Modelling and Mutagenesis of Butyrylcholinesterase Based on the X-Ray Structure of Acetylcholinesterase

Abstract

Human butyrylcholinesterase was modelled on the basis of the three-dimensional structure of Torpedo acetylcholinesterase with which it displays high sequence homology. The structure obtained was, overall, very similar to that of acetylcholinesterase. However, six aromatic amino acid residues, out of fourteen lining the active-site gorge of acetylcholinesterase, are absent in butyrylcholinesterase. Modelling revealed that two such residues, F288 and F290, which are replaced by aliphatic residues in butyrylcholinesterase, may prevent butyrylcholine from fitting into the active site of acetylcholinesterase. Their mutation to the appropriate residues, L and V, respectively, in Torpedo acetylcholinesterase, produced a double-mutant which hydrolysed butyrylthiocholine almost as well as acetylthiocholine. It was also inhibited efficiently by the bulky, butyrylcholinesterase-selective organophosphate, isoOMPA. W279, situated at the top of the aromatic gorge in Torpedo acetylcholinesterase, is absent in butyrylcholinesterase, thus being a candidate for the ‘peripheral’ anionic site which is lacking in the latter. The mutant, W279A, was much less sensitive to inhibition by the ‘peripheral’ site ligand, propidium, than wild-type Torpedo acetylcholinesterase, while its inhibition by the catalytic-site quaternary inhibitor, edrophonium, was unaffected.