Differences in active site gorge dimensions of cholinesterases revealed by binding of inhibitors to human butyrylcholinesterase

Amino acid sequence alignments of cholinesterases revealed that 6 of 14 aromatic amino acid residues lining the active center gorge of acetylcholinesterase are replaced by aliphatic amino acid residues in butyrylcholinesterase. The Y337(F330) in mammalian acetylcholinesterase, which is replaced by A328 in human butyrylcholinesterase, is implicated in the binding of ligands such as huperzine A, edrophonium, and acridines and one end of bisquaternary compounds such as BW284C51 and decamethonium. Y337 may sterically hinder the binding of phenothiazines such as ethopropazine, which contains a bulky exocyclic substitution. Inhibition studies of (-)- huperzine A with human butyrylcholinesterase mutants, where A328 (K(I) = 194.6 μM) was modified to either F (K(I) = 0.6 μM, as in Torpedo acetylcholinesterase) or Y (K(I) = 0.032 μM, as in mammalian acetylcholinesterase), confirmed previous observations made with acetylcholinesterase mutants that this residue is important for binding huperzine A. Inhibition studies of ethopropazine with butyrylcholinesterase mutants, where A328 (K(I) = 0.18 μM) was modified to either F (K(I) = 0.82 μM) or Y (K(I) = 0.28 μM), suggested that A328 was not solely responsible for the selectivity of ethopropazine. Volume calculations for the active site gorge showed that the poor inhibitory activity of ethopropazine toward acetylcholinesterase was due to the smaller dimension of the active site gorge which was unable to accommodate the bulky inhibitor molecule. The volume of the butyrylcholinesterase active site gorge is ~200 ų larger than that of the acetylcholinesterase gorge, which allows the accommodation of ethopropazine in two different orientations as demonstrated by rigid-body refinement and molecular dynamics calculations.