s that typically catalyze a reaction of the form: » ROOR' + electron donor (2 e-) + 2H+ → ROH + R'OH For many of these enzymes the optimal substrate is hydrogen peroxide, but others are more active with organic hydroperoxides such as lipid peroxides. Peroxidases can contain a heme cofactor in their active sites, or redox-active cysteine or selenocysteine residues. The nature of the electron donor is very dependent on the structure of the enzyme. For example, horseradish peroxidase can use… (More on Peroxidase)