Dynamics of protoporphyrin IX in the heme pocket of horseradish peroxidase.

The local motion of protoporphyrin IX in the heme pocket of horseradish peroxidase has been studied using fluorescence methods. The temperature dependence of the anisotropy and lifetime of a protoporphyrin IX-apo-horseradish peroxidase complex, dissolved in a solution of 80% glycerol and 20% buffer (0.1 M phosphate, pH 7.4), was determined. Anisotropy data were analyzed in terms of the thermal coefficient of the frictional resistance to fluorophore movement. The resultant 'Y' plot was characterized by three distinct slopes. The slope corresponding to the lowest temperature regime agreed with the value obtained for fluorophores not complexed with protein. The slope corresponding to an intermediate temperature was lower indicating a larger resistance to porphyrin rotation. At higher temperatures this resistance to rotation diminished as evidenced by the increased slope. These results are contrasted with those obtained with the protoporphyrin IX-apomyoglobin complex.