Porphyrin Motions in MbdesFe and HbdesFe
Porphyrin was bound to the heme pockets of apomyoglobin and apohemoglobin and the local rotations of this fluorescent probe were studied by fluorescence depolarization. Three temperature domains for the dynamics of porphyrin were observed. At very low temperatures the temperature dependence of the porphyrin rotations were found to follow the bulk solvent viscosity. At intermediate temperatures, porphyrin-heme pocket interactions were predominant. At temperatures above 0°C., the viscosity of the heme pocket appeared to be the determining factor. Large differences in the rotational parameters of the porphyrin were found between myoglobin and hemoglobin indicating different interactions between the porphyrin and these two heme pockets. For porphyrin in the hemoglobin heme cavity, the binding of the allosteric effector, inositol hexaphosphate resulted in immobilizing the probe in the low temperature regime, and in eliminating the intermediate temperature regime. This indicates that effector binding neutralizes certain porphyrin-pocket interactions, thus decoupling the fluorophore from the protein.