Abstract
A highly sensitive liquid flow micro reaction calorimetry system based on the heat conduction principle with multi-thermomodules, applicable to suspended solutions has been developed. This calorimetry system has been well suited for measurements on the enzyme kinetics of suspended membrane proteins. Binding and reaction studies were carried out by using it on the suspended Jørgensen's Na,K-ATPase - membrane sample with cations (Na +, K +, Mg 2+), ADP, ATP-γ-S and ATP. Evaluated values of enthalpy for cation bindings, and evaluated thermodynamic quantities of the bindings of substrate analogues ADP and ATP-γ-S were obtained definitely. Binding stability of this protein with these analogues was strong and enthalpic nearly to that in almost globular proteinligand binding systems. The biphasic Lineweaver-Burk plot on the reaction heat, which is proportional to the velocity of generated heat, of this ATPase with ATP was found calorimetically as a function of ATP concentration. Apparent values of the Michaelis constant Km were obtained to be about 7 μmol and 1 mmol for low- and high- ATP affinity sites, respectively. Evaluated values of reaction heat with ATP concentration coincide with the theoretical values when the free energy change of ATP decomposition is assumed to be 21 kJ mol .
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