A comparative study of the unfolding thermodynamics of vertebrate metmyoglobins.

Differential scanning microcalorimetry (DSC) of horse, rat, opossum, raccoon, carp, and armadillo metmyoglobins at alkaline pH gave data that fit the two-state unfolding model well. Monte Carlo studies were used to assess the impact of truncating DSC scans on the reliability of the calculated results when aggregation exotherms overlapped the unfolding endotherm at the high-temperature end of the scan. The DSC estimates for the conformational free energy at pH 8 and 298 K are compared to earlier results from isothermal acid and guanidinium chloride unfolding. Stability estimates at pH 8 for these six metmyoglobins obtained by DSC experiments do not agree with free energy estimates at pH 8 from guanidinium chloride unfolding. This is true for all three models used to extrapolate the free energy change to 0 M guanidinium chloride. Among these six myoglobins, significant variation appears in the temperature at which the myoglobin is half-unfolded, in the change in heat capacity upon unfolding, and in the change in enthalpy at 310 K. Calculations made with the hydrophobic model for protein folding [Baldwin, R.L. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 8069] suggest that a sizable variation exists for that portion of the unfolding enthalpy change assigned to forces other than the hydrophobic effect.