Add time:09/24/2019         Source:sciencedirect.com

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) + NADox(aq) +H2O(l)= 4-hydroxyphenylpyruvate(aq) +NADred(aq) + carbondioxide(aq).Here, NADoxand NADredare, 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 ΔrHmo = − (126  ±  5) kJ· mol−1for the reference reaction:prephenate2−(aq) = phenylpyruvate−(aq) + HCO3−(aq).Similarly, ΔrHmo =  − (74 ± 3) kJ· mol−1for the reference reaction:prephenate2−(aq) + NADox−(aq) + H2O(l) =4-hydroxyphenylpyruvate−(aq) + NADred2−(aq) + HCO3−(aq) + H+(aq).Both results pertain toT =  298.15 K and ionic strengthI = 0. Benson estimates for the entropies lead to approximate values of the equilibrium constantsK ≈ 1·1026andK ≈ 1· 1012, respectively, for the above two reference reactions.

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