Catalase

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Catalase
Identifiers
Symbol	CAT
HUGO	1516
Entrez	847
OMIM	115500
RefSeq	NM_001752
UniProt	P04040
Other data
EC number	1.11.1.6
Locus	Chr. 11 p13

Catalase (human erythrocyte catalase: PDB 1DGF, EC 1.11.1.6) is a common enzyme found in living organisms. Its functions include catalyzing the decomposition of hydrogen peroxide to water and oxygen.^[1] Catalase has one of the highest turnover rates for 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 which 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]

1 Action of catalase
2 Molecular mechanism
3 Cellular role
4 Distribution among organisms
5 Human applications
6 See also
7 References
8 Additional images
9 External links

[edit] Action of catalase

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

2 H₂O₂ → 2 H₂O + O₂^[6]

In microbiology, the catalase test is used to differentiate between bacterial species in the lab.[1] 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 form, the organism is said to be catalase-positive; if not, the organism is catalase-negative.[2] This test is particularly useful in distinguishing staphylococci and micrococci, which are catalase-positive, from streptococci and enterococci, which are catalase-negative.[3] 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.

[edit] 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^[7]

(where Fe()-E represents the iron centre of the heme group attached to the enzyme.)

As hydrogen peroxide enters the active site it interacts with the amino acids Asn147 and His74, causing a proton (hydrogen ion) to transfer between the oxygen atoms, polarizing and stretching the O-O bond, which breaks heterolytically. The free oxygen atom coordinates with the iron centre of the active site, freeing the newly formed water molecule and forming Fe(IV)=O. Next, the Fe(IV)=O reacts with a second hydrogen peroxide molecule to reform Fe(III)-E and produce water and oxygen.^[7] 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.^[7] Generally, the rate of the reaction can be determined by the Michaelis-Menten equation.[4]

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.

[edit] 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 manage this problem, the enzyme catalase is frequently used by cells to rapidly catalyse the decomposition of hydrogen peroxide into less reactive gaseous oxygen and liquid water molecules.^[8]

The true biological significance of catalase is not completely understood: mice genetically engineered to lack catalase are phenotypically normal, indicating that this enzyme is dispensable for animal life.^[9]

Catalase works at an optimum temperature of 37 degrees Celsius, which is approximately the temperature of the human body. Scientific research shows that increasing the temperature by 10 degrees Celsius will double the rate of reaction between the enzyme and the hydrogen peroxide.

Catalase is located in a cellular organelle called the peroxisome.^[10] 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 in the host cell.[5]

[edit] Distribution among organisms

All known animals use catalase in every organ, with particularly high concentrations occurring in the liver. The enzyme is also universal among plants. Very few aerobic microorganisms are known which do not use catalase.[6] One unique use of catalase occurs in bombardier beetle. These beetles allow a group of chemicals including hydrogen peroxide to gather in a chamber in its abdomen. When the beetle wishes to expel these chemicals for defensive purposes, it releases catalase and peroxidases into the chamber. These enzymes rapidly react with the hydrogen peroxide, generating a great deal of heat and a supply of oxygen. Under the pressure of the oxygen, the heated chemicals are explosively expelled through an opening in the chamber.[7]

Catalase is not universal among fungi, although some species have been found to produce the enzyme when growing in an environment with a low pH and warm temperatures.^[11] Catalase has also been observed in some anaerobic microorganisms, such as Methanosarcina barkeri.^[12]

[edit] Human applications

Catalase is used in the dairy industry for removing hydrogen peroxide from milk prior to cheese production.[8] Another use is in food wrappers, where it prevents food from oxidizing.[9] Catalase is also used in the textile industry, removing hydrogen peroxide from fabrics to make sure the material is peroxide-free.[10] 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.^[13] 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.

[edit] See also

[edit] References

^ Catalase: An Enzyme at Work. Science Education Outreach. Retrieved on 2007-02-11.
^ Catalase. Molecule of the Month. RCSB Protein Data Bank (2004-09-01). Retrieved on 2007-02-11.
^ Boon EM, Downs A, Marcey D. Catalase: H₂O₂: H₂O₂ Oxidoreductase. Catalase Structural Tutorial Text. Retrieved on 2007-02-11.
^ (1954) The Assay of Catalases and Peroxidases in Methods of Biochemical Analysis, 357. ISBN.
^ A Quantitative Enzyme Study; CATALASE. Retrieved on 2007-02-11.
^ Catalase: A Closer Look. Science Education Outreach. Retrieved on 2007-02-11.
^ ^a ^b ^c Boon EM, Downs A, Marcey D. Proposed Mechanism of Catalase in Catalase: H₂O₂: H₂O₂ Oxidoreductase. Catalase Structural Tutorial Text. Retrieved on 2007-02-11.
^ Gaetani G, Ferraris A, Rolfo M, Mangerini R, Arena S, Kirkman H (1996). "Predominant role of catalase in the disposal of hydrogen peroxide within human erythrocytes.". Blood 87 (4): 1595-9. PMID 8608252.
^ Ho YS, Xiong Y, Ma W, Spector A, Ho D (2004). "Mice Lacking Catalase Develop Normally but Show Differential Sensitivity to Oxidant Tissue Injury.". J Biol Chem 279 (31): 32804-812. PMID 15178682.
^ Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). Peroxisomes, in Molecular Biology of the Cell, 4th ed., Garland. (via NCBI Bookshelf) ISBN 0815332181.
^ K. Isobe, et al. (2006 Jan). "Production of catalase by fungi growing at low pH and high temperature.". J Biosci Bioeng 101 (1): 73-6. PMID 16503295.
^ Andrei Brioukhanov, Alexander Netrusov, and Rik Eggen. (2006). "The catalase and superoxide dismutase genes are transcriptionally up-regulated upon oxidative stress in the strictly anaerobic archaeon Methanosarcina barkeri.". Microbiology 152: 1671 - 1677. DOI:10.1099/mic.0.28542-0.
^ U.S. Patent 5521091