SIMULTANEOUS EFFECTS OF IMMOBILIZATION AND SUBSTRATE PROTECTION ON THE THERMODYNAMICS OF GLUCOSE-ISOMERASE ACTIVITY AND INACTIVATION

The influence of enzyme immobilization on the main thermodynamic quant ities of glucose isomerization to fructose and vice versa was studied in batch tests by using a commercial immobilized glucose isomerase. Th e values calculated for the immobilized system showed, when compared w ith those reported for the native enzyme, higher equilibrium constants at T > 70 degrees C and simultaneous increases in both standard entha lpy and entropy changes of reaction. Activation enthalpy and entropy c hanges of both forward and reverse enzyme-catalyzed reactions were cal culated from the initial glucose isomerase activity evaluated at diffe rent temperatures by the Briggs-Haldane model. The activation enthalpy of the forward reaction was about 24% less than that of the native en zyme. Enzyme inactivation tests performed at different temperatures an d sugar equilibrium concentrations confirmed the existence of the so-c alled substrate protection phenomenon which resulted in a 29-36% reduc tion in the decay constant. A slight increase in the activation free e nthalpy of the inactivation reaction was also shown. (C) 1997 Elsevier Science Inc.