Mechanic Insight into Aggregation of Lysozyme by Ultrasensitive Differential Scanning Calorimetry and Sedimentation Velocity

Folding and aggregation of proteins profoundly influence their functions. We have investigated the effects of thermal history, concentration and pH on the denaturation and refolding of lysozyme by using ultrasensitive differential scanning calorimetry (US-DSC) and sedimentation velocity (SV) via analytical ultracentrifugation (AUC). The former is sensitive to small energy change whereas the latter can differentiate the oligomers such as dimer and trimer from individual protein molecules. Our studies reveal that the degree of denaturation irreversibility increases as heating times increases. The denaturation temperature (T-d) and enthalpy change (Delta H) are influenced by heating rate since the denaturation is not in equilibrium during the heating. We can obtain T-d and Delta H in equilibrium by extrapolation of heating rate to zero. In a dilute solution, no aggregation but unfolding happens in the denaturation. However, when the concentration is above a critical value (similar to 15.0 mg/mL), lysozyme molecules readily form trimers or other oligomers. Lysozyme molecules unfold into stretched chains at pH > 6.0, which would further forms large aggregates. The formation of aggregates makes the refolding of lysozyme impossible.