Molecular Dynamics Simulations of the Interaction of Mouse andTorpedoAcetylcholinesterase with Covalent Inhibitors Explain Their Differential Reactivity: Implications for Drug Design

Although the three-dimensional structures of mouse andTorpedo californicaacetylcholinesterase are very similar, their responses to the covalent sulfonylating agents benzenesulfonyl fluoride and phenylmethylsulfonyl fluoride are qualitatively different. Both agents inhibit the mouse enzyme effectively by covalent modification of its active-site serine. In contrast, whereas theTorpedoenzyme is effectively inhibited by benzenesulfonyl fluoride, it is completely resistant to phenylmethylsulfonyl fluoride. A bottleneck midway down the active-site gorge in both enzymes restricts access of ligands to the active site at the bottom of the gorge. Molecular dynamics simulations revealed that the mouse enzyme is substantially more flexible than theTorpedoenzyme, suggesting that enhanced ‘breathing motions’ of the mouse enzyme relative to theTorpedoenzyme might explain why phenylmethylsulfonyl fluoride can reach the active site in mouse acetylcholinesterase, but not in theTorpedoenzyme. Accordingly, we performed docking of the two sulfonylating agents to the two enzymes, followed by molecular dynamics simulations. Whereas benzenesulfonyl fluoride closely approached the active-site serine in both mouse andTorpedoacetylcholinesterase in such simulations, phenylmethylsulfonyl fluoride was able to approach the active-site serine of mouse acetylcholinesterase - but remained trapped above the bottleneck in the case of theTorpedoenzyme. Our studies demonstrate that reliance on docking tools in drug design can produce misleading information. Docking studies should, therefore, also be complemented by molecular dynamics simulations in selection of lead compounds.Author summaryEnzymes are protein molecules that catalyze chemical reactions in living organisms, and are essential for their physiological functions. Proteins have well defined three-dimensional structures, but display flexibility; it is believed that this flexibility, known as their dynamics, plays a role in their function. Here we studied the neuronal enzyme acetylcholinesterase, which breaks down the neurotransmitter, acetylcholine. The active site of this enzyme is deeply buried, and accessed by a narrow gorge. A particular inhibitor, phenylmethylsulfonyl fluoride, is known to inhibit mouse acetylcholinesterase, but not that of the electric fish,Torpedo, even though their structures are very similar. A theoretical technique called molecular dynamics (MD) shows that the mouse enzyme is more flexible than theTorpedo enzyme. Furthermore, when the movement of the inhibitor down the gorge towards the active site is simulated using MD, the phenylmethylsulfonyl fluoride can reach the active site in the mouse enzyme, but not in theTorpedoenzyme, in which it remains trapped midway down the gorge. Our study emphasizes the importance of taking into account not only structure, but also dynamics, in designing drugs targeted towards proteins.