Anionic site of cholinesterase

Figure 6.1 Synthesis and metabolism of acetylcholine. Choline is acetylated by reacting with acetyl-CoA in the presence of choline acetyltransferase to form acetylcholine (1). The acetylcholine binds to the anionic site of cholinesterase and reacts with the hydroxy group of serine on the esteratic site of the enzyme (2). The cholinesterase thus becomes acetylated and choline splits off to be taken back into the nerve terminal for further ACh synthesis (3). The acetylated enzyme is then rapidly hydrolised back to its active state with the formation of acetic acid (4)...

Fio. 10. The arrangement of the cationic inhibitor tetrametliylammonium at the anionic site of cholinesterases. (For true ChE, the arrangement of lowest potential energy is shown, with the centers of the three ions involved lying on a straight line.)... 

On the Forces Acting Between Quaternary Ammonium Ions and the Anionic Sites of Cholinesterases... 

Free Energies and Interionic Distances for the Association of Ammonium Ion with the Anionic Sites of Cholinesterases Calculations are based on Equation (10)... 

It may be noted that many of the anticholinesterases of the competitive type are equally potent as inhibitors of a- and / -cholinesterases. We should add, however, that the existence of an anionic site in / -cholinesterases has been questioned.1... 

As given in classification, these agents are of two type e.g. reversible and irreversible. The reversible anticholinesterases have a structural resemblance to acetylcholine, are capable of combining with anionic and esteratic sites of cholinesterase as well as with acetylcholine receptor. The complex formed with the esteratic site of cholinesterase is less readily hydrolyzed than the acetyl esteratic site complex formed with acetylcholine. Edrophonium forms reversible complex with the anionic site and has shorter duration of action. Also, neostigmine and edrophonium have a direct stimulating action at cholinergic sites. 

Interionic Distances and Number of Anionic Sites in Cholinesterases... 

Comprehensive reviews (Kl, Ul) of the active sites of cholinesterase both postulated the presence not only of an esteratic site for butyrylcholinesterase but also of an anionic site. Additionally, in the region of the anionic site, there are two hydrophobic areas, one directly surrounding the anionic group and the second located at some distance from it (Kl). The presence of hydrophobic areas has been established (B32, C3, H29, H45, MIO) by the use of fluorescent probes with spectral responses which reflect the environment of the probe. Such probes can be used to monitor changes in the conformations of enzymes and can be designed to be active-site-directed, competitive inhibitors (H30). Aspects of the spectroscopy of intrinsic and extrinsic fluorescent probes have been reported (C3). 

Thus, it is probable that the active site of cholinesterase comprises anionic and esteratic subsites and also, as with most proteins, hydro-phobic areas. The anionic site determines specificity with respect to the choline moiety, while the actual catalytic process takes place at the esteratic site (F8). 

Carbamates generally act quickly. They are strongly toxic to a wide range of insect pests, but have a weak effect on the red spider mite. Some of them exhibit systemic characteristics. The duration of their action varies considerably. In a similar manner to the phosporic acid esters discussed later, they exert their action by paralysing the cholinesterase enzyme. During this process, the carbamate part of the molecule is attached to the esteratic site, and the aromatic part to the anionic site of the cholinesterase enzyme. As the distance between the esteratic and anionic sites is SO nm in the cholinesterase molecule, carbamate insecticides will be most efficient if the distance between the two groups to be bound to the two sites of the enzyme is also 50 nm. (Metcalf and Fukuto, 1965 1967 Fukuto et ai, 1967). 

Fig. 6. Scheme of interaction of acetylcholine with the active centres of cholino-receptor and cholinesterase. E the esterophilic site of ChR and the esteratic site of ChE A the anionic site of ChR and of ChE. 

The anionic site of ChE does not make such rigid demands on the structure of the cationic head of substrates and inhibitors. For instance, acetyltriethylcholine, which is completely devoid of cholinomimetic activity, is hydrolysed by the cholinesterases at nearly the same rate as ACh. 

The mechanism of action of the cholinesterases is much better undmstood than the mechanism of action of cholinoreceptors. The main function of cholinesterase i.e. the hydrolysis of ACh. is carried out by its esteratic site. In the structure of the esteratic site the hydroxyl of the amino-acid serine plays a central role When the ACh molecule attacks the active surface of cholinesterase the first event is the sorption of the trimethylammonium end of ACh on the anionic site of ChE. This sorption fixes the ACh molecule at the necessary distance from the esteratic site... 

We may now consider in a little more detail the interaction of true (or a-) cholinesterase with acetylcholine. Wilson and Berg mann1 suggest that there are two active sites in the enzyme, known as anionic site and esteratic site respectively. These sites (represented diagrammatically in fig. II)2 are not to be considered independent. The mode of attachment will be seen to depend upon (a) the quaternary nitrogen atom (N+< ) and... 

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... 

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