Properties of Protoporphyrin-Apomyoglobin Complexes and Related Compounds

The conformation of the 1:1 complex of protoporphyrin IX and apomyoglobin was studied by circular dichroism, potentiometric titration, and reaction with bromoacetate. Circular dichroism studies indicate that protoporphyrin and heme similarly affect the α-helix content of apomyoglobin. The apparent number of normal imidazoles in the protoporphyrin-apomyoglobin complex, as judged by reactivity to H+ ion and bromoacetate, is very similar to that of myoglobin and different from that in globin. These data suggest that the conformational difference between myoglobin and apomyoglobin is determined mainly by the interaction of globin with the porphyrin and not with the heme iron. The lesser stability to acid of porphyrin complexes than of myoglobin is tentatively explained by the absence of the iron-imidazole bond in the porphyrin complexes. Potentiometric titration of the 1:1 complex of globin with 1-anilino-8-naphthalenesulfonate indicates that this complex has the same number of normal imidazoles as does globin. These studies and those summarized above support a direct relationship between the α-helix content of myoglobin derivatives as determined by optical activity and the number of normally titratable imidazole side chains. The photosensitivity of porphyrin-apomyoglobin complexes was shown to be due to porphyrin-sensitized photooxidation of globin. Manometric studies and amino acid analysis indicated that it is the initial oxidation of 1 or 2 histidine residues, apparently at the binding site, which leads to impairment of porphyrin binding.