Thermodynamic parameters describing the conformational stability of the histidine-containing phosphocarrier protein from Streptomyces coelicolor, scHPr, have been determined by steady-state fluorescence measurements of isothermal urea-denaturations, differential scanning calorimetry at different guanidinium hydrochloride concentrations and, independently, by far-UV circular dichroism measurements of isothermal urea-denaturations, and thermal denaturations at fixed urea concentrations. The equilibrium unfolding transitions are described adequately by the two-state model and they validate the linear free-energy extrapolation model, over the large temperature range explored, and the urea concentrations used. At moderate urea concentrations (from 2 to 3 M), scHPr undergoes both high-and lowtemperature unfolding. The free-energy stability curves have been obtained for the whole temperature range and values of the thermodynamic parameters governing the heat-and cold-denaturation processes have been obtained. Colddenaturation of the protein is the result of the combination of an unusually high heat capacity change (1.4 ± 0.3 kcalAEmol )1 AEK )1 , at 0 M urea, being the average of the fluorescence, circular dichroism and differential scanning calorimetry measurements) and a fairly low enthalpy change upon unfolding at the midpoint temperature of heat-denaturation (59 ± 4 kcalAEmol )1 , the average of the fluorescence, circular dichroism and differential scanning calorimetry measurements). The changes in enthalpy (m DH i ), entropy (m DS i ) and heat capacity (m DC pi ), which occur upon preferential urea binding to the unfolded state vs. the folded state of the protein, have also been determined. The m DH i and the m DS i are negative at low temperatures, but as the temperature is increased, m DH i makes a less favourable contribution than m DS i to the change in free energy upon urea binding. The m DC pi is larger than those observed for other proteins; however, its contribution to the global heat capacity change upon unfolding is small.Keywords: calorimetry; denaturant-binding interactions; histidine-phosphocarrier; protein stability.A full understanding of the physical interactions underlying the structure, folding and the function of a protein requires a detailed description of its conformational stability in terms of the free energy of unfolding. Such a thermodynamic description relies on the quantitative analysis of denaturant-induced or thermally induced folding-unfolding transitions, measured either spectroscopically or calorimetrically. In both cases, data analyses involves the extrapolation of the thermodynamic parameters to standard conditions, usually 298 K in the absence of denaturant. To extrapolate thermal denaturation data, the change in DC p , and its temperature dependence must be known [1,2]. The extrapolation of data from chemical-denaturation [with either urea or guanidinium hydrochloride (Gdm Cl) as denaturants] is carried out using either the linear free energy model, LEM [3][4][5], or the binding...