{{enzyme | Name = Catalase | EC_number = 1.11.1.6 | CAS_number = 9001-05-2 | IUBMB_EC_number = 1/11/1/6 | GO_code = 0004096 | image = | width = | caption = }} Catalase is a common enzyme found in nearly all living organisms that are exposed to oxygen, where it catalyzes the decomposition of hydrogen peroxide to water and oxygen. Catalase has one of the highest turnover numbers of all enzymes; one catalase molecule can convert 40 million molecules of hydrogen peroxide to water and oxygen each second.

Catalase is a tetramer of four polypeptide chains, each over 500 amino acids long. It contains four porphyrin heme (iron) groups that allow the enzyme to react with the hydrogen peroxide. The optimum pH for human catalase is approximately 7, and has a fairly broad maximum (the rate of reaction does not change appreciably at pHs between 6.8 and 7.5). The pH optimum for other catalases varies between 4 and 11 depending on the species. The optimum temperature also varies by species.

History

Catalase was first noticed as a substance in 1818 when Louis Jacques Thénard, who discovered H₂O₂ (hydrogen peroxide), suggested that its breakdown is caused by a substance. In 1900, Oscar Loew was the first to give it the name catalase, and found its presence in many plants and animals. In 1937 catalase from beef liver was crystallised by James B. Sumner and the molecular weight was worked out in 1938.

In 1969, the amino acid sequence of bovine catalase was worked out. Then in 1981, the 3D structure of the protein was revealed.

Action

The reaction of catalase in the decomposition of hydrogen peroxide is:

: 2 H₂O₂ → 2 H₂O + O₂

This can be tested by taking a solution of the enzyme and dipping a piece of paper into it, then seeing how long it takes to rise in a tube of hydrogen peroxide

Molecular mechanism

While the complete mechanism of catalase is not currently known, the reaction is believed to occur in two stages:

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

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

: Here Fe()-E represents the iron center of the heme group attached to the enzyme. Fe(IV)-E(.+) is a mesomeric form of Fe(V)-E, meaning that iron is not completely oxidized to +V but receives some "supporting electron" from the heme ligand. This heme has to be drawn then as radical cation (.+).

As hydrogen peroxide enters the active site, it interacts with the amino acids Asn147 (asparagine at position 147) and His74, causing a proton (hydrogen ion) to transfer between the oxygen atoms. The free oxygen atom coordinates, freeing the newly-formed water molecule and Fe(IV)=O. Fe(IV)=O reacts with a second hydrogen peroxide molecule to reform Fe(III)-E and produce water and oxygen. The reactivity of the iron center may be improved by the presence of the phenolate ligand of Tyr357 in the fifth iron ligand, which can assist in the oxidation of the Fe(III) to Fe(IV). The efficiency of the reaction may also be improved by the interactions of His74 and Asn147 with reaction intermediates. In general, the rate of the reaction can be determined by the Michaelis-Menten equation.

Catalase can also oxidize different toxins, such as formaldehyde, formic acid, phenols, and alcohols. In doing so, it uses hydrogen peroxide according to the following reaction:

: H₂O₂ + H₂R → 2H₂O + R

Again, the exact mechanism of this reaction is not known.

Any heavy metal ion (such as copper cations in copper(II) sulfate) can act as a noncompetitive inhibitor of catalase. Also, the poison cyanide is a competitive inhibitor of catalase, strongly binding to the heme of catalase and stopping the enzyme's action.

Three-dimensional protein structures of the peroxidated catalase intermediates are available at the Protein Data Bank. This enzyme is commonly used in laboratories as a tool for learning the effect of enzymes upon reaction rates.

Cellular role

Hydrogen peroxide is a harmful by-product of many normal metabolic processes: to prevent damage, it must be quickly converted into other, less dangerous substances. To this end, catalase is frequently used by cells to rapidly catalyze the decomposition of hydrogen peroxide into less reactive gaseous oxygen and water molecules.

The true biological significance of catalase is not always straightforward to assess: Mice genetically engineered to lack catalase are phenotypically normal, indicating that this enzyme is dispensable in animals under some conditions. A Catalase deficiency may increase the likelihood of developing Type II Diabetes. Some human beings have very low levels of catalase (acatalasia), yet show few ill effects. It is likely that the predominant scavengers of H₂O₂ in normal mammalian cells are peroxiredoxins rather than catalase.

Human catalase works at an optimum temperature of 37°C, which is approximately the temperature of the human body. In contrast, catalase isolated from the hyperthermophile archaea ''Pyrobaculum calidifontis'' has a temperature optimum of 90°C.

Catalase is usually located in a cellular organelle called the peroxisome. Peroxisomes in plant cells are involved in photorespiration (the use of oxygen and production of carbon dioxide) and symbiotic nitrogen fixation (the breaking apart of diatomic nitrogen (N₂) to reactive nitrogen atoms). Hydrogen peroxide is used as a potent antimicrobial agent when cells are infected with a pathogen. Pathogens that are catalase-positive, such as ''Mycobacterium tuberculosis'', ''Legionella pneumophila'', and ''Campylobacter jejuni'', make catalase in order to deactivate the peroxide radicals, thus allowing them to survive unharmed within the host.

Distribution among organisms

All known animals use catalase in every organ, with particularly high concentrations occurring in the liver. One unique use of catalase occurs in bombardier beetle. The beetle has two sets of chemicals ordinarily stored separately in its paired glands. The larger of the pair, the storage chamber or reservoir, contains hydroquinones and hydrogen peroxide, whereas the smaller of the pair, the reaction chamber, contains catalases and peroxidases. To activate the noxious spray, the beetle mixes the contents of the two compartments, causing oxygen to be liberated from hydrogen peroxide. The oxygen oxidizes the hydroquinones and also acts as the propellant. The oxidation reaction is very exothermic (ΔH = −202.8 kJ/mol) which rapidly heats the mixture to the boiling point.

Catalase is also universal among plants, and many fungi are also high producers of the enzyme.

Very few aerobic microorganisms that do not use catalase are known. Catalase has also been observed in some anaerobic microorganisms, such as ''Methanosarcina barkeri''.

Applications

Catalase is used in the food industry for removing hydrogen peroxide from milk prior to cheese production. Another use is in food wrappers where it prevents food from oxidizing. Catalase is also used in the textile industry, removing hydrogen peroxide from fabrics to make sure the material is peroxide-free.

A minor use is in contact lens hygiene - a few lens-cleaning products disinfect the lens using a hydrogen peroxide solution; a solution containing catalase is then used to decompose the hydrogen peroxide before the lens is used again. Recently, catalase has also begun to be used in the aesthetics industry. Several mask treatments combine the enzyme with hydrogen peroxide on the face with the intent of increasing cellular oxygenation in the upper layers of the epidermis.

Catalase test

The catalase test is also one of the main three tests used by microbiologists to identify species of bacteria. The presence of catalase enzyme in the test isolate is detected using hydrogen peroxide. If the bacteria possess catalase (i.e., are catalase-positive), when a small amount of bacterial isolate is added to hydrogen peroxide, bubbles of oxygen are observed.

In microbiology, the ''catalase test'' is used to differentiate between bacterial species in the lab. The test is done by placing a drop of hydrogen peroxide on a microscope slide. Using an applicator stick, a scientist touches the colony and then smears a sample into the hydrogen peroxide drop.

If bubbles or froth forms, the organism is said to be ''catalase-positive''. Staphylococci and Micrococci are catalase-positive. Other catalase positive organisms include Listeria, Corynebacterium diphtheriae, Burkholderia cepacia, Nocardia, the Family Enterobacteriaceae (Citrobacter, E.Coli, Enterobacter, Klebsiella, Shigella, Yersinia, Proteus, Salmonella, Serratia, Pseudomonas), Mycobacterium tuberculosis, Aspergillus, and Cryptococcus.

If not, the organism is ''catalase-negative''. Streptococci and Enterococci are catalase-negative.

While the catalase test alone cannot identify a particular organism, combined with other tests such as antibiotic resistance, it can aid diagnosis. The presence of catalase in bacterial cells depends on both the growth condition and the medium used to grow the cells.

Grey hair

According to recent scientific studies, low levels of catalase may play a role in the greying process of human hair. Hydrogen peroxide is naturally produced by the body and catalase breaks it down. If there is a dip in catalase levels, hydrogen peroxide cannot be broken down. This causes the hydrogen peroxide to bleach the hair from the inside out. This finding may someday be incorporated into anti-greying treatments for aging hair.

Role in disease

The peroxisomal disorder acatalasia is due to a deficiency in the function of catalase. Genetic polymorphisms in catalase and its altered expression and activity are associated with oxidative DNA damage and subsequently the individual’s risk of cancer susceptibility.

Interactions

Catalase has been shown to interact with ABL2 and Abl gene.

See also

Enzyme kinetics

Peroxidases

Superoxide dismutase

References

External links

Category:EC 1.11.1 Category:Antioxidants Category:Hemoproteins Category:Enzymes Category:Catalysis

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