STABILITY OF YEAST ISO-1-FERRICYTOCHROME-C AS A FUNCTION OF PH AND TEMPERATURE

Absorbance-detected thermal denaturation studies of the C102T variant of Saccharomyces cerevisiae iso-1-ferricytochrome c were performed between pH 3 and 5. Thermal denaturation in this pH range is reversible, shows no concentration dependence, and is consistent with a 2-state model. Values for free energy (Delta G(D)), enthalpy (Delta H-D), and entropy (Delta S-D) of denaturation were determined as functions of pH and temperature. The value of Delta G(D) at 300 K, pH 4.6, is 5.1 +/- 0.3 kcal mol(-1). The change in molar heat capacity upon denaturation (Delta C-p), determined by the temperature dependence of Delta H-D as a function of pH (1.37 +/- 0.06 kcal mol(-1) K-1), agrees with the value determined by differential scanning calorimetry, pH-dependent changes in the Soret region indicate that a group or groups in the heme environment of the denatured protein, probably 1 or both heme propionates, ionize with a pK near 4. The C102T variant exhibits both enthalpy and entropy convergence with a Delta H-D of 1.30 kcal mol(-1) residue(-1) at 373.6 K and a Delta S-D of 4.24 cal mol(-1) K-1 residue(-1) at 385.2 K. These values agree with those for other single-domain, globular proteins.