Catalases (EC: 1.11.1.6) are antioxidant enzymes that catalyse the conversion of hydrogen peroxide to water and molecular oxygen. Hydrogen peroxide is produced as a consequence of oxidative cellular metabolism and can be converted to the highly reactive hydroxyl radical via transition metals, this radical being able to damage a wide variety of molecules within a cell, leading to oxidative stress and cell death. Catalases act to neutralise hydrogen peroxide toxicity, and are produced by all aerobic organisms ranging from bacteria to man. Most catalases are mono-functional, haem-containing enzymes, although there are also bifunctional haem-containing peroxidase/catalases (INTERPRO: IPR000763) that are closely related to plant peroxidases, and non-haem, manganese-containing catalases (INTERPRO: IPR007760) that are found in bacteria [PMID:14745498].

This entry represents the mono-functional, haem-containing catalases. Within a given species there can be different catalase isoforms, which sometimes have different subcellular locations. Most catalases exist as tetramers of 60-75 kD, where each subunit contains an active site haem group buried deep within the structure, but which is accessible from the surface through hydrophobic channels [PMID:11134921]. Some catalases contain NADPH as a cofactor, which functions to prevent the formation of inactive compound. Catalases are uniquely stable enzymes that are more resistant to pH, thermal denaturation and proteolysis than most enzymes, due to their very rigid, stable structure that is resistant to unfolding.

Catalases are mainly regulated by the oxidant status of a cell, but can also be regulated by the action of certain hormones, such as the neurohormone melatonin. Melatonin can increase the activity of various antioxidant enzymes and can also stimulate the expression of genes encoding these enzymes [PMID:11060493].