Kinetics of formation of antibody-ferric porphyrin complex with peroxidase activity

The antibody 2B4 combines with ferric mesoporphyrin to form an antibody-ferric mesoporphyrin complex which has a peroxidase activity. Formation of the complex was investigated by measuring the absorption in the Soret region after mixing the antibody and ferric mesoporphyrin. A rapid increase and a gradual decrease in the absorption were observed, and the respective first-order rate constants were obtained. From the dependence of values of the rate constants on the concentration of ferric mesoporphyrin, the complex formation was explained by a plausible mechanism, in which the antibody associated with ferric mesoporphyrin to form the first complex followed by a conformational change to the second complex. The first complex had almost the same peroxidase activity as that of the second complex. Our results suggests that the antibody acquires the peroxidase activity as soon as ferric mesoporphyrin is incorporated into its binding site, and that there will be no protein ligand to the iron center of ferric mesoporphyrin in the complex.