Calorimetric studies of the thermal denaturation of cytochrome c peroxidase

Two endotherms are observed by differential scanning calorimetry during the thermal denaturation of cytochrome c peroxidase at pH 7.0. The transition midpoint temperatures (tm) were 43.9 +/- 1.4 and 63.3 +/- 1.6 degrees C, independent of concentration. The two endotherms were observed at all pH values between 4 and 8, with the transition temperatures varying with pH. Precipitation was observed between pH 4 and 6, and only qualitative data are presented for this region. The thermal unfolding of cytochrome c peroxidase was sensitive to the presence and ligation state of the heme. Only a single endotherm was observed for the unfolding of the apoprotein, and this transition was similar to the high-temperature transition in the holoenzyme. Addition of KCN to the holoenzyme increases the midpoint of the high-temperature transition whereas the low-temperature transition was increased upon addition of KF. Binding of the natural substrate ferricytochrome c to the enzyme increases the low-temperature transition by 4.8 +/- 1.3 degrees C but has no effect on the high-temperature transition at pH 7. The presence of cytochrome c peroxidase decreases the stability of cytochrome c, and both proteins appear to unfold simultaneously. The results are discussed in terms of the two domains evident in the X-ray crystallographic structure of cytochrome c peroxidase.