Fluorescence study of conformational transitions in the structure of myoglobin

Fluorescence studies of myoglobin and Mb-like structures, apomyoglobin and the complex of apo-Mb with protoporphyrin IX, reveal both the similarity be tween them, which is due to a common type of polypeptide chain folding, and the distinctions imposed by the influence of the prosthetic group. Close r esemblance of structures of holomyoglobin and its metal-free analog, PPIX-a po-Mb, points to a key role of specific interactions between the protein an d the protoporphyrin macrocycle rather than the Fe-protein bond in the form ation of Mb-like structures. In PPIX-apo-Mb, both the hydrophobic core and the important ionic bonds between different structural elements ("salt brid ges") stabilizing the Mb structure are almost completely retained. The bond between Fe and proximal His-F8 allows additional integration of the struct ures of the heme cavity and the myoglobin molecule as a whole, providing it s functional activity and highly cooperative conformational transitions. In all the myoglobin-like structures studied, a certain relationship is found between conformational states of the "active center", the heme cavity, and the N-terminal part of the molecule. This is probably due to variations in the mutual orientation of the ABCDE and FGH helical domains, depending on the interactions between the protein, the prosthetic group, and the ligand in the heme crevice. The correlation between conformations of the N-termina l and heme regions found at a level of the globin tertiary structure is ver y important for understanding the mechanisms of homo- and heterotropic regu lation in tetrameric hemoglobins.