Simultaneous effects of immobilization and substrate protection on the thermodynamics of glucose isomerase activity and inactivation

Abstract

The influence of enzyme immobilization on the main thermodynamic quantities of glucose isomerization to fructose and vice versa was studied in batch tests by using a commercial immobilized glucose isomerase. The values calculated for the immobilized system showed, when compared with those reported for the native enzyme, higher equilibrium constants at T > 70°C and simultaneous increases in both standard enthalpy and entropy changes of reaction. Activation enthalpy and entropy changes of both forward and reverse enzyme-catalyzed reactions were calculated from the initial glucose isomerase activity evaluated at different temperatures by the Briggs-Haldane model. The activation enthalpy of the forward reaction was about 24% less than that of the native enzyme. Enzyme inactivation tests performed at different temperatures and sugar equilibrium concentrations confirmed the existence of the so-called substrate protection phenomenon which resulted in a 29–36% reduction in the decay constant. A slight increase in the activation free enthalpy of the inactivation reaction was also shown.