Acetylcholinesterase

"ACHE" redirects here. For other uses, see Ache.

acetylcholinesterase
File:The reaction catalyzed by acetylcholinesterase.tif Acetylcholinesterase catalyzes the hydrolysis of acetylcholine to acetate ion and choline
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EC number	3.1.1.7
CAS number	9000-81-1
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PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / QuickGO
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External IDs	GeneCards: [1]
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Species	Human	Mouse
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Acetylcholinesterase (HGNC symbol ACHE; EC 3.1.1.7), also known as AChE or acetylhydrolase, is the primary cholinesterase in the body. It is an enzyme that catalyzes the breakdown of acetylcholine and of some other choline esters that function as neurotransmitters. AChE is found at mainly neuromuscular junctions and in chemical synapses of the cholinergic type, where its activity serves to terminate synaptic transmission. It belongs to carboxylesterase family of enzymes. It is the primary target of inhibition by organophosphorus compounds such as nerve agents and pesticides.

Enzyme structure and mechanism

File:AChe mechanism of action.jpg

AChe mechanism of action

AChE is a hydrolase that hydrolyzes choline esters. It has a very high catalytic activity—each molecule of AChE degrades about 25000 molecules of acetylcholine (ACh) per second, approaching the limit allowed by diffusion of the substrate. The active site of AChE comprises 2 subsites—the anionic site and the esteratic subsite. The structure and mechanism of action of AChE have been elucidated from the crystal structure of the enzyme.

The anionic subsite accommodates the positive quaternary amine of acetylcholine as well as other cationic substrates and inhibitors. The cationic substrates are not bound by a negatively charged amino acid in the anionic site, but by interaction of 14 aromatic residues that line the gorge leading to the active site. All 14 amino acids in the aromatic gorge are highly conserved across different species. Among the aromatic amino acids, tryptophan 84 is critical and its substitution with alanine results in a 3000-fold decrease in reactivity. The gorge penetrates halfway through the enzyme and is approximately 20 angstroms long. The active site is located 4 angstroms from the bottom of the molecule.

The esteratic subsite, where acetylcholine is hydrolyzed to acetate and choline, contains the catalytic triad of three amino acids: serine 200, histidine 440 and glutamate 327. These three amino acids are similar to the triad in other serine proteases except that the glutamate is the third member rather than aspartate. Moreover, the triad is of opposite chirality to that of other proteases. The hydrolysis reaction of the carboxyl ester leads to the formation of an acyl-enzyme and free choline. Then, the acyl-enzyme undergoes nucleophilic attack by a water molecule, assisted by the histidine 440 group, liberating acetic acid and regenerating the free enzyme.

Biological function

During neurotransmission, ACh is released from the presynaptic neuron into the synaptic cleft and binds to ACh receptors on the post-synaptic membrane, relaying the signal from the nerve. AChE, also located on the post-synaptic membrane, terminates the signal transmission by hydrolyzing ACh. The liberated choline is taken up again by the pre-synaptic neuron and ACh is synthesized by combining with acetyl-CoA through the action of choline acetyltransferase.

A cholinomimetic drug disrupts this process by acting as a cholinergic neurotransmitter that is impervious to acetylcholinesterase's lysing action.

Disease relevance

Main article: Acetylcholinesterase inhibitor

For a cholinergic neuron to receive another impulse, ACh must be released from the ACh receptor. This occurs only when the concentration of ACh in the synaptic cleft is very low. Inhibition of AChE leads to accumulation of ACh in the synaptic cleft and results in impeded neurotransmission.

File:AChe inhibitors pic.jpg

Mechanism of Inhibitors of AChE

Irreversible inhibitors of AChE may lead to muscular paralysis, convulsions, bronchial constriction, and death by asphyxiation. Organophosphates (OP), esters of phosphoric acid, are a class of irreversible AChE inhibitors. Cleavage of OP by AChE leaves a phosphoryl group in the esteratic site, which is slow to be hydrolyzed (on the order of days) and can become covalently bound. Irreversible AChE inhibitors have been used in insecticides (e.g., malathion) and nerve gases for chemical warfare (e.g., Sarin and Soman). Carbamates, esters of N-methyl carbamic acid, are AChE inhibitors that hydrolyze in hours and have been used for medical purposes (e.g., physostigmine for the treatment of glaucoma). Reversible inhibitors occupy the esteratic site for short periods of time (seconds to minutes) and are used to treat of a range of central nervous system diseases. Tetrahydroaminoacridine (THA) and donepezil are FDA-approved to improve cognitive function in Alzheimer's disease. Rivastigmine is also used to treat Alzheimer's and Lewy body dementia, and pyridostigmine bromide is used to treat myasthenia gravis.

An endogenous inhibitor of AChE in neurons is Mir-132 microRNA, which may limit inflammation in the brain by silencing the expression of this protein and allowing ACh to act in an anti-inflammatory capacity.

It has also been shown that the main active ingredient in cannabis, tetrahydrocannabinol, is a competitive inhibitor of acetylcholinesterase.

Distribution

AChE is found in many types of conducting tissue: nerve and muscle, central and peripheral tissues, motor and sensory fibers, and cholinergic and noncholinergic fibers. The activity of AChE is higher in motor neurons than in sensory neurons.

Acetylcholinesterase is also found on the red blood cell membranes, where different forms constitute the Yt blood group antigens. Acetylcholinesterase exists in multiple molecular forms, which possess similar catalytic properties, but differ in their oligomeric assembly and mode of attachment to the cell surface.

AChE gene

In mammals, acetylcholinesterase is encoded by a single AChE gene while some invertebrates have multiple acetylcholinesterase genes. Note higher vertebrates also encode a closely related paralog BCHE (butyrylcholinesterase) with 50% amino acid identity to ACHE . Diversity in the transcribed products from the sole mammalian gene arises from alternative mRNA splicing and post-translational associations of catalytic and structural subunits. There are three known forms: T (tail), R (read through), and H(hydrophobic).

AChE_T

The major form of acetylcholinesterase found in brain, muscle, and other tissues, known as is the hydrophilic species, which forms disulfide-linked oligomers with collagenous, or lipid-containing structural subunits. In the neuromuscular junctions AChE expresses in asymmetric form which associates with ColQ or subunit. In the central nervous system it is associated with PRiMA which stands for Proline Rich Membrane anchor to form symmetric form. In either case, the ColQ or PRiMA anchor serves to maintain the enzyme in the intercellular junction, ColQ for the neuromuscular junction and PRiMA for synapses.

AChE_H

The other, alternatively spliced form expressed primarily in the erythroid tissues, differs at the C-terminus, and contains a cleavable hydrophobic peptide with a PI-anchor site. It associates with membranes through the phosphoinositide (PI) moieties added post-translationally.

AChE_R

The third type has, so far, only been found in Torpedo sp. and mice although it is hypothesized in other species. It is thought to be involved in the stress response and, possibly, inflammation.

Nomenclature

The nomenclatural variations of ACHE and of cholinesterases generally are discussed at Cholinesterase § Types and nomenclature.

Inhibitors

For acetylcholine esterase (AChE), reversible inhibitors are those that do not irreversibly bond to and deactivate AChE. Drugs that reversibly inhibit acetylcholine esterase are being explored as treatments for Alzheimer's disease and myasthenia gravis, among others. Examples include tacrine and donepezil.

See also

• Molecular and Cellular Biology portal
• Acetylcholinesterase inhibitor
• Cholinesterases

Further reading

• Silman I, Futerman AH (1988). "Modes of attachment of acetylcholinesterase to the surface membrane". Eur. J. Biochem. 170 (1–2): 11–22. doi:10.1111/j.1432-1033.1987.tb13662.x. PMID 3319614.
• Sussman JL, Harel M, Frolow F, Oefner C, Goldman A, Toker L, Silman I (1991). "Atomic structure of acetylcholinesterase from Torpedo californica: a prototypic acetylcholine-binding protein". Science. 253 (5022): 872–9. Bibcode:1991Sci...253..872S. doi:10.1126/science.1678899. PMID 1678899.
• Soreq H, Seidman S (2001). "Acetylcholinesterase--new roles for an old actor". Nature Reviews Neuroscience. 2 (4): 294–302. doi:10.1038/35067589. PMID 11283752.
• Shen T, Tai K, Henchman RH, McCammon JA (2003). "Molecular dynamics of acetylcholinesterase". Acc. Chem. Res. 35 (6): 332–40. doi:10.1021/ar010025i. PMID 12069617.
• Pakaski M, Kasa P (2003). "Role of acetylcholinesterase inhibitors in the metabolism of amyloid precursor protein". Current drug targets. CNS and neurological disorders. 2 (3): 163–71. doi:10.2174/1568007033482869. PMID 12769797.
• Meshorer E, Soreq H (2006). "Virtues and woes of AChE alternative splicing in stress-related neuropathologies". Trends Neurosci. 29 (4): 216–24. doi:10.1016/j.tins.2006.02.005. PMID 16516310.
• Ehrlich G, Viegas-Pequignot E, Ginzberg D, Sindel L, Soreq H, Zakut H (1992). "Mapping the human acetylcholinesterase gene to chromosome 7q22 by fluorescent in situ hybridization coupled with selective PCR amplification from a somatic hybrid cell panel and chromosome-sorted DNA libraries". Genomics. 13 (4): 1192–7. doi:10.1016/0888-7543(92)90037-S. PMID 1380483.
• Spring FA, Gardner B, Anstee DJ (1992). "Evidence that the antigens of the Yt blood group system are located on human erythrocyte acetylcholinesterase". Blood. 80 (8): 2136–41. PMID 1391965.
• Shafferman A, Kronman C, Flashner Y, Leitner M, Grosfeld H, Ordentlich A, Gozes Y, Cohen S, Ariel N, Barak D (1992). "Mutagenesis of human acetylcholinesterase. Identification of residues involved in catalytic activity and in polypeptide folding". J. Biol. Chem. 267 (25): 17640–8. PMID 1517212.
• Getman DK, Eubanks JH, Camp S, Evans GA, Taylor P (1992). "The human gene encoding acetylcholinesterase is located on the long arm of chromosome 7". Am. J. Hum. Genet. 51 (1): 170–7. PMC 1682883. PMID 1609795.
• Li Y, Camp S, Rachinsky TL, Getman D, Taylor P (1992). "Gene structure of mammalian acetylcholinesterase. Alternative exons dictate tissue-specific expression". J. Biol. Chem. 266 (34): 23083–90. PMID 1744105.
• Velan B, Grosfeld H, Kronman C, Leitner M, Gozes Y, Lazar A, Flashner Y, Marcus D, Cohen S, Shafferman A (1992). "The effect of elimination of intersubunit disulfide bonds on the activity, assembly, and secretion of recombinant human acetylcholinesterase. Expression of acetylcholinesterase Cys-580----Ala mutant". J. Biol. Chem. 266 (35): 23977–84. PMID 1748670.
• Soreq H, Ben-Aziz R, Prody CA, Seidman S, Gnatt A, Neville L, Lieman-Hurwitz J, Lev-Lehman E, Ginzberg D, Lipidot-Lifson Y (1991). "Molecular cloning and construction of the coding region for human acetylcholinesterase reveals a G + C-rich attenuating structure". Proceedings of the National Academy of Sciences of the United States of America. 87 (24): 9688–92. Bibcode:1990PNAS...87.9688S. doi:10.1073/pnas.87.24.9688. PMC 55238. PMID 2263619.
• Chhajlani V, Derr D, Earles B, Schmell E, August T (1989). "Purification and partial amino acid sequence analysis of human erythrocyte acetylcholinesterase". FEBS Lett. 247 (2): 279–82. doi:10.1016/0014-5793(89)81352-3. PMID 2714437.
• Lapidot-Lifson Y, Prody CA, Ginzberg D, Meytes D, Zakut H, Soreq H (1989). "Coamplification of human acetylcholinesterase and butyrylcholinesterase genes in blood cells: correlation with various leukemias and abnormal megakaryocytopoiesis". Proceedings of the National Academy of Sciences of the United States of America. 86 (12): 4715–9. Bibcode:1989PNAS...86.4715L. doi:10.1073/pnas.86.12.4715. PMC 287342. PMID 2734315.
• Bazelyansky M, Robey E, Kirsch JF (1986). "Fractional diffusion-limited component of reactions catalyzed by acetylcholinesterase". Biochemistry. 25 (1): 125–30. doi:10.1021/bi00349a019. PMID 3954986.
• Gaston SM, Marchase RB, Jakoi ER (1982). "Brain ligatin: a membrane lectin that binds acetylcholinesterase". J. Cell. Biochem. 18 (4): 447–59. doi:10.1002/jcb.1982.240180406. PMID 7085778.
• Ordentlich A, Barak D, Kronman C, Ariel N, Segall Y, Velan B, Shafferman A (1995). "Contribution of aromatic moieties of tyrosine 133 and of the anionic subsite tryptophan 86 to catalytic efficiency and allosteric modulation of acetylcholinesterase". J. Biol. Chem. 270 (5): 2082–91. doi:10.1074/jbc.270.5.2082. PMID 7836436.
• Maruyama K, Sugano S (1994). "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene. 138 (1–2): 171–4. doi:10.1016/0378-1119(94)90802-8. PMID 8125298.
• Ben Aziz-Aloya R, Sternfeld M, Soreq H (1994). "Promoter elements and alternative splicing in the human ACHE gene". Prog. Brain Res. 98: 147–53. doi:10.1016/s0079-6123(08)62392-4. PMID 8248502.
• Massoulié J, Pezzementi L, Bon S, Krejci E, Vallette FM (1993). "Molecular and Cellular Biology of Cholinesterases". Prog. Brain Res. 41 (1): 31–91. doi:10.1016/0301-0082(93)90040-Y. PMID 8321908.

External links

• ATSDR Case Studies in Environmental Medicine: Cholinesterase Inhibitors, Including Insecticides and Chemical Warfare Nerve Agents U.S. Department of Health and Human Services
• Proteopedia Acetylcholinesterase
• Proteopedia AChE_inhibitors_and_substrates
• Proteopedia AChE_inhibitors_and_substrates_(Part_II)
• Proteopedia AChE bivalent inhibitors AChE_bivalent_inhibitors AChE bivalent inhibitors
• Acetylcholinesterase: A gorge-ous enzyme—PDBe
• Acetylcholinesterase—RCSB PDB
• Human ACHE genome location and ACHE gene details page in the UCSC Genome Browser.