Microcalorimetry and high-performance liquid chromatography (h. p. l. c.) have been used to conduct a thermodynamic investigation of the biochemical reaction (phosphoenolpyruvate(aq) + D-erythrose 4-phosphate(aq) + H2O(1) = 2-dehydro-3-deoxy-D-arabino-heptonate 7-phosphate(aq) + phosphate(aq)), the first step in the metabolic pathway leading to the biosynthesis of chorismate. DAHP synthase, the enzyme used to carry out this reaction. was prepared by using molecular biology techniques and then isolated in a highly purified form. The h.p.l.c. was used to establish that this reaction went to completion and that the apparent equilibrium constant K'(T = 298.15 K. pH approximate to 8.2) is > 1.4 . 10(3). The calorimetrically determined molar enthalpy for this biochemical reaction is Delta(r)H(m)(cal) = - (67.7 +/- 1.5) kJ . mol(-1) at T = 298.15 K, pH = 8.18, and ionic strength I-m = 0.090 mol . kg(-1). Use of this result together with a chemical equilibrium model led to the standard molar enthalpy of reaction Delta(r)H(m)(o), = -(70.0 +/- 3.0) kJ . mol(-1) at T = 298.15 K and I-m = 0 for the chemical reference reaction {phosphoenolpyruvate(3-)(aq) + D-erythrose 4-phosphate(2-)(aq) + H2O(1) = 2-dehydro-3-deoxy-D-arabino-heptonate 7-phosphate(3-)(aq) + HPO42- (aq)}. Use of an estimated value of the standard molar entropy change Delta(r)S(m)(o) for this reaction together with the measured value of Delta(r)H(m)(o) yielded the standard molar Gibbs free energy change Delta(r)G(m)(o) approximate to -39 kJ . mol(-1) and K approximate to 7 . 10(6) for the chemical reference reaction at T = 298.15 K and I-m = 0. Some insight into the thermochemistry of this reaction has been obtained by assessing the contributions to Delta(r)H(m)(o) that correspond to the loss of a phosphate from phosphoenolpyruvate and to the addition of a pyruvate group to D-erythrose 4-phosphate.