AChE gorge

The PAS is located at the rim of the aromatic gorge, on the protein s surface. It spans six AChE residues Tyr 72, Tyr 124, Glu 285 and Tip 286, on one side of the gorge entrance, and Asp 74 and Tyr 341, on its opposite side. Its core is comprised of Tip 286 and Asp 74, which accommodates many distinct ligands. BChE also has a PAS, but its relatively aromatic content and the response upon ligand-binding differ significantly from those of AChE. 

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.

The initial solution of the crystal structure of the Torpedo enzyme [28], followed by the mammalian AChE structure [29], revealed that the active center serine lies at the base of a rather narrow gorge that is lined heavily with aromatic residues (Fig. 11-6). The enzyme carries a net negative charge, and an electrostatic dipole is oriented on the enzyme to facilitate diffusional entry of cationic ligands. Crystal structures of several inhibitors in a complex with AChE also have been elucidated [25]. 

The detailed mechanism by which AChE and BChE hydrolyze ACh has been the subject of much research, especially since the crystal structure of the Torpedo califomica AChE was elucidated by Sussman et al. in 1991 [12]. (Reviews of these enzymes and their interactions can be found in Refs. [5,13]). This mechanism will be described here only briefly, as an introduction to the reaction of the enzyme with carbamates. The active site of AChE is located at the bottom of a 20 A-deep gorge, where acetylcholine fits in by attachment of the quaternary ammonium group to the so-called anionic site (mainly through cation interaction with the n electrons of Trp84), and by dipole interactions between the ester group and Ser200 at the esteratic site . 

Other reversible inhibitors, such as propidium and the peptide toxin fasciculin, bind to the peripheral anionic site on AChE. This site resides at the lip of the gorge and is defined by tryptophan 286 and tyrosines 72 and 124 (Taylor, 2001). 

The second anionic site of AChE, the so-called peripheral anionic site, is located at the active center gorge entry, and encompasses overlapping binding sites for different... 

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. 

The three-dimensional structure of AChE from the electric organ of Torpedo californica has been established. One interesting feature is that the active site is embedded in a gorge of 20 A that reaches halfway into the protein. The postulated anionic site , theoretically invoked to bind the quaternary ammonium ion of ACh, appears to be represented by aromatic amino acids in the gorge itself these and charges in the active center are believed to stabilize the choline group. In addition, some inhibitions, such as that due... 

Figure 24. Close-up view of the active site of AChE with the triad in yellow, the aromatic side chains coating the deep gorge in red, and a model of the acetylcholine transition state in pink.

These results suggest that E2020 is a noncovalent AChE inhibitor which binds to the hydrophobic gorge but does not interact with the catalytic residues. 

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