Effect of scan rate and protein concentration on DSC thermograms of Bovine SuperoxideDismutase

DSC thermograms of bovine Cu-Zn superoxide dismutase (BSOD) have been recorded at different scan rates and protein concentrations in order to clarify the process of its unfolding. The lack of calorimetric reversibility made direct thermodynamic analysis of the thermograms impossible. The study of the effect of the scan rate on the shape of the heat capacity (C-p) profiles of BSOD has allowed the calculation of the apparent activation energy (E(app)) for the whole irreversible process. Extrapolation of the excess heat capacity curves (Cpexe) to infinite scan rate has provided the scan-rate-independent part of the thermograms. The protein concentration effect is explained by a change in molecularity that takes place before the kinetically-controlled step. The data collected suggest that, on thermal denaturation, BSOD dissociates during the thermally-induced transitions of the protein; this is followed by a kinetically controlled, exothermic step which is responsible for the global irreversibility of the entire process.