Crystal Structure and Functional Sites of Acetylcholinesterase

The crystal structure of acetylcholinesterase revealed that the active site, constituted by a catalytic triad and a so-called anionic subsite, is located at the bottom of a deep and narrow gorge. The anionic subsite recognizes the quaternary ammonium group of the substrate, while the catalytic triad, formed by Glu327, His440, and Ser200, is responsible for hydrolysis of the substrate. At the entrance of gorge, there is allosteric site called the peripheral anionic site. And residues Trp84 and Trp279 play key roles in catalytic and peripheral anionic sites, respectively. The presence of a strong electrostatic dipole directed toward the bottom of the gorge has been elucidated. Such a dipole should be unfavorable to the clearance of the cationic product choline, with an apparent contrast to the high catalytic rate of the enzyme. The opening of a back door at the bottom of the catalytic pocket is a likely alternative for release of products, but this hypothesis is still an object under great controversy. X-ray crystallograpgy and chemical modification studies on the dimensions of the active center gorge will promote the elucidation for molecular basis of the specificity of binding ligands. And the site-directed mutagenesis could play an instrumental role in identifying amino acid residues essential for steric structure and enzymic function.