THE CHLOROPEROXIDASE-CATALYZED OXIDATION OF PHENOLS - MECHANISM, SELECTIVITY, AND CHARACTERIZATION OF ENZYME-SUBSTRATE COMPLEXES

The reactivity of a series of para-substituted phenolic compounds in t he peroxidation catalyzed by chloroperoxidase was investigated, and th e results were interpreted on the basis of the binding characteristics of the substrates to the active site of the enzyme. Marked selectivit y effects are observed. These operate through charge, preventing pheno lic compounds carrying amino groups on the substituent chain to act as substrates for the enzyme, and through size, excluding potential subs trates containing bulky substituents to the phenol nucleus. Also, chir al recognition is exhibited by chloroperoxidase in the oxidation of N- acetyltyrosine, where only the L isomer is oxidized. Kinetic measureme nts show that, in general, the efficiency of chloroperoxidase in the o xidation of phenols is lower than that of horseradish peroxidase. Para magnetic NMR spectra and relaxation rate measurements of chloroperoxid ase-phenol complexes are consistent with binding of the substrates clo se to the heme, in the distal pocket, with the phenol group pointing t oward the iron atom. On the other hand, phenolic compounds which are n ot substrates for chloroperoxidase bind to the enzyme with a much diff erent disposition, with the phenol group very distant from the iron an d probably actually outside the active-site cavity.