The combination of hydrogen peroxide, which is widely produced by aerobic life, and halide anionsCl−, Br−, I− provides the equivalent of Cl2, Br2, I2.
The reaction that they catalyze is: From the perspective of thermodynamics, the Nernst equation confirms that hydrogen peroxide can oxidize chloride (E°= 1.36 V), bromide (E°= 1.09 V), and iodide (E°= 0.536 V) from a thermodynamic perspective under natural conditions, i.e., a temperature range of about 0–30 °C and a pH ranging from about 3 (humic soil layer) to about 8 (sea water).
[3] The mammalian haloperoxidases myeloperoxidase (MPO), lactoperoxidase (LPO) and eosinophil peroxidase (EPO) are also capable of oxidizing the pseudohalide thiocyanate (SCN−).
They each contain a heme prosthetic group covalently bound by two ester linkages to aspartate and/or glutamate side-chains.
Horseradish peroxidase is also capable of oxidizing these substrates, but its heme is not covalently bound and becomes damaged during turnover.