Acetylcholinesterase esteratic site

Acetylcholinesterase can be inhibited by two general mechanisms. In the first mechanism, positively charged quaternary ammonium compounds bind to the anionic site and prevent ACh from binding—a simple competitive inhibition. In the second mechanism, the agents act either as a false substrate for the cholinesterase or directly attack the esteratic site in both cases they covalently modify the esteratic site and non-competitively prevent further hydrolytic activity. Either mechanism can be effective in preventing the hydroly-... 

The OPPs inhibit acetylcholinesterase (AChE) by phosphorylating the esteratic site of the enzyme. As a result of AChE inhibition, ACh accumulates and binds to muscarinic and nicotinic receptors throughout the nervous system. Transformation of OPPs in the organisms takes place by conversion of the phosphorothioate (P=S) group to oxon (P=0) analogs. These oxo compounds are of concern because they are the activated forms of the OPPs, with a considerably stronger inhibition of acetylcholinesterase activity (27). 

The ketoxime derivative fluvoxamine (12) is a newer antidepressant thought to potentiate the action of 5-hydroxytryptamine76. Oxacillin (13), cefuroxime (14) as well as the monobactam aztreonam (15) represent potent antibacterial agents of the beta-lactam type77. The aldoxime pralidoxime (16) and a number of bi.v-quarternary oximes, such as obidoxime (17), can be used as reactivators of the phosphorylated esteratic site of acetylcholinesterase that occurs in the presence of organophosphate inhibitors78,79. 

The reversible inhibitors, which have a short to moderate duration of action, fall into two categories. Type one, exemplified by edrophonium, forms an ionic bond at the anionic site and a weak hydrogen bond at the esteratic site of acetylcholinesterase. Type two, exemplified by neostigmine, forms an ionic bond at the anionic site and a hydrolyzable covalent bond at the esteratic site. The irreversible inhibitors, exemplified by organophosphorus compounds (diisopropyl fluorophosphate, parathion,... 

Organic phosphates can cause moderate to severe acute poisoning. These substances inhibit the function of the enzyme, acetylcholinesterase by phospho-rylating or binding the enzyme at its esteratic site. The symptoms of acute toxicity... 

Although OPPs and carbamates exhibit very similar modes of action in various animal species, i.e, acetylcholinesterase inhibition in the CNS with resulting paralysis—there is an important difference between the two classes of pesticides. Carbamates do not require metabolic conversion prior to exhibiting their toxicity. Furthermore the enzyme activity may at times be rapidly regenerated by reversal of inhibition. The kinetics of the inhibition (carbamoylation) reaction have been well studied in it electrophilic carbamoyl moieties form covalent bonds with enzyme esteratic sites. This is followed by carbamate transfer of an acidic group to the site to yield the acetylated enzyme complex (ref. 176). 

Acetylcholinesterase is only cholinesterase in insects. It is mainly located in the neuropile (area of synapses between nerve fibers) of the CNS in insects (Toutant, 1989). AChE contains two active sites, the esteratic site and the anionic site. The esteratic site possesses the hydroxyl group of serine and a basic nucleophilic imidazole group of histidine. The anionic site has a free carboxyl group (aspartic acid and/or glutamic acid). The interaction of ACh with AChE can be divided into three steps, as shown in Figure 7.13. The first... 

Acyl Cholinesterases. Acetylcholinesterase (AChE EC 3.1.1.7 CAS 9000-81-1) is the serine esterase which catalyzes the hydrolysis of acetylcholine and possesses an esteratic site, and which is responsible for unspecific hydrolyses of several substrates. Also, butyrylcholinesterase (EC 3.1.1.8 CAS 9001-08-5) has been sometimes used for asymmetric hydrolysis of esters. Acetylcholinesterase has been used for... 

There are three primary cholinesterases (ChE) in the body. Acetylcholinesterase is located in the vicinity of ACh receptors at neuronal and neuromuscular junctions. Acetylcholinesterase terminates ACh activity via hydrolysis into choline and acetic acid. The positively charged choline portion of the ACh molecule attaches to the anionic site and the acetyl region attaches to the esteratic site on the AChE molecule. Eollowing the attachment of the two regions, choline is rapidly released to be recycled back into the presynaptic nerve terminal, and the acetyl group reacts with... 

It has already been mentioned that there are some doubts A26) about the existence of an anionic site in human or horse cholinesterase. Comparative kinetic studies using a series of pyridylcarbinol acetates as substrates have shown that acetylcholinesterase from T. marmorata electric organ and the plasma cholinesterases from horse and man have similar esteratic sites. It was also shown that the electric eel organ enzyme has an anionic site, whereas the second site of butyrylcholine... 

The hydrophobic area surrounding the anionic site plays a more important role for butyrylcholinesterase than for acetylcholinesterase. The greater importance of this hydrophobic area for butyrylcholinesterase could help to explain and resolve some of the opposing views of earlier workers (A26). Kabachnik et al. (Kl) also proposed that in the vicinity of the esteratic site of butyrylcholinesterase there are two hydrophobic areas separated by a hydrophilic group. Differences in length and structure of the hydrophobic areas of the active surfaces of butyryl- and... 

The quaternary ammonium compound Tris [tris-(hydroxymethyl)-aminomethane] has been found to be a competitive inhibitor of horse plasma cholinesterase using butyrylcholine as substrate (P5), with inhibition occurring at concentrations commonly used in buffer solutions. Tris is believed to compete for the esteratic site but, in the absence of Mg and Ca, the enzyme was activated. The exact role of the cations was not investigated, but a complex of the cation with an amino group could be the effective inhibitor. Similar results were obtained using acetylcholinesterase from r. marmorata. 

Figure 1. Schematic of the probable physical structure of acetylcholinesterase. One physical region carries the esteratic site, which is proximal to one anionic site (Site I) the other region would carry at least four anionic sites, and would be homologous to the acetylcholine receptor of the motor end plate excitable membrane. Sites I and II are masked by DPA, but Site II can be regenerated at alkaline pH. Decamethonium (C10) would interact at least with Sites I and II whereas curare would bind at III and TV, and perhaps at II and III. Most quaternary salt substituents bind on the anionic chain [exo-binding (26, 36, 42.

The phosphorus ester molecule, blocking the activity of acetylcholinesterase, first takes up a steric orientation determined by the anionic and acidic groups of the enzyme, and this is then followed by hydrolysis of the phosphorus ester and by phosphorylation of the hydroxyl group of serine at the esteratic site of the enzyme ... 

The phosphorylation of acetylcholinesterase is rapid while the hydrolysis of the phosphorylated enzyme is a slow process. Therefore, the enzyme is unable to function on prolonged blocking of the esteratic site. Acetylcholine accumulates which finally results in endogenic acetylcholine poisoning. 

Because the ACh receptor does not hydrolyse acetylcholine, the esteratic site of acetylcholinesterase (see Fig. 12.3) must be absent. Other fundamental differences in the two sites are indicated by the following (a) dimethylbutyl acetate 12.66) is a good substrate for the enzyme, but barely activates the receptor which requires a basic group for marked activity (b) muscarine is not a substrate for the enzyme and yet it is a powerful agonist for the muscarinic receptor (c) di-isopropyl phosphorofluoridate (isoflurophate) 13.26) binds to the active site of AChase but not to an ACh receptor (d) acetyl-bungarotoxin specifically binds to the ACh nicotinic receptor but not to the enzyme. 

The TT net charge, nucleophilic w superdelocalizability, and electrophilic tt superdelocalizability parameters were applied to the acetylcholinesterase inhibitory potencies of some 3-hydroxyphenylammonium derivatives with considerable success. Correlations indicate the inhibitory response depends upon the strength of the hydrogen bond of the 3-hydroxy group of the inhibitor and esteratic site on the acetylcholinesterase. ... 

The mechanism of the action of acetylcholinesterase purified from the electric organs of Electrophorus electricus involves the attraction of the positively charged nitrogen of acetylcholine to an anionic site on the enzyme and cleavage of the substrate at an esteratic site of a nucleophilic character. The irreversible inhibition by the alkyl phosphates, tetraethyl pyrophosphate (TEPP) and diisopropyl-fluorophosphate (DFP) may be due to phosphorylation of the nucleophilic esteratic site. The phosphorylation by DFP of the phenolic hydroxyl group of free tyrosine has been demonstrated by Ashbolt and Rydon. Chymotrypsin and citrus fruit acetylesterase are also inhibited by DFP and TEPP. ... 

Acetylcholine is a neurotransmitter at cholinergic sites and acetylcholinesterase is the esterase, which brings about the hydrolysis of acetylcholine after it has performed its purpose of neurotransmission at synapses and cholinergic effector sites. Acetylcholine binds to acetylcholinesterase at two sites, namely the anionic site and the esteratic site. The quaternary nitrogen of choline forms an electrostatic link at the anionic site, while the carbonyl group binds to a serine residue at the esteratic site. The reaction for acetylcholine can be visualized as shown in eqn (10.1) below, E being the enzyme, AX acetylcholine, EAX is a reversible Michaelis-Menten complex and A is acetate ... 

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