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 Phormi dium luridum (mesophile), are investigated by comparative thermal and denaturant unfolding, using differential scanning calorimetry, visible absorption spectrophotometry, and circular dichroism. The thermophili c 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 t he thermophilic and mesophilic proteins; in contrast, the correspondin g temperature and the enthalpy of thermal unfolding are much higher fo r the thermophilic protein. In addition, the concentration of urea at which the non-thermal (denaturant) unfolding of protein is half-comple ted, as detected by either circular dichroism or absorption spectrosco py, is significantly higher in the thermophilic protein, while the app arent free energy of unfolding only shows a moderate difference betwee n the two proteins. The distinct differences in the enthalpy of therma l unfolding and the free energy of denaturant unfolding are interprete d in terms of a significant entropy change associated with the unfoldi ng of these proteins. This entropy contribution is much higher in the thermophilic protein, and may be derived from its more rigid overall s tructure that possesses higher internal hydrophobicity and stronger in ternal packing.