Acetylcholinesterase: Mechanisms of covalent inhibition of H447I mutant determined by computational analyses

The reaction mechanisms of two inhibitor TFK+ and TFK0 binding to H447I mutant mouse acetylcholinesterase (mAChE) have been investigated by using a combined ab initio quantum mechanical/molecular mechanical (QM/MM) approach and classical molecular dynamics (MD) simulations. TFK+ binding to the H447I mutant may proceed with a different reaction mechanism from the wild-type. A water molecule takes over the role of His447 and participates in the bond breaking and forming as a “charge relayer”. Unlike in the wild-type mAChE case, Glu334, a conserved residue from the catalytic triad, acts as a catalytic base in the reaction. The calculated energy barrier for this reaction is about 8 kcal/mol. These predictions await experimental verification. In the case of the neutral ligand TFK0, however, multiple MD simulations on the TFK0/H447I complex reveal that none of the water molecules can be retained in the active site as a “catalytic” water. Taken together our computational studies confirm that TFK0 is almost inactive in the H447I mutant, and also provide detailed mechanistic insights into the experimental observations.