A definitive mechanism for chorismate mutase.

In previous research presentations, we have described the important features of the chorismate --> prephenate reaction using molecular dynamics (MD) and thermodynamic integration studies. This investigation of the reaction in Escherichia coli and water involves QM/MM procedures (SCCDFTB/MM two-dimensional reaction coordinates to identify transition state structures in the water, enzyme, and gas phase followed by B3LYP/6-31+G* single-point computations which allow the determination of activation energies in water and in the E. coli enzyme). Computed activation energies of 11.3 kcal/mol in enzyme and 20.3 kcal/mol in water may be compared to the experimental values of 12.7 and 20.7 kcal/mol, respectively. The transition state structures in the gas phase, water, and enzyme are much the same. The transition states are characteristic of a concerted pericyclic rearrangement. The very small differences in the partial charges of O13 in NAC and TS support only a small preferential (10%) electrostatic stabilization of TS. The free energy of NAC formation in water exceeds that in enzyme by 8.5 kcal/mol, and it is this favored formation of NAC that provides the major kinetic advantage to the enzymatic reaction. These findings compare most favorably with those previous observations of this laboratory employing molecular dynamics and thermodynamic integrations. A definitive mechanism for the chorismate mutase enzymes is provided.