As mentioned above, all peroxidases require hydrogen
peroxide, or other hydroperoxides, to activate the haem
cofactor yielding the so-called compound-I in a common
catalytic cycle (Dunford, 1999) (Fig. 3). Compound-I contains
a reactive Fe4+ oxo complex with a cation radical at
the porphyrin ring, formed by two-electron oxidation of
the Fe3+-containing haem of the resting enzyme. Oneelectron
oxidation of one substrate molecule yields
compound-II, where the porphyrin cation radical has been
reduced. The remaining Fe4+=O in compound-II oxidizes a
second substrate molecule, and the enzyme returns to its
ferric resting state to initiate a new catalytic cycle.