Microcalorimetry and high-performance liquid chromatography have been used to conduct a thermodynamic investigation of several biochemical reactions catalyzed by the enzymes D-hydantoinase and N-carbamoyl-D-amino acid hydrolase. Values of apparent equilibrium constants K' and calorimetrically determined molar enthalpies of reaction Delta H-r(m)(cal) were measured for the following D-hydantoinase catalyzed reactions: hydantoin(aq) + H2O(1) = ureidoacetic acid(aq); 5,6-dihydrouracil(aq) + H2O(1) = 3-ureidopropanoic acid(aq); 5-phenylhydantoin(aq) + H2O(1) = 5-phenyl D-Carbamoylate(aq); and 5-(4-hydroxyphenyl)hydantoin(aq) + H2O(1) = 5-(4-hydroxyphenyl) D-carbamoylate(aq). Calorimetric measurements, in which both D-hydantoinase and N-carbamoyl-D-amino acid hydrolase were present, were performed on the following reactions: 5-phenylhydantoin(aq) + 3 H2O(1) = D-phenylglycine(aq) + carbon dioxide(aq) + ammonia(aq); and (4-hydroxy)-phenylhydantoin(aq) + 3H(2)O(1) = D-(4-hydroxyphenyl)glycine(aq) + carbon dioxide(aq) + aminonia(aq). A chemical equilibrium model was then used to calculate thermodynamic property values (equilibrium constants K, standard molar enthalpies of reaction Delta H-r(m)degrees, standard molar Gibbs free energies of reaction Delta(r)G'degrees(m), and standard molar entropies of reaction ArS) for chemical reference reactions that correspond to the overall biochemical reactions that were studied experimentally. In several cases, we also provided estimated property values based on data for structurally similar reactions. The results can be used together with the chemical equilibrium model to calculate values of K', the standard apparent molar Gibbs free energy change Delta(r)G'degrees(m), the standard apparent molar enthalpy change Delta(r)H1 degrees(m), changes in binding of the proton Delta N-r(H+), and the position of equilibrium for the several overall biochemical reactions considered in this study over a reasonably wide range of temperature, pH, and ionic strength. Published by Elsevier Ltd.