Catalase is an antioxidant enzyme commonly found in almost all living organisms, mainly in chloroplasts, mitochondria, endoplasmic reticulum of plants, liver and red blood cells of animals. It is the marker enzyme of the peroxisome, accounting for about 40% of the total peroxisomal enzymes. Catalase is a homotetramer composed of four polypeptide chains, each containing more than 500 amino acid residues and a porphyrin heme group at the active site of each subunit, which catalyzes the reaction of hydrogen peroxide. The optimal pH of catalase is close to 7, and the optimal temperature varies by species. SOD is a naturally occurring superoxide radical scavenger in the body that converts harmful superoxide radicals into hydrogen peroxide. Although hydrogen peroxide is still reactive oxygen that is harmful to the organism, catalase (CAT) and peroxidase (POD) in the body will immediately break it down into completely harmless water. In this way, the three enzymes form a complete chain of antioxidant protection. Catalase (CAT), as the name implies, is an enzyme that catalyzes the breakdown of hydrogen peroxide into oxygen and water. First of all, as a substance, catalase was first discovered in 1811 by Louis Jacques Thénard, the discoverer of hydrogen peroxide (H2O2). 1900, Oscar Loew named this enzyme capable of degrading hydrogen peroxide "catalase", that is, catalase, and found that this enzyme exists in many plants and animals. In 1937, James B. Sumner crystallized catalase from bovine liver and obtained the molecular weight of the enzyme the following year. 1969, the amino acid sequence of catalase from bovine liver was solved. And then, in 1981, its three-dimensional structure was resolved. Catalase is present in almost all living organisms. Its prevalent in organisms that can breathe, mainly in chloroplasts, mitochondria, endoplasmic reticulum in plants, liver and erythrocytes in animals, and its enzymatic activity provides the antioxidant defense mechanism to the organism.CAT is an erythroleukinase with different structures from different sources. The level of its activity varies in different tissues. Hydrogen peroxide is broken down faster in the liver than in organs such as the brain or heart because of the high level of CAT in the liver. CAT is present in the peroxisome of cells. Catalase is the marker enzyme of the peroxisome, accounting for approximately 40% of the total peroxisomal enzymes. Catalase is present in all known animal tissues, especially in high concentrations in the liver. Catalase is used in the food industry to remove hydrogen peroxide from milk used in the manufacture of cheese. Catalase is also used in food packaging to prevent food from being oxidized. In the textile industry, it is used to remove hydrogen peroxide from textiles to ensure that the finished product is peroxide-free. It is also used in contact lens cleaning: the glasses are soaked in a detergent containing hydrogen peroxide and then the residual hydrogen peroxide is removed with hydrogen peroxidase before use. Catalase is present in all known animal tissues, especially in high concentrations in the liver. In the bombardier beetle (Bombardier beetle), catalase has a unique use. This beetle has two sets of chemicals stored separately in the glands. The larger gland stores hydroquinone and hydrogen peroxide, while the smaller gland stores catalase and horseradish peroxidase. When the beetle mixes the chemicals in the two glands together, it releases oxygen, which both oxidizes hydroquinone and acts as a booster. Catalase is also commonly found in plants, but not in fungi, although some fungi have been found to be able to produce the enzyme at low pH and in warm environments. The vast majority of aerobic microorganisms contain catalase. Exceptions include Streptococcus, an aerobic bacterium that does not have peroxidase. Some anaerobic microorganisms, such as Methanosarcina barkeri, also contain peroxidase.

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