Haloperoxidases areperoxidases that are able to mediate theoxidation ofhalides byhydrogen peroxide.[1] Both halides and hydrogen peroxide are widely available in theenvironment.[2]
Halogenations of organic compounds by free halogens (F2, Cl2, Br2, and sometimes I2) is generally favorable process. It is practiced industrially on a large scale for example. In nature, however, free halogens do not exist in appreciable amounts. The combination ofhydrogen peroxide, which is widely produced by aerobic life, and halide anionsCl−, Br−, I− provides the equivalent ofCl2, Br2, I2. The oxidation of these anions by hydrogen peroxide is slow in the absence of enzymes. These enzymes are called haloperoxidases. The reaction that they catalyze is:
From the perspective of thermodynamics, theNernst equation confirms that hydrogen peroxide can oxidizechloride (E°= 1.36 V),bromide (E°= 1.09 V), andiodide (E°= 0.536 V) from athermodynamic perspective under natural conditions, i.e., a temperature range of about 0–30 °C and apH ranging from about 3 (humic soil layer) to about 8 (sea water).Fluoride (E°= 2.87 V) cannot be oxidized by hydrogen peroxide.
The table shows the classification of haloperoxidases according to the halides whose oxidation they are able to catalyze.
The classification of these enzymes by substrate-usability does not necessarily indicate enzyme substratepreference. For example, althougheosinophil peroxidase isable to oxidize chloride, it preferentially oxidizes bromide.[3]
The mammalian haloperoxidasesmyeloperoxidase (MPO),lactoperoxidase (LPO) andeosinophil peroxidase (EPO) are also capable of oxidizing thepseudohalidethiocyanate (SCN−). They each contain aheme prosthetic group covalently bound by two ester linkages toaspartate and/orglutamate side-chains. MPO has a third covalent link through amethionine residue.Horseradish peroxidase is also capable of oxidizing these substrates, but its heme is not covalently bound and becomes damaged during turnover.[4]
A specificvanadium bromoperoxidase in marine organisms (fungi, bacteria, microalgae, perhaps other eukaryotes) usesvanadate and hydrogen peroxide to brominate electrophilic organics.[5]
Murex snails have abromoperoxidase used to produceTyrian purple dye. The enzyme is very specific to bromide and physically stable, but has not been characterized as to its active site.
| Haloperoxidase | Oxidisable halide | Origin, Notes |
|---|---|---|
| Chloroperoxidase (CPO) | Cl−, Br−, I− | neutrophils (myeloperoxidase), eosinophils (eosinophil peroxidase, can use Cl−, prefers Br−) |
| Bromoperoxidase (BPO) | Br−, I− | milk,saliva,tears (lactoperoxidase), sea urchin eggs (ovoperoxidase), |
| Iodoperoxidase (IPO) | I− | horseradish (horseradish peroxidase) |
