CAT

Catalase, an enzyme that catalyzes the breakdown of hydrogen peroxide into oxygen and water, is present in the peroxides of cells. Catalase is a marker enzyme of peroxidase, accounting for about 40% of the total peroxidase. Catalases are present in all tissues of all known animals, especially in the liver with high concentrations. Catalase is used in the food industry to remove hydrogen peroxide from milk used to make cheese. Catalase is also used in food packaging to prevent food from being oxidized.

Sources

Catalases are present in almost all living organisms. It is prevalent in respirable organisms, mainly in chloroplasts, mitochondria, endoplasmic reticulum, animal liver and red blood cells, and its enzymatic activity provides an antioxidant defense mechanism for organisms. Catalases from different sources have different structures. Enzyme activity levels are different in different tissues. Hydrogen peroxide breaks down faster in the liver than in the brain or heart, because of the high level of catalase in the liver.

Catalytic Mechanism

Although the complete catalytic mechanism of catalase has not been fully understood, the catalytic process is considered to be divided into two steps:

H₂O₂ + Fe(III)-E → H₂O + O=Fe(IV)-E(.+)
H₂O₂ + O=Fe(IV)-E(.+) → H₂O + Fe(III)-E + O₂

Here, "Fe()-E" represents the central iron atom (Fe) of the heme group (E) bound to the enzyme. Fe(IV)-E(.+) is a resonance form of Fe(V)-E that the iron atom is not fully oxidized to the +V valence but rather accepts some "supporting electron" from heme. Thus, the heme in the reaction is expressed as free radical cation (.+).

Hydrogen peroxide enters the active site and interacts with the asparagine residue (Asn 147) and the histidine residue (His 74) of the catalase to allow one proton to pass between oxygen atoms. Free oxygen atoms coordinate to form water molecules and Fe(IV)=O. Fe(IV)=O reacts with the second hydrogen peroxide molecule to form Fe(III)-E and generates water molecules and oxygen. Reactivity of the active center iron atom may be increased due to the presence of phenolic side chains (which help to oxidize Fe(III) to Fe(IV)) at tyrosine residue 357. The efficiency of the reaction may be enhanced by the interaction of His74 and Asn147 with the reaction intermediates.    

Catalase also oxidizes other cytotoxic substances such as formaldehyde, formic acid, phenol and ethanol. These oxidation processes require the use of hydrogen peroxide by the following reaction:

H₂O₂ + H₂R → 2H₂O + R

Any heavy metal ion (such as copper ions in copper sulfate) can act as a non-competitive inhibitor of catalase. In addition, the highly toxic cyanide is a competitive inhibitor of catalase that binds tightly to the heme and blocks the catalytic reaction of catalases.

Functions

Hydrogen peroxide is a waste product of the metabolic process that can cause damage to the organisms. In order to avoid this damage, hydrogen peroxide must be quickly converted to other non-toxic or less toxic substances. Catalase is a tool often used by cells to catalyze the breakdown of hydrogen peroxide. But the true biological importance of catalase is not that simple. It is found that mice that had been genetically engineered for catalase deficiency remained normal phenotypes, suggesting that catalase is essential to animals only under certain conditions. Catalase levels are very low in some populations but do not show significant pathological responses. This is most likely because the major hydrogen peroxide scavenger in normal mammalian cells is peroxiredoxin, not catalase.

Catalases are usually located in an organelle known as peroxisomes. Peroxisomes in plant cells are involved in photorespiration (using oxygen and generating carbon dioxide) and co-immobilization of nitrogen (dissociation of nitrogen (N2) into reactive nitrogen atoms). However, hydrogen peroxide can be used as an effective antimicrobial agent when cells are infected with pathogens. Some pathogens, such as Mycobacterium tuberculosis, Legionella pneumophila and Campylobacter jejuni, are able to produce catalase to degrade hydrogen peroxide so they can survive in the host.

Applications

Catalase is used in the food industry to remove hydrogen peroxide from milk used to make cheese. Catalase is also used in food packaging to prevent food from being oxidized. It is also used for the cleaning of contact lenses. After the glasses are soaked in a detergent containing hydrogen peroxide, the residual hydrogen peroxide is removed with catalase before use. In recent years, catalase began to be used in the beauty industry. Some facials have added the catalase and hydrogen peroxide to increase the amount of cellular oxygen in the upper epidermis. Catalases are also often used in laboratories as tools to understand the effect of enzymes on reaction rates. In the dyeing and finishing industry, catalase has a wide range of applications. It is widely used in textile as additives, printing and dyeing papermaking and other industries, for the removal of residual hydrogen peroxide. Catalase can effectively improve the dyeing quality, shorten the process time and reduce the emission of wastes.