Cold denaturation of the molten globule states of apomyoglobin and a profile for protein folding.

Protein folding is a process in which an extended polypeptide chain acquires compact packing through the formation of specific secondary and tertiary structures and hydrophobic interactions. Although much attention has been paid to secondary and tertiary structures, there is no definitive view about the relationship between these structures, compactness, and hydrophobic interactions during the process of protein folding. We show here that the molten globule intermediates of horse apomyoglobin exhibit cold denaturation in addition to heat denaturation, which indicates that the heat capacity change upon unfolding is positive and significant. This demonstrates a small but distinct contribution of hydrophobic interactions to the stability of the molten globule state. We determined the radius of gyration of the various conformational states of horse apomyoglobin and holomyoglobin by measuring small angle X-ray scattering. By comparing the conformational states in terms of secondary structure, radius of gyration, and change in heat capacity upon unfolding, we constructed a folding profile. The profile shows that the protein becomes more compact with formation of the secondary structure, but does not form substantial hydrophobic interactions until a later rate-limiting stage when tight packing of the protein side chains occurs. A very similar profile was also obtained with horse cytochrome c. We propose that the folding profile obtained with these proteins will be common to many globular proteins.