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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(1)}

Lookup NU author(s): Professor Alastair Hawkins, Dr Heather Lamb

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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)} 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 Im= 0.065 mol · kg-1). Calorimetric measurements led to a molar enthalpy of reaction ΔrHm(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 DAHP3- (aq) = 3-dehydroquinate- (aq) + HPO42- (aq), the standard molar enthalpy of reaction ΔrHm° = -(51.1 ± 4.5) kJ · mol-1. For the reaction 3-de-hydroqninate(aq) = 3-dehydroshikimate(aq) + H2O(1), the equilibrium constant K = (4.6 ± 1.5) and ΔrHm° = (2.3 ± 2.3) kJ · mol-1. A Benson type approach was used to estimate the standard molar entropy change ΔrSm° for the first reference reaction and led to the value K ≈ 2 · 1014 for this reaction. Values of the apparent equilibrium constants and the standard transformed Gibbs free energy changes ΔrGm° under approximately physiological conditions are given for the biochemical reactions. Published by Elsevier Science Ltd.

Publication metadata

Author(s): Tewari YB, Hawkins AR, Lamb HK, Goldberg RN

Publication type: Article

Publication status: Published

Journal: Journal of Chemical Thermodynamics

Year: 2002

Volume: 34

Issue: 10

Pages: 1671-1691

Print publication date: 01/10/2002

ISSN (print): 0021-9614

ISSN (electronic): 1096-3626

Publisher: Academic Press

URL: http://dx.doi.org/10.1016/S0021-9614(02)00226-4

DOI: 10.1016/S0021-9614(02)00226-4

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