Thermodynamic and Quantum Chemical Study of the Conversion of Chorismate to (Pyruvate + 4-Hydroxybenzoate)

A thermodynamic investigation of the conversion of chorismate 2- (aq) to {pyruvate - (aq) + 4-hydroxybenzoate - (aq)} has been performed by using microcalorimetry and high-performance liquid chromatography. The study used a genetically engineered sample of chorismate lyase that was prepared with the Escherichia coli ubiC gene. The calorimetric measurements led to a standard molar enthalpy change = −(144 ± 7) kJ mol -1 for this reaction at the temperature T = 298.15 K and ionic strength I m = 0. An estimated value of the standard molar entropy change = 222 J K -1 mol -1 for the above reaction was used together with the experimental value of to obtain a standard molar Gibbs free energy change ≈ −210 kJ mol -1 and an equilibrium constant K ≈ 10 37 for the conversion of chorismate 2- (aq) to {pyruvate - (aq) + 4-hydroxybenzoate - (aq)} at T = 298.15 K and I m = 0. Quantum mechanics ( Gaussian 94 with a B3LYP functional and a 6-31G* basis set) was used to calculate values of absolute energies for the neutral and ionic species pertinent to this reaction both in the gas phase and in aqueous solution. The bond angles and bond lengths in pyruvic acid and 4-hydroxybenzoic acid and their monoanions were also obtained along with values of thermodynamic reaction quantities. The effects of water solvation and solvent polarization were accounted for by using both a polarizable continuum model (PCM) and a self-consistent isodensity polarizable continuum model (SCI-PCM). The calculated value of for the conversion of chorismate 2- (aq) to {pyruvate - (aq) + 4-hydroxybenzoate - (aq)} at T = 298.l5 K was −154 kJ mol -1 with the PCM model and −178 kJ mol -1 with the SCI-PCM model. The relatively large discordance in the SCI-PCM calculation may arise from the ill-defined cavity size, which is derived from the solute charge distribution isosurface. However, the PCM model, which employs a parametrized cavity radius, yields a result that can be considered to be in agreement with experiment.