A thermodynamic study of the reactions: {2-dehydro-3-deoxy-d-arabino-heptanoate 7-phosphate(aq)=3-dehydroquinate(aq) + phosphate(aq)} and {3-dehydroquinate(aq)=3-dehydroshikimate(aq) + H2O(l)}

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) + <f>H2O(l)</f>}. 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 <f>K′=(4.6±1.5)</f> (Hepes buffer, temperature <f>T=298.15</f> K, pH=7.50, and ionic strength <f>Im=0.065 mol·kg−1</f>). Calorimetric measurements led to a molar enthalpy of reaction <f>ΔrHm (cal)=−(50.9±1.1) kJ·mol−1</f> (Hepes buffer, <f>T=298.15</f> K, pH=7.46, <f>Im=0.070 mol·kg−1</f>) for the first reaction and to <f>ΔrHm (cal)=(2.3±2.3) kJ·mol−1</f> (Hepes buffer, <f>T=298.15</f> K, pH=7.42, <f>Im=0.069 mol·kg−1</f>) 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 <f>T=298.15</f> K and <f>Im=0</f> for chemical reference reactions involving specific ionic forms. For the reaction <f>DAHP3−(aq)=3-dehydroquinate−(aq)+HPO42−(aq)</f>, the standard molar enthalpy of reaction <f>ΔrH°m=−(51.1±4.5) kJ·mol−1</f>. For the reaction 3-dehydroquinate(aq)=3-dehydroshikimate(aq) +<f>H2O(l)</f>, the equilibrium constant <f>K=(4.6±1.5)</f> and <f>ΔrH°m=(2.3±2.3) kJ·mol−1</f>. A Benson type approach was used to estimate the standard molar entropy change <f>ΔrS°m</f> for the first reference reaction and led to the value <f>K&ap;2·1014</f> for this reaction. Values of the apparent equilibrium constants and the standard transformed Gibbs free energy changes <f>ΔrG°m</f> under approximately physiological conditions are given for the biochemical reactions. [Copyright &y& Elsevier]