The EVB free energy profiles were calculated using the FEP umbrella sampling approach described elsewhere. These calculations utilized 51 discrete FEP windows, with a constant spacing of 0.02, between the two end-points chorismate ( = 0) and prephenate ( = 1). The EVB Hamiltonian was parametrized by fitting the relevant parameters (Δα and H12) for the uncatalyzed transformation of chorismate to prephenate in water to reproduce the energetics obtained with DFT. That is, the activation free energy, ∆G ‡ , was fitted to 24.5 kcal/mol, which is also the experimentally determined barrier, whereas the reaction free energy, ∆G0, was fitted to -12.8 kcal/mol, which is in excellent agreement with the previously estimated ∆G0 from the observed reaction enthalpy, ∆H0 = -13.3 kcal/mol, utilized by Warshel et al. Thermodynamic activation parameters were obtained from EVB/Arrhenius plots as explained elsewhere based on simulations at 6 different temperatures evenly spaced from 288-313 K. A total of 100 independent simulations were carried out at each temperature point, resulting in 600 individual EVB simulations (306 ns) each for PchB, BsCM and the uncatalyzed reaction (1800 simulations / 918 ns in total).