Investigation of Streptomyces antibioticus tyrosinase reactivity toward chlorophenols.

Tyrosinase (Ty) is a copper-containing enzyme ubiquitously distributed in nature. In recent years, Ty has attracted interest as a potential detoxifying agent for xenobiotic compounds with phenolic structure. Among these, chlorophenols are particularly relevant pollutants, commonly found in waste waters. The activity of Streptomyces antibioticus tyrosinase toward isomeric monochlorophenols was studied. Tyrosinase oxidizes both 3- and 4-chlorophenol to the same product, 4-chloro-1,2-ortho-quinone, which subsequently undergoes a nucleophilic substitution reaction at the chlorine atom by excess phenol to give the corresponding phenol-quinone adduct. By contrast, 2-chlorophenol is not reactive and acts as a competitive inhibitor. Docking calculations suggest that the substrates point to one of the copper atoms of the dinuclear center (copper B) and appear to interact preferentially with one of the two coordinated oxygen atoms. The approach of the substrate toward the active site is favored by a π-stacking interaction with one of the copper-coordinated histidines (His194) and by a hydrogen bonding interaction with the O1 oxygen. With this study, we provide the first characterization of the early intermediates in the biotechnologically relevant reaction of Ty with chlorophenols. Additionally, combining experimental evidences with molecular modeling simulations, we propose a detailed reaction scheme for Ty-mediated oxidation of monochlorophenols.