Catalase

Catalase is a common enzyme found in nearly all living organisms. Its functions include catalyzing the decomposition of hydrogen peroxide to water and oxygen.^[1] Catalase has one of the highest turnover rates of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.^[2]

Catalase is a tetramer of four polypeptide chains, each over 500 amino acids long.^[3] It contains four porphyrin heme (iron) groups that allow the enzyme to react with the hydrogen peroxide. The optimum pH for catalase is approximately neutral (pH 7.0),^[4] while the optimum temperature varies by species.^[5]

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History

Catalase was first noticed as a substance in 1811 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^[6]. In 1937 catalase from beef liver was crystallised by James B. Sumner ^[7] and the molecular weight worked out in 1938^[8]. In 1969 the amino acid sequence of bovine catalase was worked out^[9]. Then in 1981, the 3D structure of the protein was revealed^[10].

Action of catalase

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

2 H₂O₂ → 2 H₂O + O₂^[11]

In microbiology, the catalase test is used to differentiate between bacterial species in the lab.[3] 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; if not, the organism is catalase-negative.[4] This test is particularly useful in distinguishing staphylococci and micrococci, which are catalase-positive, from streptococci and enterococci, which are catalase-negative.[5] While the catalase test alone cannot identify a particular organism, combined with other tests, 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.

Molecular mechanism

While 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^[12]

Here Fe()-E represents the iron centre of the heme group attached to the enzyme. Fe(IV)-E(.+) ís 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 als 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.^[12] 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.^[12] In general, the rate of the reaction can be determined by the Michaelis-Menten equation.[6]

Catalase can also oxidize different toxins, such as formaldehyde, formic acid, 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) will act as a noncompetitive inhibitor on 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.^[13]

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.^[14]

Catalase works at an optimum temperature of 37 °C, which is approximately the temperature of the human body.

Catalase is usually located in a cellular organelle called the peroxisome.^[15] 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 .[7]

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 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. ^[16]

Catalase is also universal among plants, but not among fungi, although some species have been found to produce the enzyme when growing in an environment with a low pH and warm temperatures.^[17]

Very few aerobic microorganisms are known that do not use catalase. [8]. Streptococcus species are an example of aerobic bacteria that do not possess catalase. Catalase has also been observed in some anaerobic microorganisms, such as Methanosarcina barkeri.^[18]

Human applications

Catalase is used in the food industry for removing hydrogen peroxide from milk prior to cheese production.[9] Another use is in food wrappers, where it prevents food from oxidizing.[10] Catalase is also used in the textile industry, removing hydrogen peroxide from fabrics to make sure the material is peroxide-free.[11] 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.^[19] 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.

Pathology

The peroxisomal disorder acatalasia is due to a deficiency in the function of catalase.

See also

• Enzyme kinetics
• Peroxidases
• Superoxide dismutase

References

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