Thermodynamic investigations of cytochrome b5 unfolding. I. The tryptic fragment of cytochrome b5.

Thermal unfolding of the tryptic fragment of the membrane-bound protein, cytochrome b5, which contains the residues 1-90, was investigated by scanning calorimetry, circular dichroism and absorption spectroscopy. The fragment undergoes a reversible two-state transition at about 70 degrees C (neutral pH). The fragment exhibits all the thermodynamic properties of small globular proteins with respect to heat capacity and transitional changes of enthalpy, Gibbs energy and heat capacity. The heat capacity change at unfolding fits into the correlation with the specific amount of nonpolar contacts, which has been found to be valid for small globular proteins (Privalov, P.L. and Khechinashvili, N.N. (1974) J. Mol. Biol. 86, 665-684). The relatively low stabilization energy of the cytochrome b5 fragment (delta trsG25 degrees C = 25 kJ/mol) is discussed in terms of the functional requirements of electron-transfer proteins.