Catalase is an enzyme found in organisms exposed to oxygen. It breaks down hydrogen peroxide into water and oxygen, according the Encyclopedia Britannica. It is found in large amounts in the tissues of mammals, and it helps prevent damage to tissues from peroxide, which is a byproduct of many metabolic reactions.
According to the National Institutes of Health, oxidative stress from hydrogen peroxide is thought to be related to many chronic or late onset diseases such as diabetes, asthma, Alzheimer's and many cancers. According to the Protein Data Bank, catalase is one of the most efficient enzymes found in cells. A single catalase molecule can break down millions of hydrogen peroxide molecules. Human catalases use an iron ion to help in this very rapid reaction. Because catalase is involved in breaking down very reactive molecules, it is uncommonly stable for an enzyme. Organisms that rely on oxygen to power their cells must carefully control its presence in their systems because oxygen is easily converted into other reactive compounds that can cause significant damage. Chemical reactions involving electrons can convert oxygen into superoxide radicals, hydrogen peroxide or hydroxyl radicals, all of which are dangerous molecules that attack and mutate DNA and need to be broken down and neutralized.