Catalase is a common enzyme found in nearly all living organisms exposed to oxygen (such as bacteria, plants, and animals). It catalyzes the decomposition of hydrogen peroxide to water and oxygen. It is a very important enzyme in protecting the cell from oxidative damage by reactive oxygen species (ROS). Likewise, catalase has one of the highest turnover numbers of all enzymes; one catalase molecule can convert millions of hydrogen peroxide molecules 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.
Structure
Human catalase forms a tetramer composed of four subunits, each of which can be conceptually divided into four domains. The extensive hydrophobic core of each subunit is generated by an eight-stranded antiparallel b-barrel (b1-8), with nearest neighbor connectivity capped by b-barrel loops on one side and a9 on the other. A helical domain at one face of the b-barrel is composed of four C-terminal helices (a16, a17, a18, and a19) and four helices derived from residues between b4 and b5 (a4, a5, a6, and a7).
History
Catalase was not noticed until 1818 when Louis Jacques Thénard, who discovered H2O2 (hydrogen peroxide), suggested its breakdown is caused by an unknown substance. In 1900, Oscar Loew was the first to give it the name catalase, and found it in many plants and animals. In 1937 catalase from beef liver was crystallised by James B. Sumner and Alexander Dounce and the molecular weight was found in 1938.
In 1969, the amino acid sequence of bovine catalase was discovered. Then in 1981, the three-dimensional structure of the protein was revealed.
Function
Action
The reaction of catalase in the decomposition of hydrogen peroxide in living tissue:
- 2 H2O2 â†' 2 H2O + O2
The presence of catalase in a microbial or tissue sample can be tested by adding a volume of hydrogen peroxide and observing the reaction. The formation of bubbles, oxygen, indicates a positive result. This easy assay, which can be seen with the naked eye, without the aid of instruments, is possible because catalase has a very high specific activity, which produces a detectable response. Alternative splicing may result in different protein variants.
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 â†' H2O + O=Fe(IV)-E(.+)
- H2O2 + O=Fe(IV)-E(.+) â†' 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 the iron is not completely oxidized to +V, but receives some stabilising electron density from the heme ligand. This heme has to be drawn then as a radical cation (.+).
As hydrogen peroxide enters the active site, it interacts with the amino acids Asn148 (asparagine at position 148) and His75, 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 Tyr358 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 His75 and Asn148 with reaction intermediates. In general, the rate of the reaction can be determined by the Michaelis-Menten equation.
Catalase can also catalyze the oxidation, by hydrogen peroxide, of various metabolites and toxins, including formaldehyde, formic acid, phenols, acetaldehyde and alcohols. It does so according to the following reaction:
- H2O2 + H2R â†' 2H2O + R
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. Furthermore, the poison cyanide is a competitive inhibitor of catalase at high concentrations of hydrogen peroxide. Arsenate acts as an activator. 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 byproduct of many normal metabolic processes; to prevent damage to cells and tissues, 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.
Mice genetically engineered to lack catalase are initially phenotypically normal., however, catalase deficiency in mice may increase the likelihood of developing obesity, fatty liver, and type 2 diabetes. Some humans have very low levels of catalase (acatalasia), yet show few ill effects.
Human catalase works at an optimum temperature of 37 °C. In contrast, catalase isolated from the hyperthermophile archaeon Pyrobaculum calidifontis has a temperature optimum of 90 °C.
Catalase is usually located in a cellular, bipolar environment 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. Catalase-positive pathogens, such as Mycobacterium tuberculosis, Legionella pneumophila, and Campylobacter jejuni, make catalase to deactivate the peroxide radicals, thus allowing them to survive unharmed within the host.
Catalase contributes to ethanol metabolism in the body after ingestion of alcohol, but it only breaks down a small fraction of the alcohol in the body.
Distribution among organisms
The large majority of known organisms use catalase in every organ, with particularly high concentrations occurring in the liver. One unique use of catalase occurs in the bombardier beetle. This 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 (Î"H = âˆ'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.
Almost all aerobic microorganisms use catalase. It is also present in some anaerobic microorganisms, such as Methanosarcina barkeri.
According to a March 2015 Scientific American Special Report on Aging article, laboratory mice at a University of Washington laboratory which produced more catalase, which is an antioxidant, lived longer. Research on the topic both supports and cautions against the benefit of antioxidants for health effects on aging.
Clinical significance and application
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 â€" 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.
Bacterial identification (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.
The catalase 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 the mixture produces bubbles or froth, 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, Cryptococcus, and Rhodococcus equi.
- If not, the organism is 'catalase-negative'. Streptococcus and Enterococcus spp. are catalase-negative.
While the catalase test alone cannot identify a particular organism, combined with other tests, such as antibiotic resistance, it can aid identification. The presence of catalase in bacterial cells depends on both the growth condition and the medium used to grow the cells.
Capillary tubes may also be used. A small amount of bacteria is collected on the end of the capillary tube (it is essential to ensure that the end is not blocked, otherwise it may present a false negative). The opposite end is then dipped into hydrogen peroxide which will draw up the liquid (through capillary action), and turned upside down, so the bacterial end is closest to the bench. A few taps of the arm should then move the hydrogen peroxide closer to the bacteria. When the hydrogen peroxide and bacteria are touching, bubbles may begin to rise, giving a positive catalase result.
Bacterial virulence
Neutrophils and other phagocytes use peroxide to combat bacteria. The exact mechanism by which this is done is currently an active area of inquiry, but the most common theory is that peroxide, over several steps, is converted into hypochlorous acid (e.g., bleach)and the bleach is dumped on phagocytosed pathogens, to kill them. In individuals with chronic granulomatous disease (CGD) there is a defect in producing peroxide via mutations in phagocyte oxidase (e.g. NADPH-oxidase). Normal cellular metabolism will still produce a small amount of peroxide â€" and some would argue that bacterial metabolism would also produce tiny amounts of peroxide, which would leak from the bacteria â€" and this peroxide can be used for the same purpose (producing hypochlorous acid to eradicate the bacterial infection). However, in individuals with CGD, who have a catalase-positive bacterial infection, the catalase will destroy the excess peroxide before it can be used to produce the bleach. In these individuals the pathogen survives and becomes a chronic infection. This chronic infection is typically surrounded by macrophages in an attempt to isolate the infection. This wall of macrophages surrounding a pathogen is called a granuloma. Many bacteria are catalase positive, but some are better catalase-producers than others. The mnemonic "cats have BeeN PLACESS" (catalase positive â€" Burkholderia, Nocardia, Pasteurella, Listeria, Aspergillus, Candida, E. coli, Staphylococcus, & Serratia) is used by US medical students to remember the catalase positive organisms that are known to cause granuloma formation in patients with CGD.
Gray hair
According to recent scientific studies, low levels of catalase may play a role in the graying process of human hair. Hydrogen peroxide is naturally produced by the body and catalase breaks it down. If catalase levels decline, hydrogen peroxide cannot be broken down as well. This allows the hydrogen peroxide to bleach the hair from the inside out. This finding is under investigation in hopes of developing cosmetic treatments for graying hair based on catalase supplements.
Interactions
Catalase has been shown to interact with the ABL2 and Abl genes. Infection with the murine leukemia virus causes catalase activity to decline in the lungs, heart and kidneys of mice. Conversely, dietary fish oil increased catalase activity in the heart, and kidneys of mice.
See also
- Enzyme kinetics
- Peroxidases
- Superoxide dismutase
References
External links
- "GenAge entry for CAT (Homo sapiens)". Human Ageing Genomic Resources. Retrieved 2009-03-05.Â
- "Catalase". MadSci FAQ. madsci.org. Retrieved 2009-03-05.Â
- "Catalase and oxidase test video". Regnvm Prokaryotae. Retrieved 2009-03-05.Â
- "EC 1.11.1.6 - catalase". Brenda: The Comprehensive Enzyme Information System. Retrieved 2009-03-05.Â
- "PeroxiBase - The peroxidase database". Swiss Institute of Bioinformatics. Archived from the original on 2008-10-13. Retrieved 2009-03-05.Â
- "Catalase Procedure". MicrobeID.com. Retrieved 2009-04-22.Â
- "Catalase Molecule of the Month". Protein Data Bank. Archived from the original on 2013-05-11. Retrieved 2013-01-08.Â
