Determination By X-Ray Crystallography of the Three Dimensional- Structure of Acetylcholinesterase from Torpedo Electric Organ

Abstract : The X-ray structure of Torpedo California Acetylcholinesterase (AChE) was determined. The active site lies near the bottom of a deep cavity: the aromatic gorge. The choline-binding subsite contains one negative charge, and modelling reveals that the quaternary group of acetylcholine interacts principally with the indole of W84. X-ray structures of complexes of AChE with edrophonium and tacrine show that both interact with W84. The structure of a complex with the bisquaternary inhibitor, 1,5-bus(4- allyldimenthylammoniumphenyl) pentane-3-one, shows that one quaternary nitrogen interacts with W84, while the distal quaternary nitrogen interacts with W279, thus establishing the location of the 'peripheral' binding site. Human butyrylcholinesterase (BChE) was modelled on the basis of AChE. It closely resembles AChE; two aromatics, F288 and F290, replaced by Leu and Val, respectively, in BChE, may prevent entrance of butyrylcholine into the acyl- binding pocket. Site-directed mutagenesis produced a double mutant, F288L/F290V, which hydrolyzes both butyrylthiocholine and acetylthiocholine, and is inhibited by the BChE-specific organophosphate, isoOMPA. Another mutant, W279A, displays reduced inhibition, relative to the wild type, by the peripheral-site ligand, propidium, whereas inhibition by edrophonium is unaffected, corroborating the assignment of the peripheral anionic site.