Acetylcholinesterase binding sites

Normally, the acetylcholine concentration in the synaptic cleft is quickly lowered by the enzyme acetylcholinesterase, present in the cleft. When acetylcholine levels remain high for more than a few milliseconds, the receptor is desensitized (Fig. 12-lc). The receptor channel is converted to a third conformation (Fig. 12-4c) in which the channel is closed and the acetylcholine is very tightly bound. The slow release (in tens of milliseconds) of acetylcholine from its binding sites eventually allows the receptor to return to its resting state—closed and resensitized to acetylcholine levels. 

Acetylcholinesterase (AChE) deesterifies the neurotransmitter acetylcholine (ACh). AChE belongs to a group of enzymes considered serine esterases and has a mechanism similar to that of chymotrypsin. AChE has an anionic binding site that attracts the positively charged quaternary ammonium group of ACh. The serine then attacks and cleaves the ester.910... 

Johnson, G., Moore, S.W. (2003). Human acetylcholinesterase binds to mouse laminin-1 and human collagen IV by an electrostatic mechanism at the peripheral anionic site. Neuro-sci. Lett. 337 37-40. 

In a discovery project that is reminiscent of the discovery of captopril, scientists at Takeda created a hypothetical structure for the active site of acetylcholinesterase, based on SAR from previous biochemical and medicinal chemical work (141). The model consisted of (in addition to the serine protease-like catalytic machinery) an anionic binding site separating two discrete hydrophobic binding sites. This model was then used to design inhibitors of the enzyme (reviewed i n ref. 142). One set of analogs examined were based on the N-((o-phthalimidylalkyl)-iV-(a)-phenylalkyl)-amine (scaffold 66). An iterative process of testing. 

Simple quaternary compounds such as tetra-methylammonium cation combine with the substrate cation-binding site of the catalytic surface of acetylcholinesterase and thus deny acetylcholine s access to this site. These compounds have a short duration of action due to the facile reversibility of their binding and r id renal elimination (290), and thus they have minimal therapeutic utility. Cohen and Oosterbaan (291) tabulated a comprehensive list cf tetraalkyl quaternary ammonium acetylcholinesterase inhibitors. Homologation of... 

Marcelo F, Dias C, Martins A, Madeira PJ, Jorge T, Florencio MH et al (2013) Molecular recognition ofrosmarinic acid from Salvia scla-reoides extracts by acetylcholinesterase A new binding site detected by NMR spectroscopy. Chemistry 19 6641-6649... 

Several structures of small molecule complexes with acetylcholinesterase have been solved. They reveal a binding site next to the catalytic serine preferrentially occupied by a positively charged moiety next to a hydrophobic portion. The positively charged functional groups almost superimpose in front of a trj tophan residue at the bottom of the gorge [25-27]. 

In the enzyme acetylcholinesterase, there is evidence that the charged quaternary ammonium group of ACh interacts with an aromatic residue, tryptophan (83). The evidence concerning this interaction is much less certain in the nicotinic receptor, but the putative binding site region contains a number of aromatic amino acids that have been investigated. Several of them are labeled by photoaffinity reagents (84). 

Acetylcholinesterase binds to acetylcholine and cleaves the molecule into acetate and choline. The indirect cholinergic agonists bind to the ACh binding site on the enzyme, thus inhibiting acetylcholinesterase activity. This results in elevated levels of ACh and increased cholinergic effects. 

Taylor, P. and Lappi, S. (1975) Interaction of fluorescence probes with acetylcholinesterase. The site and specificity of propidium binding. Biochemistry 14 1989-1997. 

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