The endothermic effects during denaturation of lysozyme by temperature modulated calorimetry and an intermediate reaction equilibrium

To gain insight into the thermodynamics of protein denaturation, the complex heat capacity, C-p* (= C-p' - iC(p)") of lysozyme-water system has been measured at pH 2.5 in the 293-368 K range by using temperature-modulated scanning calorimetry (TMSC), a technique in which the thermally reversible enthalpy changes are measured separately and simultaneously with the thermally irreversible enthalpy changes. The plot of C-p' against the temperature T shows a broad peak, which is similar to that observed in C-p.DSC, measured here and elsewhere by differential scanning calorimetry (DSC), a technique which gives the sum of both the reversible and irreversible contributions in the apparent heat capacity value. This peak in C-p.DSC has been generally attributed to endothermic heat absorption on reversible and irreversible unfolding processes and irreversible thermal denaturation. It is shown that the observed C-p' peak results from heat absorption when the equilibrium constant between the native lysozyme state and a conformationally different intermediate state increases with T. The plot of C-p' versus T is subdivided into four regions, corresponding to the dominance of a certain process. Thermal denaturation of lysozyme was found to occur according to a scheme, native state - unfolded (intermediate) state - denatured state. This conclusion is consistent with the general view that the first step of denaturation of small one-domain globular protein like lysozyme is a reversible conformational (unfolding) transition, and the second step is irreversible denaturation. It is shown that when kept isothermally at T > 341 K, i.e., within the transition temperature range, C-p' of lysozyme decreases. This decrease is exponential in time and corresponds to a rate constant, which varies according to the Arrhenius-type equation, with a preexponential factor of 5 x 10(20) s(-1) and energy of 167 kJ/mol. The overall kinetics of the denaturation reaction is of the first order.