Thermochemistry of the reaction { phospho enol pyruvate(aq) + d -erythrose 4-phosphate(aq) + H₂ O(l) = 2-dehydro-3-deoxy- d - arabino -heptonate 7-phosphate(aq) + phosphate(aq) }

Abstract

Microcalorimetry and high-performance liquid chromatography (h.p.l.c.) have been used to conduct a thermodynamic investigation of the biochemical reaction {phospho enol pyruvate(aq) + -erythrose 4-phosphate(aq) + H₂ O(l) = 2-dehydro-3-deoxy- - 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 ≈ 8.2) is > 1.4 · 10³. The calorimetrically determined molar enthalpy for this biochemical reaction is ΔᵣHₘo (cal) = − (67.7 ± 1.5)kJ · mol⁻¹ at T = 298.15 K, pH = 8.18, and ionic strength Iₘ = 0.090 mol · kg⁻¹. Use of this result together with a chemical equilibrium model led to the standard molar enthalpy of reactionΔᵣHₘo = − (70.0 ± 3.0)kJ · mol⁻¹ at T = 298.15 K and Iₘ = 0 for the chemical reference reaction {phospho enol pyruvate ³⁻ (aq) + -erythrose 4-phosphate ²⁻(aq) + H₂ O(l) = 2-dehydro-3-deoxy- - arabino -heptonate 7-phosphate ³⁻ (aq) + HPO₄²⁻(aq)}. Use of an estimated value of the standard molar entropy changeΔrSₘo for this reaction together with the measured value of ΔᵣHₘo yielded the standard molar Gibbs free energy change ΔᵣGₘo ≈ − 39 kJ · mol⁻¹ and K ≈ 7 · 10⁶ for the chemical reference reaction at T = 298.15 K and Iₘ = 0. Some insight into the thermochemistry of this reaction has been obtained by assessing the contributions to ΔᵣHₘo that correspond to the loss of a phosphate from phospho enol pyruvate and to the addition of a pyruvate group to -erythrose 4-phosphate.