title: Single turnover studies of oxidative halophenol dehalogenation by horseradish peroxidase reveal a mechanism involving two consecutive one electron steps: toward a functional halophenol bioremediation catalyst.
abstract: Horseradish peroxidase (HRP) catalyzes the oxidative para-dechlorination of the environmental pollutant/carcinogen 2,4,6-trichlorophenol (2,4,6-TCP). A possible mechanism for this reaction is a direct oxygen atom transfer from HRP compound I (HRP I) to trichlorophenol to generate 2,6-dichloro 1,4-benzoquinone, a two-electron transfer process. An alternative mechanism involves two consecutive one-electron transfer steps in which HRP I is reduced to compound II (HRP II) and then to the ferric enzyme as first proposed by Wiese et al. [F.W. Wiese, H.C. Chang, R.V. Lloyd, J.P. Freeman, V.M. Samokyszyn, Arch. Environ. Contam. Toxicol. 34 (1998) 217-222]. To probe the mechanism of oxidative halophenol dehalogenation, the reactions between 2,4,6-TCP and HRP compounds I or II have been investigated under single turnover conditions (i.e., without excess H(2)O(2)) using rapid scan stopped-flow spectroscopy. Addition of 2,4,6-TCP to HRP I leads rapidly to HRP II and then more slowly to the ferric resting state, consistent with a mechanism involving two consecutive one-electron oxidations of the substrate via a phenoxy radical intermediate. HRP II can also directly dechlorinate 2,4,6-TCP as judged by rapid scan stopped-flow and mass spectrometry. This observation is particularly significant since HRP II can only carry out one-electron oxidations. A more detailed understanding of the mechanism of oxidative halophenol dehalogenation will facilitate the use of HRP as a halophenol bioremediation catalyst.
authors.affiliation: Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29073, USA.
authors.forename: Suganya
authors.initials: S
authors.lastname: Sumithran
authors.forename: Masanori
authors.initials: M
authors.lastname: Sono
authors.forename: Gregory M
authors.initials: GM
authors.lastname: Raner
authors.forename: John H
authors.initials: JH
authors.lastname: Dawson
mesh_terms: D001673:Biodegradation, Environmental; D002384:Catalysis; D002733:Chlorophenols / Q000737:chemistry*; D054879:Halogenation; D006735:Horseradish Peroxidase / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction
chemical_list: D002733:Chlorophenols; C098326:oxyferryl porphyrin pi cation radical; D006735:Horseradish Peroxidase; C024564:2,4,6-trichlorophenol; C032729:2,6-dichlorophenol