In order to understand the chemical mechanism of the interaction of the enzymes with the rare earth elements, the mechanism of the interaction between La^3＋ and the model of POD, MP-8 or MP-11 was studied using UV-Vis absorption spectroscopy and electrochemical technique. It was found that La^3＋ was bound firstly to the carbonyl oxygen of the metacetonic acid group in the heme group of the MP-8 or MP-11 molecule, resulting in the decrease of their aggregation extents and the increase of the non-planarity of the porphyrin group. The decrease extent of the aggregation of the molecules and then the increase extent of the nonplanarity of the porphyrin group for MP-8 due to La^3＋ binding were greater than those for MP-11 because the aggregation tendency of MP-8 was less than that of MP-11. The peptide of MP-11 was long enough to form the helical conformation, thus, the carbonyl oxygen in the peptide would be embedded in the hydrophobic microenvironment of the peptide, and La^3＋ was not easy to coordinate with the carbonyl oxygen in the peptide of MP-11. However, the peptide of MP-8 was relatively short, it could not form the helical conformation and therefore, La^3＋ was easy to interact with the carbonyl oxygen in the peptide of MP-8.

Key words: microperoxidase-8, microperoxidase-11, coordination