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 heat capacity vs. temperature profiles were obtained at temperature scanning rates of 0.25, 0.5, and 1 K/min. These profiles were analysed using three models of irreversible denaturation which are approximations to the whole Lumry-Eyring model, namely, the one-step model of irreversible denaturation, the Lumry-Eyring model with the fast equilibrating first step, and the model involving two consecutive irreversible steps. 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 and second steps calculated for the model involving two consecutive irreversible steps are the following: Ea,1 = 609.3 +/- 1.8 kJ/mol and Ea,2 = 446.8 +/- 3.2 kJ/mol.