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

Abstract

The reaction mechanisms of two inhibitor TFK(+) and TFK(0) 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 8kcal/mol. These predictions await experimental verification. In the case of the neutral ligand TFK(0), however, multiple MD simulations on the TFK(0)/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 TFK(0) is almost inactive in the H447I mutant, and also provide detailed mechanistic insights into the experimental observations.

Figure 1

(a) The acylation mechanism in the wild-type AChE enzyme; (b) The chemical structures of TFK⁺ and TFK⁰.

Figure 2

The “water” triad in the active site of the six [M·T⁺] models. The values of the distances in Å are averaged among six models.

Figure 3

Illustration of the reaction coordinate used for the H447I mutant and TFK⁺ reaction, which is dO_γ−H_γ+dO_w−H −dO_γ−C −dO_w−H_γ−d_H−O_δ.