Enzyme binding specificity and catalytic mechanisms in both wild-type mAChE and H447I mutant: A combined QM/MM-FE and thermodynamic integration study

COMP 50

Yuhui Cheng, ycheng@mccammon.ucsd.edu, Department of Chemistry and Biochemistry, University of Carlifornia at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0365, Xiaolin Cheng, xcheng@mccammon.ucsd.edu, Department of Chemistry and Biochemistry, University of California, San Diego, Urey Hall 4206, 9500 Gilman Drive, La Jolla, CA 92093, and J. Andrew McCammon, jmccammon@ucsd.edu, Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, Mail Code 0365, La Jolla, CA 92093-0365.
The reaction mechanism of two inhibitors TFK+ or TFK0 binding to either wildtype or H447I mutated mouse acetylcholinesterase (mAChE) has been investigated by a combined ab initio quantum mechanical/molecular mechanical (QM/MM) approach and classical molecular dynamics simulations. In the wild-type mAChE, either TFK+ or TFK0 binding is a spontaneous process. The reaction proceeds through the nucleophilic addition of the Ser203-O&gamma to the carbonyl-C of TFK+ or TFK0, and the reaction is facilitated by simultaneous proton transfer from Ser203 to His447. The free energy difference between the tetrahedral intermediate and the near-attack reactant conformation at the B3LYP(6-31G*) QM/MM level is over 20 kcal/mol, which is consistent with the experimental reaction rate. Furthermore, a TFK+ binding mechanism in the H447I mutant has been proposed. A water molecule takes over the role of His447 and participates in the breaking and forming of bonds as a "charge relayer". The potential energy barrier for this reaction at MP2(6-31+G*) QM/MM level is less than 10 kcal/mol, which is in accord with experimental measurements. In contrast with the wild type mAChE, the conserved residue of the catalytic triad, Glu334, acts as the catalytic base in the reaction. These predictions await experimental comparisons. The oxyanion hole, formed by peptidic NH groups from Gly121, Gly122, and Ala204, is also found to play an important role in catalysis as in the wild type. However, molecular dynamics simulation on the TFK0 and H447I complex suggests that a water molecule cannot replace His447 for the case of the neutral ligand. Thermodynamic integration in the complex demonstrates that the TFK0 and H447I complex is much less stable at all thermodynamically. Therefore, TFK0 is inactive in H447I mutant, which is quite consistent with experimental observations.
 

General Oral: Quantum Chemistry
1:00 PM-5:00 PM, Sunday, 10 September 2006 Moscone Center -- Room 208/210, Oral

Division of Computers in Chemistry

The 232nd ACS National Meeting, San Francisco, CA, September 10-14, 2006