A Thermodynamic Study of the Reactions: 2-Dehydro-3Deoxy-D-Arabino-Heptanoate 7-Phosphate(aq) = {3-Dehydroquinate (ag) + Phosphate (aq)} and {3-Dehydroquinate(aq) =3-Dehydroshikimate(aq) + H2O(l)}

Published

October 1, 2002

Author(s)

Yadu D. Tewari, Robert N. Goldberg, A R. Hawkins, H K. Lamb

Abstract

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)} + {3-dehydroquinate (aq) = 3-dehydroshikimate(aq) + H₂O(l)}. 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 Im = 0.065 mol kg^-1). Calorimetric measurements led to a molar enthalpy of reaction δrH_m(cal) = -(50.9 1.1) kJ mol^-1 (Hepes buffer, T = 298.15 K, pH = 7.46, Im = 0.070 mol kg^-1) for the first reaction and to δrHm(cal) = (2.3 2.3) kJ mol^-1 (Hepes buffer, T = 298.15 K, pH = 7.42, Im = 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 Im = 0 for chemical reference reactions involving specific ionic forms. For the reaction DAHP^3-(aq) = 3-dehydroquinate-(aq) + HPO2/4- (ap), the standard molar enthalpy of reaction δ_rH_2m= -(51.1 4.5) kJ mol^-1. For the reaction 3-dc-hydroquinate (aq) = 3-dehydroshikimat(aq) + H₂O(1), the equilibrium constant K = (4.6 1.5) and δ_rH m = (2.3 2.3) kJ mol-1. A Benson type approach was used to estimate the standard molar entropy change δrS m for the first reference reaction and led to the value K{nearly equal to} 2 10¹⁴ for this reaction. Values of the apparent equilibrium constants and the standard transformed Gibbs free energy changes δ rG'm under approximately physiological conditions are given for the biochemical reactions.

Citation

Journal of Chemical Thermodynamics

Volume

34

Issue

No. 10

Pub Type

Journals

Keywords

2-dehydro-3-deoxy-D-arabino-heptanoate 7, 3-dehydroquinate synthase, apparent equilibrium constant, DAHP synthase, dehydratase, DHQ, enthalpy, entropy, Gibbs free energy

Citation

Tewari, Y. , Goldberg, R. , Hawkins, A. and Lamb, H. (2002), A Thermodynamic Study of the Reactions: 2-Dehydro-3Deoxy-D-Arabino-Heptanoate 7-Phosphate(aq) = {3-Dehydroquinate (ag) + Phosphate (aq)} and {3-Dehydroquinate(aq) =3-Dehydroshikimate(aq) + H₂O(l)}, Journal of Chemical Thermodynamics (Accessed December 14, 2024)