COMPARATIVE THERMODYNAMIC ELUCIDATION OF THE STRUCTURAL STABILITY OF THERMOPHILIC PROTEINS

Differential scanning calorimetry, circular dichroism, and visible abs orption spectrophotometry were employed to elucidate the structural st ability of thermophilic phycocyanin derived from Cyanidium caldarium, a eucaryotic organism which contains a nucleus, grown in acidic condit ions (pH 3.4) at 54 degrees C. The obtained results were compared with those previously reported for thermophilic phycocyanin derived from S yechococcus lividus, a procaryote containing no organized nucleus, gro wn in alkaline conditions (pH 8.5) at 52 degrees C. The temperature of thermal unfolding (t(d)) was found to be comparable between C. caldar ium (73 degrees C) and S. lividus (74 degrees C) phycocyanins. The app arent free energy of unfolding (Delta G([urea]=0)) at zero denaturant (urea) concentration was also comparable: 9.1 and 8.7 kcal/mole for un folding the chromophore part of the protein, and 5.0 and 4.3 kcal/mole for unfolding the apoprotein part of the protein, respectively. These values of t(d) and Delta G([urea]=0) were significantly higher than t hose previously reported for mesophilic Phormidium luridum phycocyanin (grown at 25 degrees C). These findings revealed that relatively high er values of t(d) and Delta G([urea]=0) were characteristics of thermo philic proteins. In contrast, the enthalpies of completed unfolding (D elta H-d) and the half-completed unfolding (Delta H-d)(1/2) for C. cal darium phycocyanin were much lower than those for S. lividus protein ( 89 versus 180 kcal/mole and 62 versus 115 kcal/mole, respectively). Fa ctors contributing to a lower Delta H-d in C. caldarium protein and th e role of charged groups in enhancing the stability of thermophilic pr oteins were discussed.