Reaction Pathway and Free Energy Profile for Prechemical Reaction Step of Human Butyrylcholinesterase-Catalyzed Hydrolysis of (-)-Cocaine by Combined Targeted Molecular Dynamics and Potential of Mean Force Simulations

Combined targeted molecular dynamics (TMD) and potential of mean force (PMF) simulations have been carried out to uncover the detailed pathway and determine the corresponding free energy profile for the structural transformation from the nonprereactive butyrylcholinesterase (BChE)-(-)-cocaine binding to the prereactive BChE-(-)-cocaine binding associated with the (-)-cocaine rotation in the binding pocket of BChE. It has been shown that the structural transformation involves two transition states (TS1(rot) and TS2(rot)). TS1(rot) is mainly associated with the deformation of the nonprereactive complex, whereas TS2(rot) is mainly associated with the formation of the prereactive complex. It has also been demonstrated that the A328W/Y332G mutation significantly reduces the steric hindrance for (-)-cocaine rotation in the binding pocket of BChE and, thus, decreases the free energy barrier for the structural transformation from the nonprereactive binding to the prereactive binding. The calculated relative free energy barriers are all consistent with available experimental kinetic data. The new mechanistic insights obtained and the novel computational protocol tested in this study should be valuable for future computational design of high-activity mutants of BChE. The general computational strategy and approach based on the combined TMD and PMF simulations may be also valuable in computational studies of detailed pathways and free energy profiles for other similar mechanistic problems involving ligand rotation or another type of structural transformation in the binding pocket of a protein.