Unfolding pathway of myoglobin: molecular properties of intermediate forms.

The guanidine-induced unfolding of myoglobin as well as apomyoglobin has been found to involve the occurrence of at least a molecular intermediate observed at low denaturant concentrations, the molecular properties of which resemble those possessed by the acid-denatured form of the protein. The two partially unfolded forms show the same secondary structure and similar tryptophanyl fluorescence emission and polarization but exhibit marked differences in the tyrosine contributions to the near-ultraviolet circular dichroism and in the degree of solvent accessibility to tyrosyl residues. The molecular characterization of the two structural forms indicates that acids disorganize the 80-146 molecular domain identified in the myoglobin molecule to a great extent with respect to that induced by low guanidine concentration, whereas the structure of the 1-79 domain appears to be quite similar in the two molecular forms.