Dynamic Mechanism of a Fluorinated Oxime Reactivator Unbinding from AChE Gorge in Polarizable Water

A well-tempered metadynamics simulation is performed to study the unbinding process of a fluorinated oxime (FHI-6) drug from the active-site gorge of acetylcholinesterase enzyme in a polarizable water medium. Cation-π interactions and water bridge and hydrogen bridge formations between the protein and the drug molecule are found to strongly influence the unbinding process, forming basins and barriers along the gorge pathway. Distinct unbinding pathways are found when FHI-6 was compared with its recently reported nonfluorinated analogue, HI-6. For example, because of permanent positive charges on both the pyridinium rings of HI-6, it exhibits the minimum in the potential of mean force of the unbinding process in the gorge mouth (where the peripheral anion site, PAS, of the enzyme is located), which is largely caused by cation-π interactions. However, the same interaction, both in the catalytic active-site (CAS) and PAS regions, is found to be greatly enhanced in its lipophilic fluorinated analogue, FHI-6, causing a deep potential energy minimum in the bound state. This may render FHI-6 to be held more firmly in the CAS region of the gorge, as is also evidenced from the microkinetics of unbinding transitions, measured through a combination of metadynamics and hyperdynamics simulations.