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Thermodynamics of Reactions Catalyzed by PABA Synthase



Yadu D. Tewari, P Y. Jensen, N Kishore, M P. Mayhew, J F. Parsons, Edward Eisenstein, Robert N. Goldberg


Microcalorimetry and high performance liquid chromatography (HPLC) have been used to conduct a thermodynamic investigation of reactions catalyzed by PABA synthase, the enzyme located at the first step in the shikimic acid metabolic pathway leading from chorismate to 4-aminobenzoate (PABA). The overall biochemical reaction catalyzed by PAbB and PabC components of PABA snythase is: chorismate(aq) + ammonia(aq) = 4-aminobenzoate(aq) + pyruvate(aq) + H2O(l). This reaction can be divided into two partial reactions involving the intermediate 4-amino-4-deoxychorismate (ADC): chorismate(aq) + ammonia(aq) = ADC(aq) + H2O(l) and ADC(aq) = 4-aminobenzoate(aq) + pyruvate(aq). Microcalorimetric measurements were performed on all three of these reactions at a temperature of 298.15 K and pHs in the range 8.72 to 8.77. Equilibrium measurements were performed on the first partial (ADC synthase) reaction at T = 298.15 K and at pH = 8.78. The saturation molality of 4-aminobenzoate(cr) in water is (0.00382 0.0004) mol kg-1 at T = 298.15 K. The results of the equilibrium and calorimetric measurements 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 the temperature 298.15 K and an ionic strength of zero for chemical reference reactions involving specific ionic forms. For the reaction: chorismate2-(aq) + NH4+(aq) = 4-aminobenzoate-(aq) + pyruvate-(aq) + H2O(l) + H+(aq), DrH m = -(174.0 6) kJ mol-1. For the reaction: chorismate2-(aq) + NH4+(aq) = ADC-(aq) + H2O(l), K = (10.8 4.2) and DrH m = -(35 15) kJ mol-1. For the reaction: ADC-(aq) = 4-aminobenzoate-(aq) + pyruvate-(aq) + H+(aq), DrH m = -(139 23) kJ mol-1. Thermodynamic cycle calculations were used to calculate thermodynamic quantities for three additional reactions that utilize L-glutamine rather than ammonia and that are pertinent to this branch point of the shikimic acid pathway. The quantities obtained in this study permit the calculation of the position of equilibrium of these reactions as a function of temperature, pH, and ionic strength. Values of the apparent equilibrium constants and the standard transformed Gibbs energy changes DrG'm under approximately physiological conditions are given.
Biophysical Chemistry


4-amino-4-deoxychorismate, 4-aminobenzoic acid, ADC, apparent equilibrium constant, calorimetry, chorismate, enthalpy, PABA, thermodynamics


Tewari, Y. , Jensen, P. , Kishore, N. , Mayhew, M. , Parsons, J. , Eisenstein, E. and Goldberg, R. (2002), Thermodynamics of Reactions Catalyzed by PABA Synthase, Biophysical Chemistry (Accessed April 23, 2024)
Created April 10, 2002, Updated February 17, 2017