KINETIC-STUDIES ON THE OXIDATION OF PHENOLS BY THE HORSERADISH-PEROXIDASE COMPOUND-II

Oxidation of substituted phenols by horseradish peroxidase compound II were studied using slopped-flow technique. Dissociation constants (K- D) of HRP-II-phenol complexes were deduced from the kinetic data. Magn itudes of K-D fall in a relatively narrow range of 3-11 mM. These are comparable to 3-10 mM reported for the binding of substituted phenols to native HRP, suggesting that the mode of binding of phenols to nativ e HRP and HRP compound II may be similar. pH dependence of the apparen t second order rate constants (k(app)) of the reactions of all the phe nols were determined. The k(app) values of reactions other than the re action of tyrosine, were observed to increase in the acidic region but decreased in the alkaline region. The increase was attributed to the deprotonation of distal carboxylic acid residue on enzyme with pK(a) v alues of 4.2-5.2. For tyrosine, however, the apparent second-order rat e constant was observed to further increase non linearly on increasing the pH in the alkaline region. Results were interpreted quantitativel y on the basis that protonated form of the enzyme reacted with the pro tonated form of the phenol with different individual rate constants.