A thermodynamic investigation of some reactions involving prephenic acid

Calorimetric enthalpies of reaction have been measured for the following en zyme-catalysed reactions at the temperature 298.15 K: prephenate(aq) = phenylpyruvate(aq) + carbon dioxide(aq), prephenate(aq) + NAD(ox) (ag) + H2O(1) = 4-hydroxyphenylpyruvate(aq) NAD(red)(aq) + carbon dioxide(aq). Here, NAD(ox) and NAD(red) are, respectively, the oxidized and reduced form s of beta-nicotinamide adenine dinucleotide. The enzymes that catalyse thes e respective reactions, prephenate dehydratase and prephenate dehydrogenase , were prepared by expression of the appropriate plasmids using the techniq ues of molecular biology. The calorimetric measurements together with the e quilibrium modeling calculations lead to a standard molar enthalpy change D elta(r)H(m)(o) = -(126 +/- 5) kJ.mol(-1) for the reference reaction: prephenate(2-)(aq) = phenylpyruvate(-)(aq) + HCO3-(aq). Similarly, Delta(r)H(m)(o) = -(74 +/- 3) kJ.mol(-1) for the reference react ion: prephenate(2-) (ag) + NAD(ox)(-)(aq) + H2O(l) = 4-hydroxyphenylpyruvate(-) (ag) + NAD(red)(2-)(aq) + HCO3-(aq) + H+(aq). Both results pertain to T = 298.15 K and ionic strength I = 0. Benson estim ates for the entropies lead to approximate values of the equilibrium consta nts K approximate to 1 . 10(26) and K approximate to 1 . 10(12), respective ly, for the above two reference reactions. (C) 1999 Academic Press.