THERMOCHEMISTRY OF THE REACTIONS BETWEEN ADENOSINE, ADENOSINE 5'-MONOPHOSPHATE, INOSINE, AND INOSINE 5'-MONOPHOSPHATE - THE CONVERSION OF L-HISTIDINE TO (UROCANIC ACID PLUS AMMONIA)

Results of equilibrium and calorimetric measurements are reported for a series of enzymecatalyzed reactions involving the deamination of adenosine 5′-monophosphate (AMP), adenosine, and d-histidine, and the dephosphorylation of AMP and inosine 5′-monophosphate (IMP). These measurements have been analyzed with a model which describes the complex equilibria in solution. The results obtained are: ΔrHom= -(49.6±0.5) kJ·mol-1for AMP2—(aq) + H2O(l) + H+(aq) = HIMP2—(aq) + NH+4(aq); ΔrHom= -(49.4±0.7) kJ·mol-1for adenosine(aq) + H2O(l) + H+(aq) = inosine(aq) + NH+4(aq); ΔrGom= -(13.0±0.4) kJ·mol-1, ΔrHom= (0.9±0.4) kJ·mol-1, and ΔrSom= (47±2) J·K-1·mol-1for AMP2—(aq) + H2O(l) = adenosine(aq) + HPO2—4(aq); ΔrGom= -(12.6±0.1) kJ·mol-1, ΔrHom= (1.2±0.2) kJ·mol-1, and ΔrSom= (48±1) J·K-1·mol-1for HIMP2—(aq) + H2O(l) = inosine(aq) + HPO2—4(aq); and ΔrGom= -(2.5±0.7) kJ·mol-1, ΔrHom= (7.6±0.8) kJ·mol-1, and ΔrSom= (34±4) J·K-1·mol-1for d-histidine(aq) = urocanate—(aq) + NH+4(aq). These results are discussed in relation to both thermodynamic-cycle calculations involving these substances and other reactions of a chemically similar nature. © 1993 Academic Press Ltd.