function of Template:Chem2-ATPase in foot muscle and hepatopancreas in land snail Otala lactea by comparing the active and estivating states.[13] They concluded that reversible phosphorylation can control the same means of coordinating ATP use by this ion pump with the rates of the ATP generation by catabolic pathways in estivating O. lactea. 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.[14]

Protein-protein interactions play a very important role in Template:Chem2-Template:Chem2 pump-mediated signal transduction. For example, the Template:Chem2-Template:Chem2 pump interacts directly with Src, a non-receptor tyrosine kinase, to form a signaling receptor complex.[15] Src is initially inhibited by the Template:Chem2-Template:Chem2 pump. However, upon subsequent ouabain binding, the Src kinase domain is released and then activated. Based on this scenario, NaKtide, a peptide Src inhibitor derived from the Template:Chem2-Template:Chem2 pump, was developed as a functional ouabain–Template:Chem2-Template:Chem2 pump-mediated signal transduction.[16] Template:Chem2-Template:Chem2 pump also interacts with ankyrin, IP3R, PI3K, PLCgamma1 and cofilin.[17]

Controlling neuron activity states

The Template:Chem2-Template:Chem2 pump has been shown to control and set the intrinsic activity mode of cerebellar Purkinje neurons,[18] accessory olfactory bulb mitral cells[19] and probably other neuron types.[20] 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.[21] Indeed, a mutation in the Template:Chem2-Template:Chem2 pump causes rapid onset dystonia-parkinsonism, which has symptoms to indicate that it is a pathology of cerebellar computation.[22] Furthermore, an ouabain block of Template:Chem2-Template:Chem2 pumps in the cerebellum of a live mouse results in it displaying ataxia and dystonia.[23] Alcohol inhibits sodium–potassium pumps in the cerebellum and this is likely how it corrupts cerebellar computation and body coordination.[24][25] The distribution of the Template:Chem2-Template:Chem2 pump on myelinated axons in the human brain has been demonstrated to be along the internodal axolemma, and not within the nodal axolemma as previously thought.[26] The Template:Chem2-Template:Chem2 pump disfunction has been tied to various diseases, including epilepsy and brain malformations.[27]

Mechanism

File:0308 Sodium Potassium Pump.jpg
The sodium–potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.

Looking at the process starting from the interior of the cell:

Regulation

Endogenous

The Template:Chem2-ATPase is upregulated by cAMP.[28] Thus, substances causing an increase in cAMP upregulate the Template:Chem2-ATPase. These include the ligands of the Gs-coupled GPCRs. In contrast, substances causing a decrease in cAMP downregulate the Template:Chem2-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. Script error: No such module "Unsubst".

The Template:Chem2-ATPase is endogenously negatively regulated by the inositol pyrophosphate 5-InsP7, an intracellular signaling molecule generated by IP6K1, which relieves an autoinhibitory domain of PI3K p85α to drive endocytosis and degradation.[29]

The Template:Chem2-ATPase is also regulated by reversible phosphorylation. Research has shown that in estivating animals, the Template:Chem2-ATPase is in the phosphorylated and low activity form. Dephosphorylation of Template:Chem2-ATPase can recover it to the high activity form.Template:R

Exogenous

The Template:Chem2-ATPase can be pharmacologically modified by administering drugs exogenously. Its expression can also be modified through hormones such as triiodothyronine, a thyroid hormone.Template:R[30]

For instance, Template:Chem2-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 [[Calcium in biology|Template:Chem2]] concentration, which is caused by Template:Chem2 release from the muscle cells' sarcoplasmic reticulum. Immediately after muscle contraction, intracellular Template:Chem2 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.

According to the Blaustein-hypothesis,[31] this carrier enzyme (Template:Chem2 exchanger, NCX) uses the Na gradient generated by the Template:Chem2-Template:Chem2 pump to remove Template:Chem2 from the intracellular space, hence slowing down the Template:Chem2-Template:Chem2 pump results in a permanently elevated Template:Chem2 level in the muscle, which may be the mechanism of the long-term inotropic effect of cardiac glycosides such as digoxin. The problem with this hypothesis is that at pharmacological concentrations of digitalis, less than 5% of Na/K-ATPase molecules – specifically the α2 isoform in heart and arterial smooth muscle (Kd = 32 nM) – are inhibited, not enough to affect the intracellular concentration of Template:Chem2. However, apart from the population of Na/K-ATPase in the plasma membrane, responsible for ion transport, there is another population in the caveolae which acts as digitalis receptor and stimulates the EGF receptor.[32][33][34][35]

Pharmacological regulation

In certain conditions such as in the case of cardiac disease, the Template:Chem2-ATPase may need to be inhibited via pharmacological means. A commonly used inhibitor used in the treatment of cardiac disease is digoxin (a cardiac glycoside) which essentially binds "to the extracellular part of enzyme i.e. that binds potassium, when it is in a phosphorylated state, to transfer potassium inside the cell"[36] After this essential binding occurs, a dephosphorylation of the alpha subunit occurs which reduces the effect of cardiac disease. It is via the inhibiting of the Template:Chem2-ATPase that sodium levels will begin to increase within the cell which ultimately increases the concentration of intracellular calcium via the sodium-calcium exchanger. This increased presence of calcium is what allows for the force of contraction to be increased. In the case of patients where the heart is not pumping hard enough to provide what is needed for the body, use of digoxin helps to temporarily overcome this.

Discovery

Template:Chem2-ATPase was proposed by Jens Christian Skou in 1957 while working as assistant professor at the Department of Physiology, University of Aarhus, Denmark. He published his work that year.[37]

In 1997, he received one-half of the Nobel Prize in Chemistry "for the first discovery of an ion-transporting enzyme, Template:Chem2-ATPase."[38]

Genes

Template:Gene2, although closely related to ATP1B1, ATP1B2, and ATP1B3, lost its function as Template:Chem2-ATPase beta subunit.[39]

Parallel evolution of cardiotonic steroid resistance in vertebrates

Several studies have detailed the evolution of cardiotonic steroid resistance of the alpha-subunit gene family of Na/K-ATPase (ATP1A) in vertebrates via amino acid substitutions most often located in the first extracellular loop domain.[40][41][42][43][44][45][46] Amino acid substitutions conferring cardiotonic steroid resistance have evolved independently many times in all major groups of tetrapods.[44] ATP1A1 has been duplicated in some groups of frogs and neofunctionlised duplicates carry the same cardiotonic steroid resistance substitutions (Q111R and N122D) found in mice, rats and other muroids.[47][40][41][42]

In insects

In Drosophila melanogaster, the alpha-subunit of Template:Chem2-ATPase has two paralogs, ATPα (ATPα1) and JYalpha (ATPα2), resulting from an ancient duplication in insects.[48] In Drosophila, ATPα1 is ubiquitously and highly expressed, whereas ATPα2 is most highly expressed in male testes and is essential for male fertility. Insects have at least one copy of both genes, and occasionally duplications. Low expression of ATPα2 has also been noted in other insects. Duplications and neofunctionalization of ATPα1 have been observed in insects that are adapted to cardiotonic steroid toxins such as cardenolides and bufadienolides.[48][49][50] Insects adapted to cardiotonic steroids typically have a number of amino acid substitutions, most often in the first extra-cellular loop of ATPα1, that confer resistance to cardiotonic steroid inhibition.[51][52]

See also

References

Template:Reflist

External links

Template:Ion pumps Template:Acid anhydride hydrolases Template:Enzymes Template:Portal bar

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