Thermodynamics of the arginine kinase reaction

Abstract

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 K' = [ATP][Arg]/[ADP][PArg] (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 (ΔH°') is -8.19 kJ mol-1, and the corresponding standard apparent entropy of the reaction (AS°') 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 °C. We further show that the magnitude of transformed Gibbs energy (ΔG°') of -8.89 kJ mol-1 is mostly comprised of the enthalpy of the reaction, with 7.4% coming from the entropy TAS°' term (+0.66 kJ mol-1). Our results are discussed in relation to the thermodynamic properties of its evolutionary successor, creatine kinase.