IRREVERSIBLE THERMAL-DENATURATION OF URIDINE PHOSPHORYLASE FROM ESCHERICHIA-COLI K-12

Thermal denaturation of uridine phosphorylase from Escherichia coli K- 12 has been studied by differential scanning calorimetry. The excess h eat capacity vs. temperature profiles were obtained at temperature sca nning rates of 0.25, 0.5, and 1 K/min. These profiles were analysed us ing three models of irreversible denaturation which are approximations to the whole Lumry-Eyring model, namely, the one-step model of irreve rsible denaturation, the Lumry-Eyring model with the fast equilibratin g first step, and the model involving two consecutive irreversible ste ps. In terms of statistics the latter model describes the kinetics of thermal denaturation of uridine phosphorylase more satisfactorily than the two other models. The values of energy activation for the first a nd second steps calculated for the model involving two consecutive irr eversible steps are the following: E-a,E-1 = 609.3 +/- 1.8 kJ/mol and E-a,E-2 = 446.8 +/- 3.2 kJ/mol. (C) 1998 Elsevier Science B.V.