Choline acetyltransferase

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Choline acetyltransferase
Identifiers
EC number 2.3.1.6
CAS number 9012-78-6
Databases
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO
Choline O-acetyltransferase
Protein CHAT PDB 2fy2.png
PDB rendering based on 2fy2.
Available structures
PDB Ortholog search: PDBe, RCSB
Identifiers
Symbols CHAT ; CHOACTASE; CMS1A; CMS1A2
External IDs OMIM118490 MGI88392 HomoloGene40693 ChEMBL: 4039 GeneCards: CHAT Gene
EC number 2.3.1.6
RNA expression pattern
PBB GE CHAT 221197 s at tn.png
More reference expression data
Orthologs
Species Human Mouse
Entrez 1103 12647
Ensembl ENSG00000070748 ENSMUSG00000021919
UniProt P28329 n/a
RefSeq (mRNA) NM_001142929 NM_009891
RefSeq (protein) NP_001136401 NP_034021
Location (UCSC) Chr 10:
50.82 – 50.9 Mb
Chr 14:
32.41 – 32.47 Mb
PubMed search [1] [2]

Choline acetyltransferase (commonly abbreviated as ChAT, but sometimes CAT) is a transferase enzyme responsible for the synthesis of the neurotransmitter acetylcholine. ChAT catalyzes the transfer of an acetyl group from the coenzyme, acetyl-CoA, to choline yielding acetylcholine (ACh). ChAT is found in high concentration in cholinergic neurons, both in the central nervous system (CNS) and peripheral nervous system (PNS). As with all nerve terminal proteins, ChAT is produced in the body of the neuron and is transported to the nerve terminal, where its concentration is highest. Presence of ChAT in a nerve cell classifies this cell as a “cholinergic” neuron. In humans, the choline acetyltransferase enyzme is encoded by the CHAT gene.[1]

History & Discovery[edit]

Choline acetyltransferase was first described by David Nachmansohn and A. L. Machado in 1943.[2] 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.[3]

“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.” -Nachmanson & Machado, 1943[2]

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.[2] It was not until 1945 that Coenzyme A (CoA) was discovered simultaneously and independently by three laboratories,[4][5][6] Nachmansohn's being one of these. Subsequently acetyl-CoA, at the time called “active acetate,” was discovered in 1951.[7] The 3D structure of rat-derived ChAT was not solved until nearly 60 years later, in 2004.[8]

Structure[edit]

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.

Structure of choline acetyltransferase binding sites
]
Crystal structure of choline ion bound in choline acetyltransferase. Side chain residues of His324A and Tyr552A shown.PDB 2FY3 
Stereoscopic depiction of choline and acetyl-CoA in ChAT active site.(PDB 2FY3, PDB 2FY5 - overlaid). 
Stereoscopic depiction of choline and acetyl-CoA bound in ChAT active site - alternate angle. (PDB 2FY3, PDB 2FY5 - overlaid). 

Homology[edit]

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%.[9]

Forms of ChAT[edit]

There are two forms of ChAT: Soluble form& membrane-bound form.[10] 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.[11] However, there has long been a debate on how the latter form of ChAT is bound to the membrane.[12] Some researchers claim that the membrane-bound form of ChAT is associated with synaptic vesicle[13] while the others argue the opposite.[11]

Common and Peripheral Isoforms of ChAT[edit]

There exist two isoforms of ChAT, both encoded by the same sequence. The common type ChAT (cChAT) is present in both the CNS and PNS. Peripheral type ChAT (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 pChAT is missing parts of the sequence present in cChAT. The 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 cChAT and 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.

Function[edit]

synthesis and transportation of ACh in cells[14]

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.

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

Mutations[edit]

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 phonetype 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.[15] 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.[16]

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.[17]

ChAT and Diseases[edit]

Alzheimer's Disease[edit]

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.[18] 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 the metabolism of neurones and further damages the cholinergic axons in the cortex and cholinergic neurones in the basal forebrain.[19]

Amyotrophic Lateral Sclerosis[edit]

The amyotrophic lateral sclerosis(ALS) is one of the most common motor neuron diseases. A significant loss of ChAT immunoreactivity is found in ALS.[20] It is hypothesized that the cholinergic function is involved in an uncontrolled increase of intracellular calcium concentration whose reason still remains unclear.[21]

Drugs[edit]

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.[22]

Exposure to estradiol has been shown to increase ChAT in female rats.[23]

See also[edit]

References[edit]

  1. ^ Strauss WL, Kemper RR, Jayakar P, Kong CF, Hersh LB, Hilt DC, Rabin M (February 1991). "Human choline acetyltransferase gene maps to region 10q11-q22.2 by in situ hybridization". Genomics 9 (2): 396–8. doi:10.1016/0888-7543(91)90273-H. PMID 1840566. 
  2. ^ a b c Nachmansohn, D.; A. L. Machado (1943). "The Formation of Acetylcholine. A New Enzyme: Choline Acetylase". J. Physiol. 6 (5): 397–403. 
  3. ^ BERMAN, R; WILSON, IB; NACHMANSOHN, D (September–October 1953). "Choline acetylase specificity in relation to biological function.". Biochimica et Biophysica Acta 12 (1-2): 315–24. doi:10.1016/0006-3002(53)90150-4. PMID 13115440. Retrieved 2014-02-21. 
  4. ^ Lipmann, Fritz; N. O. Kaplan (1946). "A Common Factor in the Enzymatic Acetylation of Sulfanilamide and of Choline". J. Biol. Chem 162: 743–744. 
  5. ^ Lipton, M. A. (1946). Federation Proc 5: 145. 
  6. ^ Nachmansohn, David; M. Berman (1846). "Studies on choline acetylase: III. On the preparation of the coenzyme and its effects on the enzyme". J. Biol. Chem 165: 551–563. 
  7. ^ Lynen, F.; Reichert & Rueff (1951). Liebigs Ann. 574 (1). doi:10.1016/0003-2697(68)90195-4 http://www.sciencedirect.com/science/article/pii/0003269768901954 |url= missing title (help). 
  8. ^ Govindasamy, L; Pedersen, B; Lian, W; Kukar, T; Gu, Y; Jin, S; Agbandje-McKenna, M; Wu, D; McKenna, R (November 2004). "Structural insights and functional implications of choline acetyltransferase". Journal of structural biology 148 (2): 226–35. doi:10.1016/j.jsb.2004.06.005. PMID 15477102. 
  9. ^ Oda, Y (November 1999). "Choline acetyltransferase: the structure, distribution and pathologic changes in the central nervous system.". Pathology international 49 (11): 921–37. doi:10.1046/j.1440-1827.1999.00977.x. PMID 10594838. 
  10. ^ Collier, B. (23 November 2002). "Effects of Colchicine Application to Preganglionic Axons on Choline Acetyltransferase Activity and Acetylcholine Content and Release in the Superior Cervical Ganglion". Journal of Neurochemistry 66 (3): 1033. doi:10.1046/j.1471-4159.1996.66031033.x. 
  11. ^ a b Eder-Colli, Lorenza (25 May 1998). "Study of subcellular localization of membrane-bound choline acetyltransferase in Drosophila central nervous system and its association with membranes". European Journal of Neuroscience 10 (5): 1644. doi:10.1046/j.1460-9568.1998.00177.x. PMID 9751137. 
  12. ^ Hersh, LB (December 1987). "Studies on detergent released choline acetyltransferase from membrane fractions of rat and human brain.". Neurochem Res 12 (12): 1059. PMID 2450285. 
  13. ^ Carrol, Paul (1994). "Membrane-bound choline-O-acetyltransferase in rat hippocampal tissue is associated with synaptic vesicles". Brain Research 633 (1-2): 112. doi:10.1016/0006-8993(94)91529-6. 
  14. ^ Rand, James. "Acetylcholine". WormBook. 
  15. ^ Rand, JB; Russell, RL (February 1984). "Choline acetyltransferase-deficient mutants of the nematode Caenorhabditis elegans". Genetics 106 (2): 227–48. PMID 6698395. 
  16. ^ Greenspan, Ralph J. (1980). "Mutations of choline acetyltransferase and associated neural defects". Journal of Comparative Physiology ? A 137 (1): 83–92. doi:10.1007/BF00656920. 
  17. ^ Ohno, K.; Tsujino, A.; Brengman, J. M.; Harper, C. M.; Bajzer, Z.; Udd, B.; Beyring, R.; Robb, S.; Kirkham, F. J.; Engel, A. G. (13 February 2001). "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans". Proceedings of the National Academy of Sciences 98 (4): 2017–2022. doi:10.1073/pnas.98.4.2017. 
  18. ^ Lippa, AS. (30 July 1982). "The cholinergic hypothesis of geriatric memory dysfunction". Science 217 (4558): 408. PMID 7046051. 
  19. ^ Geula, C (January 1998). "Relationship between plaques, tangles, and loss of cortical cholinergic fibers in Alzheimer disease". J Neuropathol Exp Neurol 57 (1): 63. doi:10.1097/00005072-199801000-00008. PMID 9600198. 
  20. ^ Oda, J (December 1995). "Immunohistochemical study on choline acetyltransferase in the spinal cord of patients with amyotrophic lateral sclerosis". Pathol Int 45 (12): 933. doi:10.1111/j.1440-1827.1995.tb03418.x. PMID 8808298. 
  21. ^ Morrison, BM (January 1999). "Amyotrophic lateral sclerosis associated with mutations in superoxide dismutase: a putative mechanism of degeneration". Brain Res Brain Res Rev 29 (1): 121. doi:10.1016/s0165-0173(98)00049-6. PMID 9974153. 
  22. ^ Greer, M; Schotland, M (July 1960). "Myasthenia gravis in the newborn". Pediatrics 26: 101–8. PMID 13851666. 
  23. ^ Luine, VN (August 1985). "Estradiol increases choline acetyltransferase activity in specific basal forebrain nuclei and projection areas of female rats". Experimental neurology 89 (2): 484–90. doi:10.1016/0014-4886(85)90108-6. PMID 2990988. 

Further reading[edit]

External links[edit]