Haloperoxidase
Haloperoxidasesareperoxidasesthat are able to mediate theoxidationofhalidesbyhydrogen peroxide.[1]Both halides and hydrogen peroxide are widely available in theenvironment.[2]
Mechanistic and thermodynamic considerations
[edit]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:
- X−+ H2O2+ H++ R−H → X + 2 H2O + R−X
From the perspective of thermodynamics, theNernst equationconfirms that hydrogen peroxide can oxidizechloride(E°= 1.36 V),bromide(E°= 1.09 V), andiodide(E°= 0.536 V) from athermodynamicperspective under natural conditions, i.e., a temperature range of about 0–30 °C and apHranging from about 3 (humicsoil layer) to about 8 (sea water).Fluoride(E°= 2.87 V) cannot be oxidized by hydrogen peroxide.
Classification
[edit]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 peroxidaseisableto 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 ahemeprosthetic group covalently bound by two ester linkages toaspartateand/orglutamateside-chains. MPO has a third covalent link through amethionineresidue.Horseradish peroxidaseis also capable of oxidizing these substrates, but its heme is not covalently bound and becomes damaged during turnover.[4]
A specificvanadium bromoperoxidasein marine organisms (fungi, bacteria, microalgae, perhaps other eukaryotes) usesvanadateand hydrogen peroxide to brominate electrophilic organics.[5]
Murexsnails have abromoperoxidaseused to produceTyrian purpledye. 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 urchineggs (ovoperoxidase), |
| Iodoperoxidase(IPO) | I− | horseradish(horseradish peroxidase) |
See also
[edit]References
[edit]- ^S.L. Neidleman, J. Geigert (1986) Biohalogenation - principles, basic roles and applications; Ellis Horwood Ltd Publishers; Chichester;ISBN0-85312-984-3
- ^Vaillancourt, Frédéric H.; Yeh, Ellen; Vosburg, David A.; Garneau-Tsodikova, Sylvie; Walsh, Christopher T. (2006). "Nature's Inventory of Halogenation Catalysts: Oxidative Strategies Predominate".Chemical Reviews.106(8): 3364–3378.doi:10.1021/cr050313i.PMID16895332.
- ^[1]Archived2009-05-26 at theWayback MachineEosinophils preferentially use bromide to generate halogenating agents - Mayeno et al. 264 (10): 5660 - Journal of Biological Chemistry
- ^[2] Role of Heme-Protein Covalent Bonds in Mammalian Peroxidases
- ^Winter, JM; Moore, BS (July 2009)."Exploring the chemistry and biology of vanadium-dependent haloperoxidases".J. Biol. Chem.284(28): 18577–81.doi:10.1074/jbc.R109.001602.PMC2707250.PMID19363038.
