choline acetyltransferase

Choline acetyltransferase was first described by David Nachmansohn and A. L. Machado in 1943.{{cite journal | vauthors = Nachmansohn D, Machado AL | title = The Formation of Acetylcholine. A New Enzyme: Choline Acetylase | journal = J. Neurophysiol. | year = 1943 | volume = 6 | issue = 5 | pages = 397–403 | doi = 10.1152/jn.1943.6.5.397 }} A German biochemist, Nachmansohn had been studying the process of nerve impulse conduction and utilization of energy-yielding chemical reactions in cells, expanding upon the works of Nobel laureates Otto Warburg and Otto Meyerhof on fermentation, glycolysis, and muscle contraction. Based on prior research showing that "acetylcholine's actions on structural proteins" were responsible for nerve impulses, Nachmansohn and Machado investigated the origin of acetylcholine.{{cite journal | vauthors = Berman R, Wilson IB, Nachmansohn D | title = Choline acetylase specificity in relation to biological function | journal = Biochimica et Biophysica Acta | volume = 12 | issue = 1–2 | pages = 315–24 | date = September–October 1953 | pmid = 13115440 | doi = 10.1016/0006-3002(53)90150-4 }}

{{Blockquote|An enzyme has been extracted from brain and nervous tissue which forms acetylcholine. The formation occurs only in presence of adenosinetriphosphate (ATP). The enzyme is called choline acetylase.|source=Nachmanson & Machado, 1943}}

The acetyl transferase mode of action was unknown at the time of this discovery, however Nachmansohn hypothesized the possibility of acetylphosphate or phosphorylcholine exchanging the phosphate (from ATP) for choline or acetate ion. It was not until 1945 that Coenzyme A (CoA) was discovered simultaneously and independently by three laboratories,{{cite journal | vauthors = Lipmann F, Kaplan NO | title = A Common Factor in the Enzymatic Acetylation of Sulfanilamide and of Choline | journal = J. Biol. Chem. | year = 1946 | volume = 162 | issue = 3 | pages = 743–744 | doi = 10.1016/S0021-9258(17)41419-0 | doi-access = free }}{{cite journal | vauthors = Lipton MA | title = Mechanism of the enzymatic synthesis of acetylcholine | journal = Fed. Proc. | volume = 5 | issue = 1 Pt 2 | pages = 145 | year = 1946 | pmid = 21066687 }}{{cite journal | vauthors = Nachmansohn D, Berman M | title = Studies on choline acetylase; on the preparation of the coenzyme and its effect on the enzyme | journal = J. Biol. Chem. | volume = 165 | issue = 2 | pages = 551–63 | year = 1946 | doi = 10.1016/S0021-9258(17)41168-9 | pmid = 20276121 | doi-access = free }} Nachmansohn's being one of these. Subsequently, acetyl-CoA, at the time called “active acetate,” was discovered in 1951.{{cite journal | vauthors = Jones DH, Nelson WL | title = A method for isolation of coenzyme A products | journal = Anal. Biochem. | volume = 26 | issue = 3 | pages = 350–7 | year = 1968 | pmid = 5716187 | doi = 10.1016/0003-2697(68)90195-4 }} The 3D structure of rat-derived ChAT was not solved until nearly 60 years later, in 2004.{{cite journal | vauthors = Govindasamy L, Pedersen B, Lian W, Kukar T, Gu Y, Jin S, Agbandje-McKenna M, Wu D, McKenna R | title = Structural insights and functional implications of choline acetyltransferase | journal = Journal of Structural Biology | volume = 148 | issue = 2 | pages = 226–35 | date = November 2004 | pmid = 15477102 | doi = 10.1016/j.jsb.2004.06.005 }}

The 3D structure of ChAT has been solved by X-ray crystallography {{PDB|2FY2}}. Choline is bound in the active site of ChAT by non-covalent interactions between the positively charged amine of choline and the hydroxyl group of Tyr552, in addition to a hydrogen bond between choline's hydroxyl group and a histidine residue, His324.

The choline substrate fits into a pocket in the interior of ChAT, while acetyl-CoA fits into a pocket on the surface of the protein. The 3D crystal structure shows the acetyl group of acetyl-CoA abuts the choline binding pocket – minimizing the distance between acetyl-group donor and receiver.

{{Gallery

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|File:Crystal structure of choline ion bound in choline acetyltransferase..png|Crystal structure of choline ion bound in choline acetyltransferase. Side chain residues of His324A and Tyr552A shown.{{PDB|2FY3}}

|File:Stereoscopic depiction of choline and acetyl-CoA in ChAT active site..png| Stereoscopic depiction of choline and acetyl-CoA in ChAT active site.({{PDB|2FY3}}, {{PDB|2FY5}} - overlaid).

|File:Stereoscopic depiction of choline and acetyl-CoA bound in ChAT active site - alternate angle.png|Stereoscopic depiction of choline and acetyl-CoA bound in ChAT active site - alternate angle. ({{PDB|2FY3}}, {{PDB|2FY5}} - overlaid).}}

ChAT is very conserved across the animal genome. Among mammals, in particular, there is very high sequence similarity. Human and cat (Felis catus) ChAT, for example, have 89% sequence identity.

Sequence identity with Drosophila is about 30%.{{cite journal | vauthors = Oda Y | title = Choline acetyltransferase: the structure, distribution and pathologic changes in the central nervous system. | journal = Pathology International | volume = 49 | issue = 11 | pages = 921–37 | date = November 1999 | pmid = 10594838 | doi = 10.1046/j.1440-1827.1999.00977.x | s2cid = 23621617 }}

There are two forms of ChAT: Soluble form and membrane-bound form.{{cite journal | vauthors = Tandon A, Bachoo M, Weldon P, Polosa C, Collier B | title = Effects of colchicine application to preganglionic axons on choline acetyltransferase activity and acetylcholine content and release in the superior cervical ganglion | journal = J. Neurochem. | volume = 66 | issue = 3 | pages = 1033–41 | year = 1996 | pmid = 8769864 | doi = 10.1046/j.1471-4159.1996.66031033.x | s2cid = 44586742 }} The soluble form accounts for 80-90% of the total enzyme activity while the membrane-bound form is responsible for the rest of 10-20% activity.{{cite journal | vauthors = Pahud G, Salem N, van de Goor J, Medilanski J, Pellegrinelli N, Eder-Colli L | title = Study of subcellular localization of membrane-bound choline acetyltransferase in Drosophila central nervous system and its association with membranes | journal = European Journal of Neuroscience | volume = 10 | issue = 5 | pages = 1644–53 | date = 25 May 1998 | pmid = 9751137 | doi = 10.1046/j.1460-9568.1998.00177.x | s2cid = 24196247 }} However, there has long been a debate on how the latter form of ChAT is bound to the membrane.{{cite journal | vauthors = Bruce G, Hersh LB | title = Studies on detergent released choline acetyltransferase from membrane fractions of rat and human brain. | journal = Neurochem Res | volume = 12 | issue = 12 | pages = 1059–66 | date = December 1987 | pmid = 2450285 | doi = 10.1007/bf00971705 | s2cid = 4336737 }} The membrane-bound form of ChAT is associated with synaptic vesicles.{{cite journal | vauthors = Carroll PT | s2cid = 1139292 | title = Membrane-bound choline-O-acetyltransferase in rat hippocampal tissue is associated with synaptic vesicles | journal = Brain Res. | volume = 633 | issue = 1–2 | pages = 112–8 | year = 1994 | pmid = 8137149 | doi = 10.1016/0006-8993(94)91529-6 }}

There exist two isoforms of ChAT, both encoded by the same sequence. The common type ChAT ({{proper name|cChAT}}) is present in both the CNS and PNS. Peripheral type ChAT ({{proper name|pChAT}}) is preferentially expressed in the PNS in humans, and arises from exon skipping (exons 6–9) during post-transcriptional modification. Therefore, the amino acid sequence is very similar; however, {{proper name|pChAT}} is missing parts of the sequence present in {{proper name|cChAT}}. The {{proper name|pChAT}} isoform was discovered in 2000 based on observations that brain-derived ChAT antibodies failed to stain peripheral cholinergic neurons as they do for those found in the brain. This gene splicing mechanism which leads to {{proper name|cChAT}} and {{proper name|pChAT}} differences has been observed in various species, including both vertebrate mammals and invertebrate mollusks, suggesting this mechanism leads to some yet-unidentified evolutionary advantage.

File:Cholinergic enzymes and transporters.png

Cholinergic systems are implicated in numerous neurologic functions. Alteration in some cholinergic neurons may account for the disturbances of Alzheimer disease. The protein encoded by this gene synthesizes the neurotransmitter acetylcholine. Acetylcholine acts at two classes of receptors in the central nervous system – muscarinic and nicotinic – which are each implicated in different physiological responses. The role of acetylcholine at the nicotinic receptor is still under investigation. It is likely implicated in the reward/reinforcement pathways, as indicated by the addictive nature of nicotine, which also binds to the nicotinic receptor. The muscarinic action of acetylcholine in the CNS is implicated in learning and memory. The loss of cholinergic innervation in the neocortex has been associated with memory loss, as is evidenced in advanced cases of Alzheimer's disease. In the peripheral nervous system, cholinergic neurons are implicated in the control of visceral functions such as, but not limited to, cardiac muscle contraction and gastrointestinal tract function.

Image:Choline-skeletal.svg|Choline

Image:Acetylcholine.svg|Acetylcholine

It is often used as an immunohistochemical marker for motor neurons (motoneurons).

Mutants of ChAT have been isolated in several species, including C. elegans, Drosophila, and humans. Most non-lethal mutants that have a non-wild type phenotype exhibit some activity, but significantly less than wild type.

In C. elegans, several mutations in ChAT have been traced to the cha-1 gene. All mutations result in a significant drop in ChAT activity. Percent activity loss can be greater than 98% in some cases. Phenotypic effects include slowed growth, decreased size, uncoordinated behavior, and lack of sensitivity toward cholinesterase inhibitors.{{cite journal | vauthors = Rand JB, Russell RL | title = Choline acetyltransferase-deficient mutants of the nematode Caenorhabditis elegans | journal = Genetics | volume = 106 | issue = 2 | pages = 227–48 | date = February 1984 | doi = 10.1093/genetics/106.2.227 | pmid = 6698395 | pmc = 1202253 }} Isolated temperature-sensitive mutants in Drosophila have all been lethal. Prior to death, affected flies show a change in behavior, including uncontrolled movements and a change in electroretinogram activity.{{cite journal | vauthors = Greenspan RJ | s2cid = 45897606 | title = Mutations of choline acetyltransferase and associated neural defects | journal = Journal of Comparative Physiology | year = 1980 | volume = 137 | issue = 1 | pages = 83–92 | doi = 10.1007/BF00656920 }}

The human gene responsible for encoding ChAT is CHAT. Mutations in CHAT have been linked to congenital myasthenic syndrome, a disease which leads to general motor function deficiency and weakness. Further symptoms include fatal apnea. Out of ten isolated mutants, 1 has been shown to lack activity completely, 8 have been shown to have significantly decreased activity, and 1 has an unknown function.{{cite journal | vauthors = Ohno K, Tsujino A, Brengman JM, Harper CM, Bajzer Z, Udd B, Beyring R, Robb S, Kirkham FJ, Engel AG | title = Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans | journal = Proceedings of the National Academy of Sciences | volume = 98 | issue = 4 | pages = 2017–2022 | date = 13 February 2001 | pmid = 11172068 | pmc = 29374 | doi = 10.1073/pnas.98.4.2017 | bibcode = 2001PNAS...98.2017O | doi-access = free }}

The Alzheimer's disease (AD) involves difficulty in memory and cognition. The concentrations of acetylcholine and ChAT are remarkably reduced in the cerebral neocortex and hippocampus.{{cite journal | vauthors = Bartus RT, Dean RL, Beer B, Lippa AS | title = The cholinergic hypothesis of geriatric memory dysfunction | journal = Science | volume = 217 | issue = 4558 | pages = 408–14 | date = 30 July 1982 | pmid = 7046051 | doi = 10.1126/science.7046051 | bibcode = 1982Sci...217..408B }} Although the cellular loss and dysfunction of the cholinergic neurones is considered a contributor to Alzheimer disease, it is generally not considered as a primary factor in the development of this disease. It is proposed that the aggregation and deposition of the Beta amyloid protein, interferes with the metabolism of neurones and further damages the cholinergic axons in the cortex and cholinergic neurones in the basal forebrain.{{cite journal | vauthors = Geula C, Mesulam MM, Saroff DM, Wu CK | title = Relationship between plaques, tangles, and loss of cortical cholinergic fibers in Alzheimer disease | journal = J Neuropathol Exp Neurol | volume = 57 | issue = 1 | pages = 63–75 | date = January 1998 | pmid = 9600198 | doi = 10.1097/00005072-199801000-00008 | doi-access = free }}

The amyotrophic lateral sclerosis (ALS) is one of the most common motor neuron diseases. A significant loss of ChAT immunoreactivity is found in ALS.{{cite journal | vauthors = Oda Y, Imai S, Nakanishi I, Ichikawa T, Deguchi T | title = Immunohistochemical study on choline acetyltransferase in the spinal cord of patients with amyotrophic lateral sclerosis | journal = Pathol Int | volume = 45 | issue = 12 | pages = 933–9 | date = December 1995 | pmid = 8808298 | doi = 10.1111/j.1440-1827.1995.tb03418.x | s2cid = 23763400 }} It is hypothesized that the cholinergic function is involved in an uncontrolled increase of intracellular calcium concentration whose reason still remains unclear.{{cite journal | vauthors = Morrison BM, Morrison JH | title = Amyotrophic lateral sclerosis associated with mutations in superoxide dismutase: a putative mechanism of degeneration | journal = Brain Res Brain Res Rev | volume = 29 | issue = 1 | pages = 121–35 | date = January 1999 | pmid = 9974153 | doi = 10.1016/s0165-0173(98)00049-6 | s2cid = 28937351 }}

Neostigmine methylsulfate, an anticholinesterase agent, has been used to target ChAT. In particular, use of neostigmine methylsulfate has been shown to have positive effects against congenital myasthenic syndrome.{{cite journal | vauthors = Greer M, Schotland M | title = Myasthenia gravis in the newborn | journal = Pediatrics | volume = 26 | pages = 101–8 | date = July 1960 | doi = 10.1542/peds.26.1.101 | pmid = 13851666 | s2cid = 8672902 }}

Exposure to estradiol has been shown to increase ChAT in female rats.{{cite journal | vauthors = Luine VN | title = Estradiol increases choline acetyltransferase activity in specific basal forebrain nuclei and projection areas of female rats | journal = Experimental Neurology | volume = 89 | issue = 2 | pages = 484–90 | date = August 1985 | pmid = 2990988 | doi = 10.1016/0014-4886(85)90108-6 | s2cid = 1525252 }}

• Acetyltransferase

{{reflist|35em}}

{{refbegin|35em}}

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• {{cite journal | vauthors = Quirin-Stricker C, Nappey V, Simoni P, Toussaint JL, Schmitt M | title = Trans-activation by thyroid hormone receptors of the 5' flanking region of the human ChAT gene | journal = Brain Res. Mol. Brain Res. | volume = 23 | issue = 3 | pages = 253–65 | year = 1994 | pmid = 8057782 | doi = 10.1016/0169-328X(94)90232-1 }}
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• {{cite journal | vauthors = Misawa H, Matsuura J, Oda Y, Takahashi R, Deguchi T | title = Human choline acetyltransferase mRNAs with different 5'-region produce a 69-kDa major translation product | journal = Brain Res. Mol. Brain Res. | volume = 44 | issue = 2 | pages = 323–33 | year = 1997 | pmid = 9073174 | doi = 10.1016/S0169-328X(96)00231-8 }}
• {{cite journal | vauthors = Lönnerberg P, Ibáñez CF | title = Novel, testis-specific mRNA transcripts encoding N-terminally truncated choline acetyltransferase | journal = Mol. Reprod. Dev. | volume = 53 | issue = 3 | pages = 274–81 | year = 1999 | pmid = 10369388 | doi = 10.1002/(SICI)1098-2795(199907)53:3<274::AID-MRD3>3.0.CO;2-8 | s2cid = 39464614 }}
• {{cite journal | vauthors = Sakakibara A, Hattori S | title = Chat, a Cas/HEF1-associated adaptor protein that integrates multiple signaling pathways | journal = J. Biol. Chem. | volume = 275 | issue = 9 | pages = 6404–10 | year = 2000 | pmid = 10692442 | doi = 10.1074/jbc.275.9.6404 | doi-access = free }}

{{refend}}

• {{MeshName|Choline+Acetyltransferase}}

{{PDB Gallery|geneid=1103}}

{{Neurotransmitter metabolism enzymes}}

{{Acyltransferases}}

{{Enzymes}}

{{Acetylcholine metabolism and transport modulators}}

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Category:EC 2.3.1