A thermodynamic investigation of some reactions involving prephenic acid

Abstract Calorimetric enthalpies of reaction have been measured for the following enzyme-catalysed reactions at the temperature 298.15 K: prephenate(aq) = phenylpyruvate(aq) + carbon dioxide(aq), prephenate(aq) + NAD ox (aq) +H 2 O(l) = 4-hydroxyphenylpyruvate(aq) + NAD red ( aq ) + carbon dioxide ( aq ). Here, NAD ox and NAD red are, respectively, the oxidized and reduced forms of β-nicotinamide adenine dinucleotide. The enzymes that catalyse these respective reactions, prephenate dehydratase and prephenate dehydrogenase, were prepared by expression of the appropriate plasmids using the techniques of molecular biology. The calorimetric measurements together with the equilibrium modeling calculations lead to a standard molar enthalpy change Δ r H m o  = − (126  ±  5) kJ· mol −1 for the reference reaction: prephenate 2− (aq) = phenylpyruvate − (aq) + HCO 3 − (aq). Similarly, Δ r H m o  =  − (74 ± 3) kJ· mol −1 for the reference reaction: prephenate 2− (aq) + NAD ox − (aq) + H 2 O(l) = 4- hydroxyphenylpyruvate − ( aq ) + NAD red 2− ( aq ) + HCO 3 − ( aq ) + H + ( aq ). Both results pertain to T  =  298.15 K and ionic strength I  = 0. Benson estimates for the entropies lead to approximate values of the equilibrium constants K  ≈ 1·10 26 and K  ≈ 1· 10 12 , respectively, for the above two reference reactions.