Thermodynamics of the arginine kinase reaction

The effect of temperature, pH, free [Mg2+], and ionic strength on the appar ent equilibrium constant of arginine kinase (EC 2.7.3.3) was determined. At equilibrium, the apparent K' was defined as K' = [ATP][Arg]/[ADP][PArg] (Eq. 1) where each reactant represents the sum of all the ionic and metal complex s pecies. The K' at pH 7.0, 1.0 mM free [Mg2+], and 0.25 M ionic strength was 29.91 +/- 0.59, 33.44 +/- 0.46, 35.44 +/- 0.71, 39.64 +/- 0.74, and 45.19 +/- 0.65 (n = 8) at 40, 33, 25, 15, and 5 degrees C, respectively. The stan dard apparent enthalpy (Delta H degrees') is -8.19 kJ mol(-1), and the corr esponding standard apparent entropy of the reaction (Delta S degrees') is 2.2 J K-1 mol(-1) in the direction of ATP formation at pH 7.0, free [Mg2+] = 1.0 mM, ionic strength (I) =0.25 M at 25 degrees C, We further show that the magnitude of transformed Gibbs energy (Delta G degrees') of -8.89 kJ m ol(-1) is mostly comprised of the enthalpy of the reaction, with 7.4% comin g from the entropy T Delta S degrees' term (+0.66 kJ mol(-1)). Our results are discussed in relation to the thermodynamic properties of its evolutiona ry successor, creatine kinase.