Effect of polyglutamate on the thermal stability of ferricytochrome c.

The effect of saturated solutions of polyglutamate on the thermal stabilities of the Met-80-heme iron bond and of the ferricytochrome c as a whole were studied by absorption spectroscopy and differential scanning calorimetry at pH 7.0. According to spectral data the midtransition temperature of the cleavage of the sulfur-iron bond was 57.4 +/- 0.5 degrees C and 66.8 +/- 0.5 degrees C for cytochrome c and cytochrome c-polyglutamate complex, respectively. Addition of polyglutamate to cytochrome c at pH 7.0 alters the denaturation properties of the protein. As follows from DSC scans, the denaturation temperature for cytochrome c is decreased from 85.4 +/- 0.2 degrees C to 68.7 +/- 0.2 degrees C in the presence of the saturated amount of polyglutamate. The protein stability in terms of Gibbs energy change at protein unfolding amount to delta G(25 degrees C) = 22.7 +/- 2.7 and 32.0 +/- 2.2 kJ/mol, for cytochrome c and cytochrome c-polyglutamate complex, respectively, at pH 7.0. It is evident that polyglutamate increases the thermal stability of the sulfur-iron bond and decreases the denaturation temperature of the cytochrome c molecule as a whole. The complex thermal stability in terms of Gibbs energy is not lower than that of cytochrome c in the range of physiological temperatures.