J Biol Chem, Vol. 274, Issue 32, 22459-22463, August 6, 1999

Thermodynamics of the Arginine Kinase Reaction

From the Department of Physiology and Pharmacology, James Cook University, Townsville, Queensland, Australia 4811

The effect of temperature, pH, free [Mg²⁺], and ionic strength on the apparent equilibrium constant of arginine kinase (EC 2.7.3.3) was determined. At equilibrium, the apparent K' was defined as

(Eq. 1)

where each reactant represents the sum of all the ionic and metal complex species. 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 °C, respectively. The standard apparent enthalpy (H°') is 8.19 kJ mol¹, and the corresponding standard apparent entropy of the reaction (S°') is + 2.2 J K¹mol¹ in the direction of ATP formation at pH 7.0, free [Mg²⁺] =1.0 mM, ionic strength (I) =0.25 M at 25 °C. We further show that the magnitude of transformed Gibbs energy (G°') of 8.89 kJ mol¹ is mostly comprised of the enthalpy of the reaction, with 7.4% coming from the entropy TS°' term (+0.66 kJ mol¹). Our results are discussed in relation to the thermodynamic properties of its evolutionary successor, creatine kinase.