Fluorescence study of the conformational properties of myoglobin structure. 3. pH-dependent changes in porphyrin and tryptophan fluorescence of the complex of sperm whale apomyoglobin with protoporphyrin IX; the role of the porphyrin macrocycle and iron in formation of native myoglobin structure

The porphyrin and tryptophan fluorescence of sperm whale apomyoglobin complexed with protoporphyrin IX has been studied in the pH range 2-13. It has been shown that the fluorescence and absorption spectra of protoporphyrin incorporated into the heme crevice remain constant in the pH range 5.5-10.8 but change significantly at pH less than 5.5 and pH greater than 10.8, due to the acid and alkaline denaturation, respectively, of the complex accompanied by dissociation of protoporphyrin IX. At the same pH ranges, the quantum yield of tryptophanyl fluorescence increases sharply as a result of removal of protoporphyrin, acting as a quencher, from the complex. Other parameters of tryptophanyl fluorescence (maximum position, halfwidth and spectrum shape) change in the alkaline region as well. In the acidic pH range, these parameters change only at pH less than 4.3, indicating that the Trp surroundings are more stable to denaturation than the heme crevice region. Between pH 5.5 and 10.9, where the complex of apomyoglobin with protoporphyrin IX is in its native state, the main parameters of tryptophan fluorescence remain unchanged except for the ratio I325/I350 which diminishes at pH greater than 9.5. Its alteration precedes the alkaline denaturation of the complex and can be explained by a local conformational change induced by the break of the 'salt bridges' essential for the maintenance of the native Mb structure in the N-terminal region. The fluorescence data obtained for apomyoglobin, myoglobin and the complex between protoporphyrin IX and apomyoglobin enable one to compare their structures and to evaluate the role of the porphyrin macrocycle and the iron atom in the formation of the native myoglobin structure and its functioning.