Hydrogen potassium ATPase
ATPase, H+/K+ exchanging, alpha polypeptide | |||||||
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Identifiers | |||||||
Symbol | ATP4A | ||||||
Chr. 19 q13.1 | |||||||
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ATPase, H+/K+ exchanging, beta polypeptide | |||||||
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Identifiers | |||||||
Symbol | ATP4B | ||||||
Chr. 13 q34 | |||||||
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Gastric hydrogen potassium ATPase, also known as H+/K+ ATPase, is an enzyme which functions to acidify the stomach.[1] It is a member of the P-type ATPases, also known as E1-E2 ATPases due to its two states.[2]
Biological function and location
The gastric hydrogen potassium ATPase or H+/K+ ATPase is the proton pump of the stomach. It exchanges potassium from the intestinal lumen with cytoplasmic hydronium[2] and is the enzyme primarily responsible for the acidification of the stomach contents and the activation of the digestive enzyme pepsin[3] (see gastric acid).
The H+/K+ ATPase is found in
Genes and protein structure
The H+/K+ ATPase is a heterodimeric protein, the product of 2 genes. The gene ATP4A[4] encodes the H+/K+ ATPase α subunit, and is a ~1000-amino acid protein that contains the catalytic sites of the enzyme and forms the pore through the cell membrane that allows the transport of ions. Hydronium ions bind to two active sites present in the α subunit.[5] The α subunit also has a phosphorylation site (Asp385).[6] The gene ATP4B[7] encodes the β subunit of the H+/K+ ATPase, which is a ~300-amino acid protein with a 36-amino acid N-terminal cytoplasmic domain, a single transmembrane domain, and a highly glycosylated extracellular domain.
The H+/K+ ATPase β subunit stabilizes the H+/K+ ATPase α subunit and is required for function of the enzyme. The β subunit prevents the pump from running in reverse,[8] and it also appears to contain signals that direct the heterodimer to membrane destinations within the cell, although some of these signals are subordinate to signals found in H+/K+ ATPase α subunit.
The structure of H+/K+ ATPase has been determined for humans, dogs, hogs, rats, and rabbits and is 98% homologous across all species.[2]
Enzyme mechanism and activity
H+/K+ ATPase is a P2-type ATPase, a member of the eukaryotic class of P-type ATPases.[9] Like the Ca2+ and the Na+/K+ ATPases, the H+/K+ ATPase functions as an α, β protomer.[10] Unlike other eukaryotic ATPases, the H+/K+ ATPase is electroneutral, transporting one proton into the stomach lumen per potassium ion retrieved from the gastric lumen.[9] As an ion pump the H+/K+ ATPase is able to transport ions against a concentration gradient using energy derived from the hydrolysis of ATP. Like all P-type ATPases, a phosphate group is transferred from adenosine triphosphate (ATP) to the H+/K+ ATPase during the transport cycle. This phosphate transfer powers a conformational change in the enzyme that helps drive ion transport.[citation needed]
The hydrogen potassium ATPase is activated indirectly by
The E1 conformation binds a phosphate from ATP and hydronium ion on the cytoplasmic side. The enzyme then changes to the E2 conformation, allowing hydronium to be released in the lumen. The E2 conformation binds potassium, and reverts to the E1 conformation to release phosphate and K+ into the cytoplasm where another ATP can be hydrolyzed to repeat the cycle.[2] The β subunit prevents the E2-P conformation from reverting to the E1-P conformation, making proton pumping unidirectional.[8] The number of ions transported per ATP varies from 2H+/2K+ to 1H+/1K+depending on the pH of the stomach.[12]
Disease relevance and inhibition
Inhibiting the hydrogen potassium pump to decrease stomach acidity has been the most common method of treating diseases including
Three drug classes have been used to inhibit H+/K+-ATPases.
Inactivation of the proton pump can also lead to health problems. A study in mice by Krieg et al.[18] found that a mutation of the pump's α-subunit led to achlorhydria, resulting in problems with iron absorption, leading to iron deficiency and anemia. The use of PPIs has not been correlated with an elevated risk of anemia, so the H+/K+-ATPase is thought to aid iron absorption but is not necessarily required.[18]
Current association of dementia and PPIs have been documented in Germany and in research articles denoting how Benzimidazole derivatives, Astemizole (AST) and Lansoprazole (LNS) interact with anomalous aggregates of tau protein (neurofibrillary tangles).[19][20][21] Current theories include the non-selective blockade of sodium-potassium pumps in the brain causing osmotic imbalances or swelling in the cells. [auth opinion] Interaction of PPIs with other drug affecting the sodium-potassium pump, e.g., digoxin, warfarin etc., has been well documented.[22] Memory has been associated with astrocytes and the alpha3 subunit of adenosine receptor found in hydrogen/Sodium-potassium pumps may be a focal point in dementia.[23][24][25] Chronic use of PPIs may cause down regulation of alpha3 subunit increasing damage to astrocytes.[26] Osteopetrosis via TCIRG1 gene has a strong association with pre-senile dementia.[27][28]
See also
References
- PMID 26860309.
- ^ PMID 18536934.
- ^ Berg, J. M., Tymoczko, J. L., Stryer, L. (2012). Biochemistry (7th ed.). New York: W.H. Freeman and Company.
- ^ ATP4A ATPase H+/K+ transporting alpha subunit
- PMID 16157306.
- PMID 15096097.
- ^ ATP4B ATPase H+/K+ transporting beta subunit
- ^ PMID 19387495.
- ^ ISBN 978-0-470-01617-6.
- PMID 23055529.
- PMID 1341065.
- PMID 23091039.
- PMID 17928953.
- S2CID 42592868.
- ^ PMID 17575528.
- PMID 19006606.
- ^ PMID 21132072.
- ^ PMID 21976678.
- PMID 26882076.
- PMID 20110603.
- PMID 24896980. Epub 2014 Jun 16.
- PMID 16822281.
- PMID 18727925. Epub 2008 Aug 13.
- PMID 19592509.
- PMID 26283915. eCollection 2015
- PMID 23969284. Epub 2013 Aug 19.
- ^ "TCIRG1". Genetics Home Reference.
- PMID 12569157.
External links
- Yao X, Forte JG (2003). "Cell biology of acid secretion by the parietal cell". Annu. Rev. Physiol. 65: 103–31. PMID 12500969.
- Kühlbrandt W (April 2004). "Biology, structure and mechanism of P-type ATPases". Nat. Rev. Mol. Cell Biol. 5 (4): 282–95. S2CID 24927167.
- Dunbar LA, Caplan MJ (August 2001). "Ion pumps in polarized cells: sorting and regulation of the Na+, K+- and H+, K+-ATPases". J. Biol. Chem. 276 (32): 29617–20. PMID 11404365.
- Sachs G, Shin JM, Briving C, Wallmark B, Hersey S (1995). "The pharmacology of the gastric acid pump: the H+,K+ ATPase". Annu. Rev. Pharmacol. Toxicol. 35: 277–305. PMID 7598495.
- Potassium+Hydrogen+ATPase at the U.S. National Library of Medicine Medical Subject Headings (MeSH)