Catalase

Enzymes are proteins that act as biological catalysts. Catalysts accelerate some of the chemical reactions. The molecules through which enzymes may act are called substrates, and the enzyme that converts the substrates into different molecules is called products. There are six kinds of enzymes such as hydrolases, oxidoreductases, lyases, transferases, ligases, and isomerases. Where catalase belongs to the class oxidoreductase. Catalase meaning is, it is the crystalline enzyme that consists of the protein complex. Catalase was first noticed in the year 1818 by Louis Jacques Thénard. When he discovered hydrogen peroxide, suggested that its breakdown is caused by an unknown substance. In the year 1900, Oscar Loew was the first person to give it the name catalase where he found it in many plants and animals. In the year 1937, the beef liver was used to catalase through the process of crystallization by James B. Sumner and Alexander Dounce and its molecular weight was found in the year 1938. The amino acid sequence of bovine catalase was determined in the year 1969, and the three-dimensional structure in 1981.

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What is Catalase Enzyme?

Catalase is a common enzyme that is found in nearly all living organisms that are exposed to oxygen such as bacteria, plants, and animals. The catalase location in mammals is found in the liver. This catalyzes the decomposition of the substance hydrogen peroxide to form water and oxygen. It is a very important enzyme in protecting the cell from oxidative damage by reactive oxygen species. Catalase has one of the highest turnover numbers of all enzymes whereas one catalase molecule can convert millions of hydrogen peroxide molecules each second to water and oxygen.

Catalase is a tetramer molecule consisting of four polypeptide chains, each of these chains are over 500 amino acids long. It contains four iron-containing heme groups that allow the enzyme to react with hydrogen peroxide. The optimum pH for human catalase is approximately seven is neutral and the rate of reaction does not change in between the pH range of 6.8 and 7.5. The optimum pH and optimum temperature for other catalases vary between four and eleven depending on the species.

Human catalase forms a tetramer that is composed of four subunits, each of which can be conceptually divided into four domains. The extensive core of each subunit is consists of an eight-stranded antiparallel b-barrel, where the nearest neighbor connectivity is by b-barrel loops on one side and a₉ loops on the other side. A helical domain at one face of the b-barrel is composed of four helices of the C-terminal and four helices that are derived from residues between b₄ and b₅. Different protein variants are caused by the alternative splicing process.

Catalase Function

The presence of catalase in a microbial or tissue sample can be demonstrated by adding hydrogen peroxide and observing the reaction. The production of oxygen can be seen by the formation of bubbles. This easy test, which can be viewed with the naked eye, without the aid of instruments. It is possible only because catalase has a very high specific activity that helps to produce a detectable response, as well as the fact that one of the products is a gas.

Cellular Role: Hydrogen peroxide is a harmful byproduct that is formed from many of the normal metabolic processes. It is done in order to prevent damage that occurs to cells and tissues, these byproducts must be quickly converted into other forms or to less dangerous substances. By the end, catalase is frequently used by cells to catalyze rapidly the decomposition of hydrogen peroxide into less-reactive gaseous oxygen and water molecules. Peroxisomes in plant cells are involved in the process of photorespiration in which oxygen is used in the production of carbon dioxide and symbiotic nitrogen fixation. Hydrogen peroxide is used as a potent antimicrobial agent when the cells in the organisms are infected with a pathogen. Catalase-positive pathogens, such as Mycobacterium tuberculosis, Campylobacter jejuni, and Legionella pneumophila. These pathogens can make catalase deactivate the peroxide radicals, which allows them to survive unharmed within the host.

Molecular Mechanism: 

H₂O₂ + Fe (111) - EH₂O + O = Fe (1V) - E(.+)

H₂O₂ + O = Fe (1V) - E(.+) → H₂O+Fe (111)-E+O₂

Here, Fe()-E represents the center of iron of the heme group that is attached to the enzyme. Fe(IV)-E(.+) is a mesomeric form of Fe(V)-E, which means that the iron is not completely oxidized to +V. But it receives some of the stabilizing electron density from the heme ligand, which is then shown as a radical cation (.+).

As hydrogen peroxide enters the active site, it interacts with the amino acids Asn148 and His75, causing a proton to transfer between the oxygen atoms. The free oxygen atom coordinates, freeing the newly formed water molecule and Fe(IV)=O. Fe(IV)=O. It 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 coordination position. This can assist in the oxidation of Fe(III) to Fe(IV). The efficiency of the reaction may also be improved by the interactions of His75 and Asn148 with reaction intermediates. The decomposition of hydrogen peroxide by catalase proceeds according to first-order kinetics, the rate is proportional to the hydrogen peroxide concentration.

Catalase Uses

Catalase enzyme is used mostly in the food industry for removing the hydrogen peroxide content from the milk prior to cheese production. It is widely used in the production of food wrappers which helps to prevent the food from oxidizing. Catalase is also used in the textile industry, in order to remove the hydrogen peroxide content from the fabrics to make sure the material is peroxide-free.

A minor use is in the hygiene of contact lens, few of the lens-cleaning products can disinfect the lens using a hydrogen peroxide solution. A solution that contains catalase is then used to decompose the hydrogen peroxide from the solution before the lens is used again.

Catalase test: The catalase test is one of the three main tests that are used by microbiologists to identify the presence of species of bacteria. If the bacteria possess the enzyme catalase, then bubbles of oxygen are observed when a small amount of bacterial isolate is added to the hydrogen peroxide. This catalase test is done by adding a drop of hydrogen peroxide to a microscope slide. An applicator stick is touched to the colony, and the tip is then smeared onto the hydrogen peroxide drop.

Bacterial virulence: Neutrophils and some other phagocytes use hydrogen peroxide to kill the bacteria. The enzyme called NADPH oxidase generates the superoxide within its phagosome, this is converted through the hydrogen peroxide to other oxidizing substances such as hypochlorous acid. This acid kills the phagocytosed pathogens that are present. In individuals with a disorder called chronic granulomatous disease (CGD), has a defect in producing hydrogen peroxide with the help of mutations in phagocyte oxidases such as myeloperoxidase. Normal cellular metabolism can still produce a small amount of hydrogen peroxide and this peroxide can be used in order to produce the hypochlorous acid to eradicate the infection that is caused due to bacteria. However, if the individuals affected with the CGD are infected with catalase-positive bacteria, this bacterial catalase can destroy the excess peroxide that is present before it can be used to produce the other oxidizing substances. In these individuals, the pathogen survives and converts as a chronic infection. This chronic infection is surrounded typically by macrophages as an attempt to isolate the infections. This wall of macrophages that are surrounding a pathogen is called a granuloma.

Acatalasia: Acatalasia is a condition that is caused by the homozygous mutations in CAT, that results in a lack of catalase. Symptoms that are mild and include oral ulcers. A heterozygous CAT mutation is found lesser but still can have the presence of catalase.

Gray hair: Low levels of catalase may play a vital role in the graying process of human hair. Hydrogen peroxide is naturally produced within the body and can be broken down by catalase. If these catalase levels decline, then the hydrogen peroxide cannot be broken down into smaller molecules. This unbroken hydrogen peroxide interferes with the production of melanin, which is the pigment that is involved in giving the hair its color.

Catalase Information

The large majority of known organisms use catalase in every organ, with particularly high concentrations that are occurring in the liver of mammals. Catalase production in the body is majorly found in the liver. Catalase is found primarily in the cell organelle called peroxisomes and the cytosol of erythrocytes. Almost all aerobic microorganisms make use of catalase. It is also found to be present in some of the anaerobic microorganisms, such as Methanosarcina barkeri. Catalase is also universal among plants and occurs in most fungi.

One of the unique uses of catalase is that it occurs in the bombardier beetle. This beetle has two sets of liquids that are separately stored in the two paired glands. The larger the pair of glands, the storage chamber or reservoir that contains the hydroquinones and hydrogen peroxide. The smaller will be the reaction chamber that contains catalases and peroxidases. To activate the noxious spray, the beetle mixes the contents of these two compartments that causes the 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 = -202.8KJ mol ^-1) and this rapidly heats the mixture to the boiling point.

Long-lived queens of the termite Reticulitermes speratus species have significantly lower oxidative damage to their DNA than that of the non-reproductive individuals. Queens have more than two times higher catalase activity and seven times higher expression levels of the catalase gene such as RsCAT1 than that of the workers. It appears that the efficient antioxidant capability of these termite queens can partly explain how they attain long life.

Conclusion

Catalase acts as the catalyzing enzyme in the decomposition of hydrogen peroxide. This enzyme, like many others, aids in the decomposition of one substance into another. Catalase reaction involves the decomposition or break down of the hydrogen peroxide into water and oxygen.