Catalytic triad cholinesterases

FIGURE 52.2 Anti-ChEs and the cholinergic system. (A) Cholinesterase catalytic triad interactions with acetylcholine and anti-ChEs. (B) Schematic representation of the long-term effects of exposure to anti-ChEs. Source (B) is based on Arbel et al. (2014). 

Fig. 11.2. Schematic representation of the primary structure of secreted AChE B of N. brasiliensis in comparison with that of Torpedo californica, for which the three-dimensional structure has been resolved. The residues in the catalytic triad (Ser-His-Glu) are depicted with an asterisk, and the position of cysteine residues and the predicted intramolecular disulphide bonding pattern common to cholinesterases is indicated. An insertion of 17 amino acids relative to the Torpedo sequence, which would predict a novel loop at the molecular surface, is marked with a black box. The 14 aromatic residues lining the active-site gorge of the Torpedo enzyme are illustrated. Identical residues in the nematode enzyme are indicated in plain text, conservative substitutions are boxed, and non-conservative substitutions are circled. The amino acid sequence of AChE C is 90% identical to AChE B, and differs only in the features illustrated in that Thr-70 is substituted by Ser.

Other serine hydrolases such as cholinesterases, carboxylesterases, lipases, and fl-lactamases of classes A, C, and D have a hydrolytic mechanism similar to that of serine peptidases [25-27], The catalytic mechanism also involves an acylation and a deacylation step at a serine residue in the active center (see Fig. 3.3). All serine hydrolases have in common that they are inhibited by covalent attachment of diisopropyl phosphorofluoridate (3.2) to the catalytic serine residue. The catalytic site of esterases and lipases has been less extensively investigated than that of serine peptidases, but much evidence has accumulated that they also contain a catalytic triad composed of serine, histidine, and aspartate or glutamate (Table 3.1). 

The covalent bond between OP and the active site serine of intact cholinesterase is stable, but not irreversible. Hydrolysis can occur with a half-life of between 10 and 35,000 min, depending on the enzyme, OP, temperature, pH, and buffer composition. The adduct becomes irreversibly bound to the enzyme after one of the alkyl groups on the OP is lost in a step called aging (Benschop and Keijer, 1966 Michel et al., 1967). The dealkylated OP makes a stable salt bridge with the protonated histidine of the catalytic triad, so that histidine is no longer available for the dephosphorylation step that would otherwise have restored the enzyme to an uninhibited state. Hundreds of scientists have contributed to this understanding... 

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. 

The enzymatic nucleophile is similar in kind and reactivity to the ultimate solution acceptor. Examples of this class include the serine proteases and the alkahne phosphatase. The serine hydroxyl group is similar in chemical reactivity to the hydroxyl group of water, the final acceptor in these group transfer reactions (Fersht, 1985). For example, the active site Ser200 and His444 of cholinesterase are involved in a putative catalytic triad to effect acyl transfer (Taylor, 1991) ... 

The human lysosmal serine carboxypeptidase CatA is a member of the a/p hydrolase enzyme family and therefore shares structural homology with yeast carboxypeptidase Y and cholinesterases [4] (Figure 23.1). The common feature shared by members of this family is a three-dimensional structure of eight p sheets connected by a-heUces resulting in a typical topological localization of the catalytic triad, in which only the His residue is conserved [2]. 

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