THERMODYNAMIC AND QUANTUM-CHEMICAL STUDY OF THE CONVERSION OF CHORISMATE TO (PYRUVATE PLUS 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. Th e study used a genetically engineered sample of chorismate lyase that was prepared with the Escherichia coli ubiC gene. The calorimetric mea surements led to a standard molar enthalpy change Delta(r)H(m)degrees = -(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 m olar entropy change Delta(r)S(m)degrees = 222 J K-1 mol(-1) for the ab ove reaction was used together with the experimental value of Delta(r) H(m)degrees to obtain a standard molar Gibbs free energy change 4G(m)d egrees approximate to -210 kJ mol(-1) and an equilibrium constant K ap proximate to 10(37) for the conversion of chorismate(2-)(aq) to {pyruv ate(-)(aq) + 4-hydroxybenzoate(-)(aq)} at T = 298.15 K and I-m = 0. Qu antum mechanics (Gaussian 94 with a B3LYP functional and a 6-31G basi s set) was used to calculate values of absolute energies for the neutr al and ionic species pertinent to this reaction both in the gas phase and in aqueous solution. The bond angles and bond lengths in pyruvic a cid 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 isodens ity polarizable continuum model (SCI-PCM). The calculated value of Del ta(r)H(m)degrees for the conversion of chorismate(2-)(aq) to {pyruvate (-)(aq) + 4-hydroxybenzoate(-)(aq)} at T = 298.15 K was -154 kJ mol(-1 ) with the PCM model and -178 kJ mol(-1) with the SCI-PCM model. The r elatively large discordance in the SCI-PCM calculation may arise from the ill-defined cavity size, which is derived from the solute charge d istribution isosurface. However, the PCM model, which employs a parame trized cavity radius, yields a result that can be considered to be in agreement with experiment.