Thermodynamics of the hydrolysis reactions of adenosine 3 ′,5 ′-(cyclic)phosphate(aq) and phospho enolpyruvate(aq); the standard molar formation properties of 3 ′,5 ′-(cyclic)phosphate(aq) and phospho enolpyruvate(aq)

Abstract

Molar calorimetric enthalpy changes Δ r H m(cal) have been measured for the biochemical reactions {cAMP(aq) + H 2O(l)=AMP(aq)} and {PEP(aq) + H 2O(l)=pyruvate(aq) + phosphate(aq)}. The reactions were catalyzed, respectively, by phosphodiesterase 3 ′,5 ′-cyclic nucleotide and by alkaline phosphatase. The results were analyzed by using a chemical equilibrium model to obtain values of standard molar enthalpies of reaction Δ r H m ∘ for the respective reference reactions {cAMP −(aq) + H 2O(l)=HAMP −(aq)} and {PEP 3−(aq) + H 2O(l)=pyruvate −(aq) + HPO 2− 4(aq)}. Literature values of the apparent equilibrium constants K ′ for the reactions {ATP(aq)=cAMP(aq) + pyrophosphate(aq)}, {ATP(aq) + pyruvate(aq)=ADP(aq) + PEP(aq)}, and {ATP(aq) + pyruvate(aq) + phosphate(aq)=AMP(aq) + PEP(aq) + pyrophosphate(aq)} were also analyzed by using the chemical equilibrium model. These calculations yielded values of the equilibrium constants K and standard molar Gibbs free energy changes Δ r G m ∘ for ionic reference reactions that correspond to the overall biochemical reactions. Combination of the standard molar reaction property values ( K, Δ r H m ∘, and Δ r G m ∘) with the standard molar formation properties of the AMP, ADP, ATP, pyrophosphate, and pyruvate species led to values of the standard molar enthalpy Δ f H m ∘ and Gibbs free energy of formation Δ f G m ∘ and the standard partial molar entropy S m ∘ of the cAMP and PEP species. The thermochemical network appears to be reasonably well reinforced and thus lends some confidence to the accuracy of the calculated property values of the variety of species involved in the several reactions considered herein.