Catalase is a common enzyme found in nearly all living organisms which are exposed to oxygen, where it functions to catalyze (to accelerate a chemical reaction without the catalyst itself being affected) the decomposition of hydrogen peroxide to water and oxygen.
Catalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a chemical substance known as a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself.
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Catalase has one of the highest turnover numbers of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.
Catalase is a tetramer (a protein with four subunits) 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 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.
The reaction of catalase in the decomposition of hydrogen peroxide is:
2 H2O2 → 2 H2O + O2
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. 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 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
Catalase is also universal among plants, and many fungi are also high producers of the enzyme.
Very few aerobic microorganisms are known that do not use catalase. 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.
Applications: Hydrogen peroxideCatalase 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.
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. Scientists believe this finding may someday lead to "anti" greying treatments for aging hair.
Enzyme kinetics is the study of the chemical reactions that are catalysed by enzymes. In enzyme kinetics the reaction rate is measured and the effects of varying the conditions of the reaction investigated. Studying an enzyme's kinetics in this way can reveal the catalytic mechanism of this enzyme, its role in metabolism, how its activity is controlled, and how a drug or a poison might inhibit the enzyme.
Catalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a chemical substance known as a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. The catalyst may participate in multiple chemical transformations. Catalysts that speed the reaction are called positive catalysts. Catalysts that slow down the reaction are called negative catalysts or inhibitors. Substances that increase the activity of catalysts are called promoters and substances that deactivate catalysts are called catalytic poisons.
The general feature of catalysis is that the catalytic reaction has a lower rate-limiting free energy change to the transition state than the corresponding uncatalyzed reaction, resulting in a larger reaction rate at the same temperature. However, the mechanistic origin of catalysis is complex. Catalysts may affect the reaction environment favorably, e.g. acid catalysts for reactions of carbonyl compounds form specific intermediates that are not produced naturally, such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause lysis of reagents to reactive forms, such as atomic hydrogen in catalytic hydrogenation.
Kinetically, catalytic reactions behave like typical chemical reactions, i.e. the reaction rate depends on the frequency of contact of the reactants in the rate-determining step. Usually, the catalyst participates in this slow step, and rates are limited by amount of catalyst. In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts.
Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid-liquid system or evaporate in a solid-gas system.
Enzyme catalysis is the catalysis of chemical reactions by specialized proteins known as enzymes. Catalysis of biochemical reactions in the cell is vital due to the very low reaction rates of the uncatalysed reactions.
A single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the catalyst for that reaction.
A phase transfer catalyst or PTC in chemistry is a catalyst which facilitates the migration of a reactant in a heterogeneous system from one phase into another phase where reaction can take place. Ionic reactants are often soluble in an aqueous phase but insoluble in an organic phase unless the phase transfer catalyst is present. Phase transfer catalysis (also PTC) refers to the acceleration of the reaction by the phase transfer catalyst.
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