Acetylcholinesterase active-site gorge

Harel, M., Sussman, J.L. Quaternary Hgand Binding to Aromatic Residues in the Active Site Gorge of Acetylcholinesterase. Proc. Natl. Acad. Sci. USA 1993, 90, 9031-9035. 

Hard M, Schalk I, Ehretsabatier L, Bouet F, Goeldner M, Hirth C, Axelsen PH, Silman I, Sussman JL (1993) Quaternary ligand binding to aromatic residues in the active-site gorge of acetylcholinesterase. Proc Natl Acad Sci USA 90 9031-9035... 

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. 

Axelsen, P.H., Harel, M., Silman, I., and Sussman, J.L. 1994. Structure and dynamics of the active-site gorge of acetylcholinesterase synergistic use of molecular-dynamics simulation and x-ray crystallography. Protein Sci., 3, 188-197. 

Koellner, G., Kryger, G., Millard, C.B., Silman, I., Sussman, J.L., and Steiner, T. 2000. Active-site gorge and buried water molecules in crystal structures of acetylcholinesterase from Torpedo californica.. Mol. Biol., 296, 713-735. 

Xu YC, Colletier JP, Weik M, Jiang HL, Moult J, Silman I, Sussman JL (2008) Flexibility of aromatic residues in the active-site gorge of acetylcholinesterase X-ray versus molecular dynamics. Biophys J 95(5) 2500-2511. doi 10.1529/biophysj.108.129601... 

Saltmarsh, J. R. Boyd, A. E. Rodriguez, O. R Radic, Z. Taylor, R Thompson, C. M. Synthesis of fluorescent probes directed to the active site gorge of acetylcholinesterase. Bioorg. Med. Chem. Lett. 2000, 10, 1523-1526. 

Fig. 2. The acetylcholinesterase gorge showing regions where binding takes place and interaction of acetylcholine with active site.

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... 

Acetylcholinesterase inhibition has been widely used for pesticide detection [88-94], but less exploited than protein phosphatase inhibition for cyanobacterial toxin detection. Nevertheless, the anatoxin-a(s) has more inhibition power than most insecticides, as demonstrated by the higher inhibition rates [95]. In order to detect toxin concentrations smaller than usually, mutant enzymes with increased sensitivity were obtained by genetic engineering strategies residue replacement, deletion, insertion and combination of mutations. Modifications close to the active site, located at the bottom of a narrow gorge, made the entrance of the toxin easier and enhanced the sensitivity of the enzyme. 

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. 

Fig 5-2. This schematic ribbon diagram shows the structure of Torpedo californica acetylcholinesterase. The diagram is color-coded green the 537-amino acid polypeptide of the enzyme monomer pink the 14 aromatic residues that line the deep aromatic gorge leading to the active site and gold and blue a model of the natural substrate for acetylcholinesterase, the neurotransmitter acetylcholine, docked in the active site. Reprinted with permission from Sussman JL, Silman I. Acetylcholinesterase Structure and use as a model for specific cation-protein interactions. Curr Opin Struct Biol. 1992 2 724. 

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