Haem iron-containing peroxidases.

Peroxidases are enzymes that utilize hydrogen peroxide to oxidize substrates. A histidine residue on the proximal side of the haem iron ligates most peroxidases. The various oxidation states and ligand complexes have been spectroscopically characterized. HRP-I is two oxidation states above ferric HRP. It contains an oxoferryl (= oxyferryl) iron with a pi-radical cation that resides on the haem. HRP-II is one oxidation state above ferric HRP and contains an oxoferryl iron. HRP-III is equivalent to the oxyferrous state. Only compounds I and II are part of the peroxidase reaction cycle. CCP-ES contains an oxoferryl iron but the radical cation resides on the Trp-191 residue and not on the haem. CPO is the only known peroxidase that is ligated by a cysteine residue rather than a histidine residue, on the proximal side of the haem iron. CPO is a more versatile enzyme, catalysing numerous types of reaction: peroxidase, catalase and halogenation reactions. The various CPO species are less stable than other peroxidase species and more elusive, thus needing further characterization. The roles of the amino acid residues on the proximal and distal sides of the haem need more investigation to further decipher their specific roles. Haem proteins, especially peroxidases, are structure-function-specific.