Fig. 1.

Panel A. Free energy profile for a two-state model of protein folding/unfolding. The free energy landscape changes by varying temperature making one state more favorable than the other. At the melting temperature T_M the probabilities for the protein to be in the folded or unfolded state are equal. The change in the free energy difference between the unfolded and folded state as a function of temperature produces the so-called stability curve (black curve in the right panel). Three distinct mechanisms that shift the melting temperature toward higher values are represented: the up-shift (red), the right-shift (orange), the broadening (indigo). Panel B. Activity curve for mesophilic, thermophilic and hyperthermophilic species. Increasing temperature the activity increases since it is more favorable for the system to cross the barrier related to the catalytic reaction, beyond an optimal temperature the degradation of the active site is due to the onset of unfolding causing the reduction in activity. Panel C. Typical thermogram from DSC experiments. The heat capacity of unfolding is given by the difference between the two baselines corresponding to the folded and unfolded states. At the melting temperature T_M, C_p shows a peak due to the enhanced transitions between folded and unfolded states.