Predicting relative binding free energies of tacrine-huperzine A hybrids as inhibitors of acetylcholinesterase.

The binding of the 9-methyl derivative of tacrine-huperzine A hybrid to Torpedo californica acetylcholinesterase (AChE) has been studied by computational methods. Molecular dynamics simulations have been performed for the AChE-drug complex considering two different ionization states of the protein and two different orientations of the drug in the binding pocket, which were chosen from a previous screening procedure. Analysis of structural fluctuations and of the pattern of interactions between drug and enzyme clearly favor one binding mode for the tacrine-huperzine A hydrid, which mixes effectively some of the binding features of tacrine and huperzine A. The differences in inhibitory activity for a series of related derivatives have been successfully predicted by free energy calculations, which reinforces the confidence in the binding mode and its usefulness for molecular modeling studies. The same techniques have been used to make de novo predictions for a new 3-fluoro-9-ethyl derivative, which can be used to verify a posteriori the goodness of the binding mode. Finally, we have also investigated the effect of replacing Phe300 in the Torpedo californica enzyme by Tyr, which is present in the human AChE. The results indicate that the Phe330-->Tyr mutation is expected to have little effect on the binding affinities. Overall, the whole of results supports the validity of the putative binding model to explain the binding of tacrine-huperzine A hybrids to AChE.