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Peroxidase-catalyzed halide ion oxidation.

H B Dunford1

  • 1Department of Chemistry, University of Alberta, Edmonton, Canada.

Redox Report : Communications in Free Radical Research
|September 20, 2000
PubMed
Summary
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This study details the two-electron oxidation of iodide by horseradish peroxidase, forming hypoiodous acid. It also explores other peroxidases and the complex mechanisms of halide oxidation, including chloride.

Area of Science:

  • Enzymology and biochemical mechanisms
  • Oxidation-reduction reactions catalyzed by heme peroxidases

Background:

  • Horseradish peroxidase (HRP) was the first enzyme studied for complete mechanistic analysis of halide ion oxidation.
  • Previous studies indicated a two-electron oxidation of iodide by HRP compound I, suggesting oxygen atom transfer to form hypoiodous acid (HOI).

Purpose of the Study:

  • To analyze the mechanistic pathways of halide ion oxidation by various peroxidases.
  • To investigate the potential for two-electron oxidation mechanisms beyond iodide by HRP.
  • To clarify the reactive species involved in peroxidase-catalyzed halogenation, particularly chloride oxidation.

Main Methods:

  • Mechanistic analysis of iodide oxidation by horseradish peroxidase.
  • Investigation of two-electron oxidation mechanisms for other substrates like sulfite, nitrite, and sulfoxides.

Related Experiment Videos

  • Comparative studies of mammalian peroxidases (lactoperoxidase, myeloperoxidase) and chloroperoxidase.
  • Analysis of reaction kinetics and product identification in enzymatic and non-enzymatic halogenation reactions.

    • Main Results:

    • Confirmed a two-electron oxidation of iodide by HRP compound I, yielding hypoiodous acid (HOI).
    • Identified sulfite as another substrate undergoing two-electron oxidation, while nitrite and sulfoxides did not.
    • Demonstrated that compound II competes for halide, leading to slower iodine atom formation.
    • Highlighted that only chloroperoxidase and myeloperoxidase can oxidize chloride ion.
    • Discussed the ambiguity in identifying the chlorinating agent (HOCl, enzyme-bound HOCl, or Cl2) due to equilibria.

    Conclusions:

    • Peroxidase-catalyzed halide oxidation involves complex mechanistic pathways, including one- and two-electron transfers.
    • The specific halogenating agent in chloride oxidation by myeloperoxidase remains an area of active investigation, with evidence suggesting enzyme-bound hypochlorous acid.
    • Understanding these mechanisms is crucial for elucidating the roles of peroxidases in biological systems and for developing targeted therapeutic strategies.