Interaction between lanthanum ion and horseradish peroxidase in vitro

The interaction between lanthanum ion (La3+) and horseradish peroxidase (HRP) in vitro was investigated using a combination of biophysical and biochemical methods. When the molar ratio of La3+ and HRP is low, it was found that the interaction between La3+ and HRP mainly depends on the electrostatic attraction, van der waals force and hydrogen bond etc. Thus, the interaction is weak and the La–HRP complex cannot be formed in vitro. As expected, the interaction can change the conformation of HRP molecule, leading to the increase in the non-planarity of the porphyrin ring in the heme group of HRP molecule, and then in the exposure degree of the active center, Fe(III) of the porphyrin ring of HRP molecule. Therefore, the catalytic activity of HRP for the H2O2 reduction is improved. When the molar ratio of La3+ and HRP is high, La3+ can strongly coordinate with O and/or N in the amide group of the polypeptide chain of HRP molecule, forming the La–HRP complex. The formation of the La–HRP complex causes the change in the conformation of HRP molecule, leading to the decrease in the non-planarity of the porphyrin ring in the heme group of HRP molecule, and then in the exposure degree of the active center, Fe(III) of the porphyrin ring of HRP molecule. Thus, the catalytic activity of HRP for the H2O2 reduction is decreased comparing with that of HRP in the absence of La3+. The results can provide some references for understanding the interaction mechanism between trace elements ions and peroxidase in living organisms.