Catalytic triad acetylcholinesterase

The esteratic subsite contains the catalytic machinery of the enzyme. The catalytic triad residues - Ser 200, His 440 and Glu 327 (the residue numbering in this section refers to Torpedo californica acetylcholinesterase, TcAChE) - are identical in both enzymes and basically in the same positions. 

FIGURE 10.3 Acetylcholinesterase structure of catalytic triad. The structure of the catalytic triad of the active center of the enzyme is shown (from Sussman et al. 1991). 

Zhang Y, Kua J, McCammon JA (2002) Role of the catalytic triad and oxyanion hole in acetylcholinesterase catalysis an ab initio QM/MM study. J Am Chem Soc 124 10572—10577... 

All of these esterases appear to act by mechanisms closely related to those of proteases. Acetylcholinesterase contains an active site serine that reacts with organophosphorus compounds (Box 12-E) and is part of an Asp-His-Ser catalytic triad which lies in a deep "gorge" as well as an oxyanion hole.637 A surprise is the absence of an essential carboxylate group that might bind the positively charged trimethylammonium... 

Figure 9.15. Catalytic mechanism of acetylcholinesterase, a A catalytic triad yields a deprotonated serine, which attacks acetylcholine and winds np as an acetylated intermediate. Transfer of the acetyl gronp to the enzyme involves a tetrahedral transition state, b Hydrolysis of the acetyl group is facilitated by an analogous mechanism, which involves a hydroxide anion as the nucleophile.

Phosphylation of the OH moiety of the serine residue, being part of the catalytic triad in the esteratic center of acetylcholinesterase (AChE), represents the pathophysio-logically most important reaction resulting in enzyme deactivation. Inhibition of AChE was proven to be the predominant major reaction in vivo that causes death... 

Millard, C.B., Koellner, G., Ordentlich, A., Shafferman, A., Silman, I., Sussman, J. (1999b). Reaction products of acetylcholinesterase and VX reveal a mobile histidine in the catalytic triad. J. Am. Chem. Soc. 121 9883-4. 

The discovery of catalytic triads in lipases and in related esterases, such as acetylcholinesterase (AChE) (Sussman et al., 1991) and cutinase (Martinez et al., 1992), revived interest in this otherwise well-known constellation of amino acids (like GcL, the AChE triad includes a glutamate). It should also be remembered that there are other functionally... 

Acetylcholinesterase (AChE) catalyses the hydrolysis of the ester bond of acetylcholine to yield choline and acetate (Sussman et al., 1991). This is a critical reaction for the termination of impulses transmitted through cholinergic synapses. It is a highly efficient catalyst, with reaction rates approaching the diffusion limit. Its overall structure resembles the lipases with an active site gorge. Above the base of the gorge is the reactive serine to be activated by the classical (Ser-200...His-440...Glu-327) catalytic triad. 

Figure 34-18 Reactivation of phosphoryiated acetylcholinesterase by pralidoxime formation of aged phosphoryiated enzyme, which does not reactivate.The active site catalytic triad of serine, histidine, and glutamate is depicted by —OH, =NH—, and a negative charge, respectively.

The action of OP nerve agents on the nervous system results from their effects on enzymes, particularly esterases. The most notable of these esterases is acetylcholinesterase. The active site of acetylcholinesterase comprises a catalytic triad of serine, histidine and glutamic acid residues and other important features of the enzyme are a gorge connecting the active site to the surface of the protein and a peripheral anionic site (Bourne etal., 1995,1999 Sussman etal., 1991 Thompson and Richardson, 2004), The OPs phosphy-late1 the serine hydroxyl group in the active site of the enzyme. 

Acetylcholinesterase is mechanistically related to serine proteases and involves acylation of Ser and contains the catalytic triad Ser -His -Glu. Precise data for acetylation by the natural substrate acetylcholine is difficult to obtain due to lack of a convenient assay method, so the mechanistically equivalent surrogate acetylthio-choline has frequently been used to probe mechanistic aspects of this enzyme [15]. To explore proton transfer reactions that accompany acetylation by this substrate, solvent isotope effect measurements and proton inventories on k /Km have been conducted [16, 17]. The isotope effects are near unity and the proton inventories bowed-upwards, suggesting that the transition state for kjKm is a virtual transi-... 

Acetylcholinesterase is a serine esterase whose catalytic mechanism is similar to that of the serine proteases. As with chymotrypsin and trypsin, the active site of acetylcholinesterase has serine as part of a Ser-His-Asp catalytic triad. The mechanism -will involve covalent tetrahedral and acyl enzyme intermediates in which the substrate is bonded covalently to the active-site Ser. The reaction starts with nucleophilic attack on... 

One group of esterases has an a,p-fold and is prominent in the liver cytosol (Quinn, 1997). Acetylcholinesterase, butyl cholinesterase, and lipases have been used as models for these esterases. Generally esterases also have amidase activity (and vice versa, due to the basic mechanisms). All esterases appear to use a catalytic triad to activate a nucleophile, which is used to form an enzyme-acyl intermediate. The triad consists of a nucleophile, a general base catalyst, and an acidic residue. 

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