Thermodynamics elucidation of the structural stability of a thermophilic protein

The structural stability of the protein, phycocyanin isolated from two strains of cyanophyta, Synechococcus lividus (thermophile) and Phormidium luridum (mesophile), are investigated by comparative thermal and denaturant unfolding, using differential scanning calorimetry, visible absorption spectrophotometry, and circular dichroism. The thermophilic protein exhibits a much higher temperature and enthalpy of unfolding from the native to the denatured state. The concentration of urea at half-completion of thermal unfolding is essentially the same between the thermophilic and mesophilic proteins; in contrast, the corresponding temperature and the enthalpy of thermal unfolding are much higher for the thermophilic protein. In addition, the concentration of urea at which the non-thermal (denaturant) unfolding of protein is half-completed, as detected by either circular dichroism or absorption spectroscopy, is significantly higher in the thermophilic protein, while the apparent free energy of unfolding only shows a moderate difference between the two proteins. The distinct differences in the enthalpy of thermal unfolding and the free energy of denaturant unfolding are interpreted in terms of a significant entropy change associated with the unfolding of these proteins. This entropy contribution is much higher in the thermophilic protein, and may be derived from its more rigid overall structure that possesses higher internal hydrophobicity and stronger internal packing.