COMPARATIVE-STUDIES ON THE INTERACTION OF PROTOPORPHYRIN WITH HEMOGLOBIN AND MYOGLOBIN

The binding parameters of protoporphyrin IX (PPIX) with hemoglobin (Hb ) were studied spectrofluorimetrically and the results were compared w ith those of PPIX interacting with myoglobin (Mb). Two concentration r anges of PPIX (0.3 mu M -1.5 mu M and 1.5 mu M -3.0 mu M) were used. F or both hemoglobin and myoglobin, the binding affinity constant (K) de creased while the number of binding sites (p) increased as the concent ration range of PPIX increased. The interactions occurred in noncooper ative mode. Over a particular PPIX range, the interaction of PPIX with hemoglobin decreased significantly with increasing NaCl molarity indi cating a trend in electrostatic interaction, whereas PPIX binding with myoglobin did not change significantly indicating mostly non-electros tatic mode of interaction. Total bound charge (z Psi) decreased signif icantly with increased PPIX concentration range in case of hemoglobin- PPIX interaction, but remained almost same in case of myoglobin-PPIX i nteractions. Thermodynamic analysis revealed that binding of PPIX to h emoglobin was mostly electrostatic at lower concentration range of PPI X but became less electrostatic at higher concentration range and myog lobin-PPIX interaction, predominantly hydrophobic in nature, became mo re hydrophobic with increased range of PPIX concentration. The differe nce in binding modality between PPIX-Hb and PPIX-Mb has been discussed in relation to the state of aggregation of porphyrin as well as the s ubunit interaction property present and absent in hemoglobin and myogl obin, respectively.