Haloperoxidase activities halogenation

The standard assay for haloperoxidase activity is the halogenation of monochloro-dimedone (mcd) (2-chloro-5,5-dimethyl-l,3-dimedone) using dihydrogen peroxide as the oxidant of the halide (Figure 4) [48],... 

Recently the amino acid sequence of vanadium chloroperoxidase was determined to have similar stretches with three families of acid phosphatases, which were previously considered unrelated [72], This sequence raises questions about the phosphatase activity of apo-V-ClPO and whether the acid phosphatases can coordinate vanadate and carry out peroxidative halogenation chemistry. In fact, apo-V-C1PO does have phosphatase activity, catalyzing the hydrolysis of/i-nitrophe-nol phosphate (p-NPP). In addition, /i-NPP displaces vanadate from V-CIPO. At this point, the haloperoxidase activity of the acid phosphatases containing coordinated vanadium(V) has not been reported. 

Heme haloperoxidases can also use peroxide and halide ions to halogenate an activated (benzylic/allylic) carbon. The halide is first oxidized to an active halogenating intermediate (Fig. 10.4, pathway (2)). The substrate is halogenated in the next step. The overall reaction is... 

For the same systems haloperoxidase activity has been reported with H2O2 / O2 as oxidant and HC1 / HBr as halogen source [22]. In this way oxychlorination / oxybromination of benzene, toluene, phenol, aniline, anisolc and resorcinol could be achieved. 

Haloperoxidases are peroxidases capable of halogenating substrates in the presence of halide and hydrogen peroxide [14] or other reactions such as sulfoxidation, epoxidation and aromatic hydroxylation. Here, the halide ion is initially bound to the active site which may incorporate heme or vanadium or be metal free. The halide ion is incorporated into the substrate after electron transfer... 

Haloperoxidases. Many specialized peroxidases are active in halogenation reactions. Chloroperoxi-dases from fungi236 237 catalyze chlorination reactions like that of Eq. 16-11 using H202 and Cl as well as the usual peroxidase reaction. 

Latumus F, Mehrtens G, Gr0n C (1995) Haloperoxidase-Like Activity in Spruce Forest Soil - A Source of Volatile Halogenated Organic Compounds Chemosphere 31 3709... 

There are several examples of haloperoxidases containing metals other than iron, and many of these are non-heme enzymes. For example, a series of bromoperoxidases have been isolated recently from seaweed17,18 and an actinomycete19 that require vanadium for halogenating activity. 

Ley and Barton s observation that di-4-methoxyphenyltelluride could be used catalytically was the first entry into the use of in situ generated selenoxides or telluroxides as catalysts. As shown in Fig. 8, a variety of different nucleophiles can be introduced via the selenoxide or telluroxide followed by reductive elimination to generate oxidized product and reduced selenide or telluride. If the nucleophile is relatively inert to oxidation by hydrogen peroxide, then the reduced selenide or telluride can be reoxidized by hydrogen peroxide and the overall oxidation of the nucleophile becomes catalytic in the selenide or telluride. In the case of thiols, disulfides are the final product and the selenides or tellurides exhibit thiolperox-idase-like activity 60-62 64 82 83 If halide salts (chloride, bromide, iodide) are the nucleophiles, then positive halogen sources are the oxidized products and the selenides and tellurides exhibit haloperoxidase-like activity.84-88 The phenoxypro-pyltelluride 59 has been used as a catalyst for the iodination and bromination of a variety of organic substrates as shown in Fig. 24.87... 

A striking array of halogenated products produced by natural sources is found in the biosphere. The majority are probably formed by the enzymatic activity of haloperoxidases. The detailed mechanism by which haloperoxidases catalyze the insertions of a halogen into organic molecules is still a point of debate. However, for most of these enzymes a simple reaction mechanism has been observed that consists of a two-electron oxidation of the electron donor (Cl", Br", I") by H2O2 to hypohalous acids (equation 1) ... 

In contrast to FADH2-dependent halogenases, haloperoxidases have no substrate specificity. The enzymatic iodination, bromination, and chlorination of a number of different aromatic compounds by haloperoxidases have been reported in the last few years. All aromatic substrates halogenated successfully by haloperoxidases are aromatic compounds activated for electrophilic substitution (Table 16.9-1). 

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