Catalases perform their rapid destruction of hydrogen peroxide in two steps. Firstly, they bind onto a hydrogen peroxide molecule to break it apart into water and oxygen, with the latter being put together with an iron atom. A second hydrogen peroxide molecule then binds onto the catalase, where it is also broken apart into pieces which combine with the iron-bound oxygen atom to release more water and oxygen gas. The oxygen atom is again bound to the iron, ready for the next hydrogen peroxide molecule to attach. The enzyme catalase exhibits two types of enzymatic activities, each involving H₂O₂.One of these functions is the disproportionation of H₂O₂. This reaction predominates with increasingly higher concentrations of H₂O₂:

1. Decomposition of H₂O₂.

Catalase disproportionates:

2H₂O₂ to 2H₂O+O₂

H₂O₂ is a destructive waste product that is produced in the human body when peroximes partial oxidise fatty acids. Other enzymes, such as plasma amine oxidase and xantine oxidase, also produce hydrogen peroxide as part of their normal catalytic cycle. As H₂O₂ is a powerful oxidizing agent which would be toxic if allowed to accumulate. By preventing excessive H₂O₂ build up, catalases allow important cellular processes which produce H₂O₂ as a by product to take place safely.

2. The Peroxidative Reaction

In the second type of function – the peroxidative reaction, the enzyme catalase takes the toxins and the potentially harmful H₂O₂ and recombine them to produce water and harmless or useful products such as phenols, formic acid, formaldehyde and alcohols.

H₂O₂ + RH₂ to 2H₂O + R

In both reactions catalyzed by catalases, hydrogen peroxide breakdown and the peroxidative reaction consume lethal hydrogen peroxide Catalase activity in the cell is therefore important for some of the following processes:

1. Peroxisomes partially oxidise fatty acids producing H₂O₂ as a by product.
2. Other Enzymes that produce H₂O₂ are enzymes that undergo oxidation and reduction make H₂O₂ and other reactive oxygen species of autooxidation, a kind of side reaction that is not part of their catalytic cycle. This happens mostly in the mitochondria. Many of these antioxidation reactions do not produce H₂O₂ directly, but rather superoxide. In order to get rid of superoxide which is more toxic than hydrogen peroxide, the body also contains lots of superoxide dismutase, that converts superoxide into water and hydrogen peroxide, which will then be decomposed by catalases.
3. One other source of superoxide comes from the white blood cells. In their action of fighting of harmful organisms, the white blood cells produce very large amount of superoxide, hydrogen peroxide and even hydrochloric acid.

There are two main classes of catalases, the HPI and HPII catalases. Some catalases are better at the peroxidative reaction than others. HPII catalases catalyse just the decomposition of H₂O₂ whereas HPI catalases have both of the described activities. HPI catalases exist as two isozymes, HPI-A and HPI-B and these sediment at slightly different densities.

Top | Back