In our body the enzyme catalase catalyses the reaction 2H₂O₂ = 2H₂O + O₂, the decomposition of hydrogen peroxide into water and oxygen. So, how does catalase work? The protein has a certain 3D structure when it is active, which contains a channel into which the hydrogen peroxide can diffuse. In the channel is a heme group which is a iron molecule bound to the center of a ring-like structure called a porphyrin ring.

Heme groups are common in biology and are frequently involved with electron transfer reactions (as in cytochrome c) or with ligand binding (as in hemoglobin). The heme group in catalase is very important to the reaction, because it can be oxidized from Fe(III) to the very oxidized and less common Fe(IV) form. This is the first part of the reaction:

H₂O₂ + Fe(III)- Enzyme -- H₂O +O = Fe(IV)-Enzyme (1)

Hydrogen peroxide has bound to the heme group and oxidized it to Fe(IV).

Now for the second part of the reaction: The enzyme has to go back to the Fe (III) form and reduce the second molecule of hydrogen peroxide to water.

H₂O₂ + O= Fe(IV)-Enzyme -- H₂O + Fe(III)- Enzyme (2)

The highly-oxidizing Fe(IV) form now reacts with the second peroxide molecule, releasing water and an oxygen molecule.