Sodium-Potassium Pump

Na + / K + -ATPase
Band model of the Na + / K + -ATPase according to 3B8E
Existing structural data : 3B8E 1MO7
Mass / length primary structure	1018 + 303 + 66 amino acids
Secondary to quaternary structure	α + β + γ, multipass membrane protein
Cofactor	Mg 2+
Isoforms	α 1/2 , β 1/2 , γ 1/2
Identifier
Gene name (s)	ATP1A1 , ATP1A2 , ATP1A3 , ATP1A4
Transporter classification
TCDB	3.A.3.1.1
designation	P-ATPase
Enzyme classification
EC, category	3.6.3.9 ,  hydrolase
Response type	hydrolysis
Substrate	ATP + H 2 O + 3 Na + inside + 2 K + outside
Products	ADP + phosphate + 3 Na + outside + 2 K + inside
Occurrence
Parent taxon	Creature

The sodium-potassium-ATPase (more precisely: Na ⁺ / K ⁺ -ATPase ), also known as the sodium-potassium pump or sodium pump , is a transmembrane protein anchored in the cell membrane . The enzyme catalyses the hydrolysis of ATP ( ATPase ) the transport of sodium - ions from the cell and the transport of potassium ions into the cell against the electrochemical gradient and thus serves its maintenance.

history

The Na ⁺ / K ⁺ -ATPase was discovered in 1957 by the Danish physician Jens Christian Skou . In 1997 Skou received the Nobel Prize in Chemistry " for the discovery of the ion-transporting enzyme sodium-potassium-ATPase ".

Structure of the protein

According to the TCDB classification for membrane transport proteins, the sodium-potassium pump belongs to the family of P-type ATPases (type 2) .

The sodium-potassium-ATPase consists of one α- and one β- protein subunit , which together form a stable and functional enzyme. The α subunit consists of 10 transmembrane passages connected by intra- and extracellular loops. It hydrolyzes ATP during the reaction cycle, is phosphorylated in the area of ​​the large cytoplasmic M4-M5 loop and transports the sodium and potassium ions. That is why it is also called the catalytic unit. Four isoforms are known of the α subunit: α ₁ - α ₄ .

The β subunit is a type II glycoprotein with a single membrane passage. In the functional enzyme it is located near the transmembrane passages M7 and M10 of the α-subunit and interacts with its extracellular M7 / M8 loop M7 / M8 and with its intracellular regions. During the maturation of the protein, the β-subunit fulfills the function of a molecular chaperone by supporting the correct folding and membrane integration of the α-subunit and protecting it from degradation. It also influences the affinity with which the α-subunit binds potassium and sodium ions. Four isoforms of the β subunit are known to date: β ₁ , β ₂ , β ₃ and β _m .

A third molecule, which associates with the α and β subunits and was formerly known as the γ subunit, belongs to the protein family FXYD and modulates the affinity of the sodium pump for sodium and potassium ions as well as ATP. It also plays a role in stabilizing the enzyme. The presence of a γ subunit is not necessary for the formation of the functional enzyme.

Sodium-potassium ATPase occurs in several combinations of isoforms of the α and β subunits, which differ in distribution, affinity for cardiac glycosides, and function. The α ₁ isoenzyme type is found in all cells in humans, α ₂ and α ₃ types in nerve cells and heart muscle cells ( myocardium ). Rare mutations in the ATP1A2 - and ATP1A3 - genes can be inherited migraine , alternating hemiplegia and dystonia lead.

function

The opposite transport ( antiport ) of 3 Na ⁺ against 2 K ⁺ across the cell membrane takes place against the respective concentration gradient and in total against the electrical resting membrane potential ( electrogenic ); it is therefore twice dependent on externally supplied energy: energy-dependent / active transport . In this case, it is made available as chemical binding energy through the hydrolysis of ATP .

The phenomenon of ATP-driven transport is best studied for Na ⁺ / K ⁺ transport across the plasma membrane. Both cations are unevenly distributed in cells :

• The Na ⁺ concentration inside the cell is low (7–11 mmol / l);
• the K ⁺ concentration inside is high (120–150 mmol / l).

This vital concentration gradient is caused on the one hand by potassium channels (see blood sugar sensor system ), on the other hand by the electrogenic sodium-potassium-ATPase.

mechanism

Scheme

ATP hydrolysis and Na ⁺ / K ⁺ transport are strictly coupled (electrogenic principle):

• For each molecule of ATP , three Na ⁺ ions are transported outwards and two K ⁺ ions inwards. In the balance, a positive charge carrier is therefore withdrawn from the intracellular space. This mechanism is the driving force for the maintenance of the functional resting membrane potential, which is particularly important for nerve and muscle cells . However, it is not the charge balance of the pump alone that leads to the generation of the resting membrane potential. The high permeability of the cell membrane for K ⁺ ions, which is given by potassium channels , and the low permeability for Na ⁺ ions during the resting membrane potential are also necessary for its maintenance.
• the pumping mechanism requires conformational changes that are generated by phosphorylation of an aspartate (Asp) residue of the α-subunit.
  • During this process, three Na ⁺ ions are initially included;
  • their release to the outside takes place in exchange for two K ⁺ ions, the binding of which activates a phosphatase that dephosphorylates the Asp residue again.

Effect of cardiac glycosides

→ Main article : Cardiac glycosides

The operating principle of the sodium-potassium pump (Na ⁺ / K ⁺ -ATPase). A functional unit consisting of an α and a β chain is shown. The phosphorylation-dephosphorylation cycle of an aspartate residue, i.e. H. the driving force of the pumping process is shown schematically; this process can be blocked by vanadate and thus detected. The calcium transporter (Na ⁺ / Ca ²⁺ antiporter) shown in the upper section uses the energy of the ATP indirectly by reducing an existing Na ⁺ gradient. Digitalis derivatives block the (Na ⁺ / K ⁺ -ATPase), thus paralyzing the entire chain of effects and causing the intracellular Ca ²⁺ level to rise.

Glycosides of the digitalis group ( digoxin , digitoxin and its aglycon digitoxigenin ) and the group of the strophanthus family ( g-strophanthin (synonym: ouabain ) and k-strophanthin ) - the latter, however, only in high concentrations - block the K ⁺ conformation of the ATPase still in the phosphorylated state. In doing so, they inhibit ion transport.

This indirectly increases the concentration of intracellular Ca ²⁺ and thus the contraction of the heart muscle, because

• the transport of Ca ²⁺ depends on the Na ⁺ concentration gradient according to the antiport principle ( sodium-calcium exchanger ).
• If this is reduced, more and more Ca ²⁺ remains in the muscle cells, whereby their contraction increases.

Since the heart muscle cells of a person with heart failure contain too much calcium ( calcium overload , which leads to a reduction in contractility), it was until recently incomprehensible why a further increase in the cellular calcium content can lead to an increase in contractility. A possible explanatory hypothesis: The α ₂ - and α ₃ - isoforms of the sodium-potassium pumps, together with the sodium-calcium exchangers, are located directly above the outgrowth of the cell's calcium store ( sarcoplasmic reticulum ). This functional unit is called a plasma mosome. As a result, the local sodium or calcium concentration can only be increased by inhibiting relatively few sodium-potassium pumps through cardiac glycosides, which stimulates the sarcoplasmic reticulum to release significantly larger amounts of calcium to the contractile proteins (for example with every heartbeat) without the total concentration of sodium and calcium in the cell changing significantly. This is more likely to be regulated by the α1 isoform of the sodium-potassium pump. The plasma mosomes have already been demonstrated for nerve cells and arterial muscle cells and are probably also present in skeletal and heart muscle cells.

g-strophanthin is an endogenous cardiac glycoside that is produced by the adrenal cortex in mammals. Endogenous synthesis, oral ingestion or low-dose, slow intravenous administration of g-strophantine only lead to low plasma concentrations, which initially lead to a stimulation of the sodium-potassium pump with a resulting reduction in the cellular sodium and calcium content. This can lead to a negative inotropic effect as with a nitro preparation or to a positive inotropic effect (probably depending on the initial situation of the calcium concentration of the heart muscle cells).

K-strophanthin can also stimulate the Na-K pump, but digoxin cannot. This explains, for example, the opposing effects of strophanthin and digoxin in angina pectoris , whereby strophanthin has a positive effect on the ECG and the frequency of seizures, but digoxin is known to have a negative effect.

Other substances affecting the transporter

In addition to digitalis, ouabain can also inhibit sodium-potassium-ATPase .

Conversely, the transporter is activated by the influence of insulin or adrenaline . The former is often used in everyday clinical practice to achieve a shift in extracellular potassium to intracellularly by increasing the activity of sodium-potassium-ATPase in the event of hyperkalemia by administering insulin at the same time as glucose. However, this effect only helps for a short time in hyperkalaemia, as the potassium is only shifted intracellularly and returns to the blood after a certain time.

See also

• Membrane-bound ATPases

Web links

• Animation of the Na-K pump in relation to cell physiology
• Na-K-ATPase and active transport (English)

Individual evidence

1. ↑ Skou, JC (1957): The influence of some cations on an adenosine triphosphatase from peripheral nerves. In: Biochim. Biophys. Acta . Vol. 23, pp. 394-401. PMID 13412736
2. ↑ Information from the Nobel Foundation on the 1997 award to Jens Christian Skou (English)
3. ↑ Entry of the P-type ATPase family on the TCDB homepage . Retrieved January 4, 2014.
4. ↑ G. Blanco: N a, K-ATPase subunit heterogeneity as a mechanism for tissues-specific ion regulation. In: Seminars in Nephrology . 2005, pp. 292-303.
5. ↑ N. Pestov, N. Ahmad, T. Korneenko, et al .: Evolution of Na, K-ATPase β _m -subunit into a coregulator of transcription in placental mammals . In: Proceedings of the National Academy of Sciences . 2007, pp. 11215-11220.
6. ^ D. Jones, T. Li, E. Arystarkhova et al .: Na, K-ATPase from mice lacking the γ-subunit (FXYD2) exhibits altered Naþ affinity and decreased thermal stability. In: Journal of Biological Chemistry. 2005 pp. 19003-19011.
7. ↑ G. Scheiner-Bobis, R. Farley: Subunit requirements for expression of functional sodium pumps in yeast cells. In: Biochimica et Biophysica Acta. 1994, Aug 3; 1193 (2), pp. 226-234.
8. ↑ Orphanet: Hemiplegic Migraine
9. ↑ Orphanet: Alternating Hemiplegia of Childhood
10. ↑ Orphanet: Dystonia-Parkinsonism
11. ↑ Blaustein MP, Juhaszova M, Golovina VA, Church PJ, Stanley EF: Na / Ca exchanger and PMCA localization in neurons and astrocytes: functional implications . In: Ann. NY Acad. Sci. . 976, November 2002, pp. 356-66. PMID 12502582 .
12. ↑ Gao J, Wymore RS, Wang Y, et al. : Isoform-specific stimulation of cardiac Na / K pumps by nanomolar concentrations of glycosides . In: J. Gen. Physiol. . 119, No. 4, April 2002, pp. 297-312. PMID 11929882 . PMC 2238186 (free full text).
13. ↑ ^{a b c} Saunders R, Scheiner-Bobis G: Ouabain stimulates endothelin release and expression in human endothelial cells without inhibiting the sodium pump . In: Eur. J. Biochem. . 271, No. 5, March 2004, pp. 1054-62. PMID 15009217 .
14. ↑ Belz GG, Matthews J, Sauer U, Stern H, Schneider B: Pharmacodynamic effects of ouabain following single sublingual and intravenous doses in normal subjects . In: Eur. J. Clin. Pharmacol. . 26, No. 3, 1984, pp. 287-92. PMID 6428911 .
15. ↑ Kubicek F, Reisner T: Hypoxia tolerance in coronary heart disease under the influence of digoxin, beta-methyl-digoxin and g-strophanthin . In: Therapy of the Present . 112, No. 5, May 1973, pp. 747-9 passim. PMID 4708582 .