Microcalorimetry and high-performance liquid chromatography (h.p.l.c.) have been used to conduct a thermodynamic investigation of reactions catalyzed by 3-dehydroquinate synthase and by 3-dehydroquinate dehydratase. These are the second and third reactions in the metabolic pathway leading to the formation of chorismate. The two reactions are: {DAHP(aq) = 3-dehydroquinate(aq) + phosphate(aq)} and {3-dehydroquinate(aq) = 3-dehydroshikimate(aq) + H2O(1)}. The h.p.l.c. measurements showed that the first reaction proceeded to completion and that the value of the apparent equilibrium constant for the second reaction was K' = (4.6 +/- 1.5) (Hepes buffer, temperature T = 298.15 K, pH = 7.50, and ionic strength I-m = 0.065 mol.kg(-1)). Calorimetric measurements led to a molar enthalpy of reaction Delta(r)H(m) (cal) = -(50.9 +/- 1.1) kJ.mol(-1) (Hepes buffer, T = 298.15 K, pH = 7.46, I-m = 0.070 mol.kg(-1)) for the first reaction and to Delta(r)H(m) (cal) = (2.3 +/- 2.3) kJ.mol(-1) (Hepes buffer, T = 298.15 K, pH = 7.42, I-m = 0.069 mol.kg(-1)) for the second reaction. These results were analyzed in terms of a chemical equilibrium model that accounts for the multiplicity of ionic states of the reactants and products. These calculations gave thermodynamic quantities at T = 298.15 K and I-m = 0 for chemical reference reactions involving specific ionic forms. For the reaction DAHP(3)-(aq) = 3-dehydroquinate(-)(aq) + HPO42-(aq), the standard molar enthalpy of reaction Delta(r)H(m) = -(51.1 +/- 4.5) kJ.mol(-1). For the reaction 3-de-hydroquinate(aq) = 3-dehydroshikimate (aq) + H2O(1), the equilibrium constant K = (4.6 +/- 1.5) and Delta(r)H(m)degrees = (2.3 +/- 2.3) kJ.mol(-1). A Benson type approach was used to estimate the standard molar entropy change Delta(r)S(m)degrees for the first reference reaction and led to the value K approximate to 2.10(14) for this reaction. Values of the apparent equilibrium constants and the standard transformed Gibbs free energy changes Delta(r)G(m)degrees under approximately physiological conditions are given for the biochemical reactions. Published by Elsevier Science Ltd.