DYNAMICS OF PROTOPORPHYRIN-IX IN THE HEME POCKET OF HORSERADISH-PEROXIDASE

The local motion of protoporphyrin IX in the heme pocket of horseradis h peroxidase has been studied using fluorescence methods. The temperat ure dependence of the anisotropy and lifetime of a protoporphyrin IX-a po-horseradish peroxidase complex, dissolved in a solution of 80% glyc erol and 20% buffer (0.1 M phosphate, pH 7.4), was determined. Anisotr opy data were analyzed in terms of the thermal coefficient of the fric tional 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 fluoroph ores not complexed with protein. The slope corresponding to an interme diate temperature was lower indicating a larger resistance to porphyri n rotation. At higher temperatures this resistance to rotation diminis hed as evidenced by the increased slope. These results are contrasted with those obtained with the protoporphyrin IX-apomyoglobin complex.