Denaturation thermodynamics of chicken cardiac metmyoglobin.

The unfolding at pH 8 of chicken cardiac aquometmyoglobin was examined as a function of temperature and concentration of guanidinium chloride using the two-state model. The isothermal unfolding data at 25 degrees C were fitted to Tanford's transfer model and the binding model of Aune and Tanford. The estimates obtained for delta GD were virtually identical, viz., 8.3 +/- 0.3 kcal mol-1. The chicken metmyoglobin is thus some 5.3 kcal mol-1 less stable than that of sperm whale metmyoglobin. The unfolding parameters alpha and delta n were decreased 20% from those of mammalian myoglobins thus far examined, suggesting nonidentity of native conformations. The apparent enthalpy change on unfolding was dependent on both temperature and denaturant concentration. The decreases in the isothermal unfolding parameters from those of sperm whale are principally assigned to three of the 46 sequence changes.