Acetylcholinesterase

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[[Image:AChE_ACh.jpg|left|150px]]<br />
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<StructureSection load='' size='350' side='right' scene='22/22/Ache_with_ach/2' caption='Torpedo california AChE (PDB code [[2ace]])'>
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<applet load='1ea5_rot.pdb' size='250' color='white' frame='true' spin='on' caption='AChE' align='right' script='Acetylcholinesterase/Ache_rot_down_gorge/1'
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[[Image:small_wh_ray0001.gif|left|150px]]<br />
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'''3D structure of acetylcholinesterase'''<br />
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'''Acetylcholinesterase''' (AChE) is key enzyme in the nervous system of animals. By rapid hydrolysis of the neurotransmitter, [[Acetylcholine|acetylcholine]] (ACh), AChE terminates neurotransmission at cholinergic synapses. It is a very fast enzyme, especially for a serine hydrolase, functioning at a rate approaching that of a diffusion-controlled reaction. AChE inhibitors are among the key drugs approved by the FDA for management of Alzheimer's disease (AD). The powerful toxicity of organophosphorus (OP) poisons is attributed primarily to their potent AChE inhibitors.
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== Key Enzyme in the Nervous System ==
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'''Acetylcholinesterase''' (AChE) is key enzyme in the cholinergic pathways in the animal nervous system. By rapid hydrolysis of the neurotransmitter, '''acetylcholine''' (ACh), AChE terminates neurotransmission at cholinergic synapses. AChE is a very fast enzyme, especially for a serine hydrolase, functioning at a rate approaching that of a diffusion-controlled reaction. The powerful toxicity of organophosphorus (OP) poisons is attributed primarily to their potent AChE inhibitors. AChE inhibitors are utilized in the treatment of various neurological and are the key drugs approved by the FDA for management of Alzheimer's disease (AD). Many carbamates and OPs serve as potent insecticides, by selectively inhibiting insect AChE.
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See also [[Acetylcholinesterase (Hebrew)]]
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In 1991, at the Weizmann Institute, Joel Sussman & Israel Silman, and thier colleagues, determined the 3D structure of AChE from ''Torpedo californica'' (''Tc''AChE), permitting visualization, for the first time, at atomic resolution, of a binding pocket for ACh. It also allowed identification of the active site of AChE, which, unexpectedly, is located at the bottom of a <scene name='Acetylcholinesterase/Ache_rot_down_gorge/2'>deep gorge</scene>, lined largely by aromatic residues. This unusual structure permitted them to work out structure-function relationships for AChE. The so-called 'anionic' binding site for the quaternary moiety of ACh does not contain several negative charges, as earlier postulated. However, AChE shows a remarkable asymmetric charge distribution resulting in an unusually large dipole moment (~1,700 Debye) aligned along the active-site gorge. Modeling studies suggested that the quaternary group interacts primarily with the indole ring of the conserved tryptophan residue, W84, via cation-π interaction, as well as with F330. Crystallographic studies on several AChE-ligand complexes confirmed this. This was in agreement with labeling studies in solution and theoretical studies on the cation-π interaction. From the various inhibitor/AChE complexes they have studied, including most currently available and potential drugs for treatment of AD, they see that although many interact very tightly with AChE (binding constants ''10''<sup>''10</sup>''), interaction is mediated mostly via waters, and van der Waals interactions, with few direct interactions with the protein.
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== Key Enzyme in the Nervous System ==
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Solution of the three-dimensional (3D) structure of [http://en.wikipedia.org/wiki/Pacific_electric_ray ''Torpedo californica''] [[acetylcholinesterase]] (''Tc''AChE) in 1991 opened up new horizons in research on an [http://en.wikipedia.org/wiki/Enzyme enzyme] that had already been the subject of intensive investigation.<ref>PMID:1678899</ref> The unanticipated structure of this extremely rapid enzyme, in which the [http://en.wikipedia.org/wiki/Active_site active site] was found to be buried at the bottom of a <scene name='2ace/Active_site/3'>deep and narrow gorge</scene>, lined by <scene name='2ace/Active_site/4'>14 aromatic residues</scene> <font color='darkmagenta'><b>(colored dark magenta)</b></font>, led to a revision of the views then held concerning [http://en.wikipedia.org/wiki/Substrate_(biochemistry) substrate] traffic, recognition and hydrolysis.<ref>PMID:10545346</ref> To understand how those aromatic residues behave with the enzyme, see [[Flexibility of aromatic residues in acetylcholinesterase]]. Solution of the 3D structure of acetylcholinesterase led to a series of theoretical and experimental studies, which took advantage of recent advances in theoretical techniques for treatment of [http://en.wikipedia.org/wiki/Protein proteins], such as
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[http://en.wikipedia.org/wiki/Molecular_dynamics molecular dynamics] and [http://en.wikipedia.org/wiki/Electrostatics electrostatics] and to [http://en.wikipedia.org/wiki/Site-directed_mutagenesis site-directed mutagenesis], utilizing suitable expression systems.
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The increasing longevity of people's lifespans, and the resulting increased
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[[Image:Synapse_Schematic.jpg|thumb|Cholinergic Synapse|300px|left]]
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prevelance of dementias such as Alzheimers Disease, led scientists to study the animal enzyme ''Acetylcholinesterase'' ('''AChE''') as a possible cause. This enzyme rapidly degrades or hydrolizes the neurotransmitter acetylcholine in synapses (junctions between nerve cells) of cholinergic nerve pathways into acetic acid and choline, to turn off the chemical signal for the nerve to fire. Should something happen to deactivate or kill this vital enzyme, nervous paralysis of vital functions occurs, leading to rapid death. Although AChE is does not the cause Alzheimers, it does seem to play a key role due to the acetylcholine deficit often seen in AD patients. Thus drugs that are mild inhibitors of AChE, like Tacrine, E2020 (Aricept) and the natural Chinese produce Huperzine appear to retard symptoms of AD.
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[http://en.wikipedia.org/wiki/Acetylcholinesterase Acetylcholinesterase] [http://en.wikipedia.org/wiki/Hydrolysis hydrolysizes] the [http://en.wikipedia.org/wiki/Neurotransmitter neurotransmitter] [http://en.wikipedia.org/wiki/Acetylcholine acetylcholine] <scene name='2ace/Cv/2'>(ACh)</scene>, producing <scene name='2ace/Cv/3'>choline and an acetate</scene> group. ACh directly binds <scene name='22/22/Cv/1'>Ser200</scene> (via its [http://en.wikipedia.org/wiki/Nucleophile nucleophilic] Oγ atom) within the <scene name='2ace/Cv/5'>catalytic triad (Ser200, His440, and Glu327)</scene> (ACh/''Tc''AChE structure [[2ace]]). The residues <scene name='2ace/Cv/6'>Trp84 and Phe330</scene> are also important in the [http://en.wikipedia.org/wiki/Ligand ligand] recognition <ref name="Raves">PMID:8989325</ref>. After this binding acetylcholinesterase <scene name='2ace/Cv/7'>hydrolysizes</scene> ACh. <br />
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See also [[Acetylcholinesterase with acetylcholine]].
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Acetylcholinesterase is a fairly large protein, consisting of a single polypeptide chain made of ~537 amino acid residues, that folds into a. <scene name='Acetylcholinesterase/Ache_rot_down_gorge/3'> single protein domain </scene>, with a large Beta sheets (orange), surrounded by a canopy of about 26 alpha helices (violet).
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== Treatment of Alzheimer's disease ==
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The active site region of this enzyme has two sites, a catalytic site and a peripheral site, which helps prebind the substrate and direct it toward the active site. When the 3-D structure was first determined, the big surprise was
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[http://en.wikipedia.org/wiki/Alzheimer's_disease Alzheimer's disease] (AD) is a disorder that attacks the [http://en.wikipedia.org/wiki/Central_nervous_system central nervous system] through progressive degeneration of its neurons. AD occurs in around 10% of the elderly and, as yet, there is no known cure. Patients with this disease develop [http://en.wikipedia.org/wiki/Dementia dementia] which becomes more severe as the disease progresses. It was suggested that symptoms of AD are caused by decrease of activity of [http://en.wikipedia.org/wiki/Cholinergic cholinergic] [http://en.wikipedia.org/wiki/Neocortex neocortical] and [http://en.wikipedia.org/wiki/Hippocampus hippocampal] neurons. Treatment of AD by ACh precursors and [http://en.wikipedia.org/wiki/Cholinergic cholinergic] [http://en.wikipedia.org/wiki/Agonist agonists] was ineffective or caused severe side effects. ACh hydrolysis by AChE causes termination of cholinergic neurotransmission. Therefore, compounds which inhibit AChE might significantly increase the levels of ACh depleted in AD. Indeed, it was shown that [http://en.wikipedia.org/wiki/Acetylcholinesterase_inhibitor AChE inhibitors] improve the cognitive abilities of AD patients at early stages of the disease development. The way in which the various cholinesterase inhibitors interact with AChE can be see at:<br />
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that the active site was deep inside the protein, at the end or base of a
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*[[Acetylcholinesterase: Treatment of Alzheimer's disease]].<br />
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<scene name='Acetylcholinesterase/AChE_gorge/1'>long tunnel or gorge </scene>,
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*[[Acetylcholinesterase complexed with N-9-(1',2',3',4'-tetrahydroacridinyl)-1,8-diaminooctane]].<br />
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lined with aromatic residues, with the peripheral site at the top or lip of this gorge. Amazingly, there were no acidic or negatively charged
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residues anywhere in these 2 sites or along this gorge, as would be expected to
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help attract and bind the basic, positively charged acetylcholine substrate, although are are some acidic residues nearby. Instead, bulky aromatic residues <scene name='Acetylcholinesterase/1ea5_279_84/2'>Trp 279 and Tyr 121 dominate the peripheral site, and Trp 84 and Phe 330 the active site, together with His 440. </scene> (These numbers are the sequential numbering
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of the residues, starting from the N-terminus, according to the '''Torpedo Californica''' form of the enzyme.)
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== Organophosphorus acid anhydride nerve agents ==
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==AChE inhibitors and substrates==
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[http://en.wikipedia.org/wiki/Organophosphorus Organophosphorus] (OP) [http://en.wikipedia.org/wiki/Acid_anhydride acid anhydride] [http://en.wikipedia.org/wiki/Nerve_agent nerve agents] are potent inhibitors which rapidly phosphonylate AChE and then may undergo an internal dealkylation reaction (called "aging") to produce an OP-enzyme conjugate that cannot be reactivated.
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As was mentioned above, AChE hydrolysizes the neurotransmitter <scene name='2wfz/Al/2'>ACh</scene>, producing <scene name='2wfz/Al/3'>choline and an acetate</scene> group. <scene name='2wfz/Al/2'>ACh</scene> directly binds catalytic <scene name='2wfz/Al/4'>Ser200</scene> (via its nucleophilic Oγ atom). <scene name='2wfz/Al/5'>Soman</scene>, [http://en.wikipedia.org/wiki/Soman O-(1,2,2-trimethylpropyl) methylphosphonofluoridate] (<font color='violet'><b>fluorine atom is colored violet</b></font> and <font color='darkmagenta'><b>phosphorus atom is colored darkmagenta</b></font>), is one of the most toxic OPs. Soman inhibits AChE by <scene name='2wfz/Al/6'>covalent binding</scene> to catalytic Ser200, <scene name='2wfz/Al/7'>mimicking ACh</scene>. This process <scene name='2wfz/Al/8'>(phosphonylation)</scene> implicates nucleophilic attack of the Ser200 nucleophilic Oγ atom on the phosphorus atom of soman, with concomitant departure of its fluoride atom. After that AChE catalyzes the <scene name='2wfz/Al/9'>dealkylation</scene> ("aging") of the soman or other OP. This causes irreversible inhibition of AChE, "aged" soman/AChE conjugate can not be reactivated. However, before “aging”, at the step of <scene name='2wfz/Al/8'>phosphonylation</scene>, AChE can be <scene name='2wfz/Al/11'>reactivated</scene> by nucleophiles, such as pralidoxime (2-PAM), resulting in <scene name='2wfz/Al/12'>cleavage</scene> of the phosphonyl adduct from Ser200 Oγ.
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At the <scene name='2wfz/Ali/3'>active site of the nonaged soman/TcAChE conjugate</scene> ([[2wfz]]) the catalytic His440 forms hydrogen bonds with Ser200 Oγ and Glu327 Oε1 via its Nε2 and Nδ1 nitrogens, respectively. The O2 atom of soman is within hydrogen bonding distance of His440 Nε2. Soman O1 mimicks carbonyl oxygen of ACh. A water molecule 1001 interacting with soman O2 is represented as a <font color='red'><b>red ball</b></font>. The active site residues of the nonaged soman/TcAChE are colored <span style="color:yellow;background-color:black;font-weight:bold;">yellow</span>. The O2 atom of the <scene name='2wfz/Ali/4'>dealkylated (aged) soman</scene> ([[2wg0]]) forms a salt bridge with His440 Nε2. The active site residues of the aged soman/TcAChE are colored <span style="color:pink;background-color:black;font-weight:bold;">pink</span>. <scene name='2wfz/Ali/5'>Alignment</scene> of the structures of the nonaged ([[2wfz]]) and aged ([[2wg0]]) conjugates reveals a small, but important, change within the active site - the imidazole ring of His440 is tilted back to a native-like conformation after dealkylation. The water molecule 1001, which interacts with soman O2 in the nonaged crystal structure, is not within hydrogen bonding distance of O2 in the aged crystal structure. 2-PAM binds poorly to the nonaged phosphonylated enzyme (its electron density was not found) and binds in an <scene name='2wfz/Ali/7'>unfavorable and nonfunctional conformation</scene> after soman aging to ''Tc''AChE ([[2wg1]]) <ref name="Sanson">PMID:19642642</ref>.
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<applet load='1eve' size='400' color='white' frame='true' spin='off' caption='Acetycholinesterase Binding E2020' align='right' />
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==Additional resources==
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Among the most interesting drugs that have been designed to inhibit this enzyme are those that
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see: [[Acetylcholinesterase_Additional_Resources]]
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have two binding sites that bind both the peripheral and catatylic sites simultaneously. Such
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drugs bind highly specificly and strongly. A good example is
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<scene name='Acetylcholinesterase/1eve_e2020/1'>the E2020 (Aricept) complex</scene>.
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It appears that the principal interaction between the aceylcholine and the enzyme is relatively newly discovered cation-pi interactions between the cationic moiety of the substrate and the many aromatic residues lining the catalytic gorge. Unlike most
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==Movies==
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interatomic interactions in chemistry, cation-pi interactions are unusual in that their energy hardly changes as the cationic and
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see: [[Acetylcholinesterase_Movies]]
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aromatic ring centers vary between 4 and 7 Angstroms apart, and for a wide variety of relative orientations of the aromatic rings.
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This gives the substrate an energetically smooth ride down the gorge with few bumps or barriers to impede passage down the gorge.
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Most acetylcholinesterases have a net negative charge and a large patch of negative potential around the entrance to the active site gorge, which may be useful to attract the positively charged acetycholine substrate to the site. As one travels down the gorge, this potential becomes increasingly more and more negative, reaching a peak at the active site at the base. Because of this potential, the peripherial site is thought to act like a substrate trap, that forces practically molecule of substrate that reaches
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==Acetylcholinesterase 3D structures==
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the peripheral site to travel down the gorge to the active site, that probably contributes greatly to the extremely rapid rate of degrading the substrate. This whole enzyme therefore acts like a brilliantly designed natural vacuum cleaner that clears the
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neurotransmitter out of the synapse extremely quickly. Yet to be solved, however, is how the products clear the active site rapidly,
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whether back through the gorge, or out a back door on the other side of the protein that quickly opens each catalytic cycle (Try 84
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is actually near the surface of the 'underside' of the protein.)
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==Selected PDB structures of AChE==
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[[Acetylcholinesterase 3D structures]]
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* [[2ace]] This is the original solved structure for '''Torpedo Californica'''
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* [[1ea5]] This is one of the highest quality representative X-ray structures in the PDB.
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* [[1eve]] The E2020 (Aricept) complex.
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* [[1ax9]] Endrophonium complex.
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* [[1vot]] Complex with Huperzine, a Chinese folk medicine.
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* [[1fss]] Complex with snake venum toxin Fasciculin-II.
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* [[1vzj]] Model complex of the Cholinesterase tetramer.
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More structures can be obtained by searching for 'AChE' at the left <-.
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==References==
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<references/>
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</StructureSection>
[[Category: catalytic triad]]
[[Category: catalytic triad]]
[[Category: cholinesterase]]
[[Category: cholinesterase]]
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[[Category: cholinesterases]]
 
[[Category: acetylcholine]]
[[Category: acetylcholine]]
[[Category: cation-pi]]
[[Category: cation-pi]]
[[Category: Alzheimers]]
[[Category: Alzheimers]]
[[Category: nerve gasses]]
[[Category: nerve gasses]]
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[[Category:Topic Page]]

Current revision

Torpedo california AChE (PDB code 2ace)

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