Calorimetric evidence for a native-like conformation of hen egg-white lysozyme dissolved in glycerol

Abstract

Hen egg-white lysozyme, lyophilized from aqueous solutions of different pH (from pH 2.5 to 10.0) and then dissolved in water and in anhydrous glycerol, has been studied by high-sensitivity differential scanning microcalorimetry over the temperature range from 10 to 150°C. All lysozyme samples exhibit a cooperative conformational transition in both solvents occurring between 10 and 100°C. The transition temperatures in glycerol are similar to those in water at the corresponding pHs. The transition enthalpies in glycerol are substantially lower than in water but follow similar pH dependences. The transition heat capacity increment in glycerol does not depend on the pH and is 1.25±0.31 kJ mol −1 K −1, which is less than one fifth of that in water (6.72±0.23 kJ mol −1 K −1). The thermal transition in glycerol is reversible and equilibrium, as demonstrated for the pH 8.0 sample, and follows the classical two-state mechanism. In contrast to lysozyme in water, the protein dissolved in glycerol undergoes an additional, irreversible cooperative transition with a marginal endothermic heat effect at temperatures of 120–130°C. The transition temperature of this second transition increases with the heating rate which is characteristic of kinetically controlled processes. Thermodynamic analysis of the calorimetric data reveals that the stability of the folded conformation of lysozyme in glycerol is similar to that in water at 20–80°C but exceeds it at lower and higher temperatures. It is hypothesized that the thermal unfolding in glycerol follows the scheme: N⇔ho-MG⇒U, where N is a native-like conformation, ho-MG is a highly ordered molten globule state, and U is the unfolded state of the protein.