Chloroperoxidases, natural formation

Aquatic Sources of Halocarbons. One of the main pathways for the natural formation of halocarbons involves peroxidase enzymes (8, 68-70). Chloroperoxidases can catalyze the formation of chlorinated, brominated, and iodinated organic compounds bromoperoxidases can produce only the latter two types of halocarbons (8, 69). The Fe-heme peroxidase enzymes, which are activated by hydrogen peroxide and organic peroxides, produce methyl halides as well as polyhalogenated compounds (Scheme II). 

X-ray absorption spectroscopy has revealed the formation of organochlorine compounds from chloride and chloroperoxidase in weathering plant material (172-174). Moreover, this technique has uncovered the bromide-to-organobromine conversion in environmental samples (174). In addition to chloroperoxidase mediated chlorination, the abiotic chlorination in soils and sediments involving the alkylation of halides during Fe(III) oxidation of natural organic phenols in soils and sediments has been discovered (175-177). 

All three chloroacetic acids (chloroacetic acid [MCA], dichloroacetic acid [DCA], and trichloroacetic acid [TCA]) are naturally occurring (7), with TCA being identified in the environment most frequently (reviews (278, 405 108)). However, these chlorinated acetic acids also have anthropogenic sources. The major source of natural TCA appears to be the enzymatic (chloroperoxidase) or abiotic degradation of humic and fulvic acids, which ultimately leads to chloroform and TCA. Early studies (409) and subsequent work confirm both a biogenic and an abiotic pathway. Model experiments with soil humic and fulvic acids, chloroperoxidase, chloride, and hydrogen peroxide show the formation of TCA, chloroform, and other chlorinated compounds (317, 410-412). Other studies reveal an abiotic source of TCA (412, 413). 

Hoekstra EJ, Lassen P, van Leeuwen JGE, de Leer EWB, Carlsen L (1995) Formation of Organic Chlorine Compounds of Low Molecular Weight in the Chloroperoxidase-Mediated Reaction Between Chloride and Humic Material. In Grimvall A, de Leer EWB (eds) Naturally-Produced Organohalogens. Kluwer, Dordrecht, p 149... 

It has been hypothesized on the basis of the formation of trichlo-roacetate from aliphatic compounds, especially acetate, by the action of chloroperoxidase in the presence of hydrogen peroxide and chloride that this might be a naturally occurring metabolite (Haiber et al. 1996). Plausible mechanisms for the formation of trichloroacetic acid by atmospheric reactions involving trichloro-ethane and tetrachloroethene are discussed in Section 4.1.2. 

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