Catechol oxidase

Catechol oxidase is an enzyme found in many sources, including plant tissues and red blood cells. It is a copper oxidase. Catechol oxidase enzymes are members of the oxidoreductase class, which includes polyphenol oxidase and diphenol oxidase.

The enzyme oxidizes, or removes hydrogen from, the substrate catechol, which is a crystalline compound in plant tissue and is manufactured for use in food additives and other products. This oxidation of the catechol results in benzoquinone derivatives, or melanin, which are brown. The effect of oxidation is easily seen when a fruit or vegetable, such as an apple or potato, is cut and turns brown. Methods such as refrigeration and heating can inactivate the enzyme. The use of an acid, such as lemon juice, can inactivate the enzyme by changing the pH level. Oxidation is often desirable, however; tea, coffee, and cocoa get their brown coloration, and flavor, from the enzymatic process.

The enzymes are found in a wide variety of sources. For example, fresh green tea leaves are high in polyphenol oxidase. This enzyme changes the green leaves to black tea. The enzymes help plants to fend off disease. Plants with higher catechol oxidase activity have been found to have greater disease resistance.

Background

Catechol is also known as pyrocatechol or benzene-1, 2-diol. It was first extracted by H. Reinsch in 1839 when he distilled catechin, which is the juice of Mimosa catechu, an herb. Catechol is poisonous and should not be allowed to contact the skin.

Much research into the catechol oxidase enzyme is related to its use in fermenting tea leaves to manufacture black tea. The importance of the enzymatic process to tea fermentation was known during the late nineteenth century, but it was not until the 1930s that scientists at the Tea Research Institute were able to describe catechol oxidase and rule out other enzymes. By 1947, researchers in California had located the tea oxidase in leaf chloroplasts, the plastids that contain chlorophylls, in the cytoplasm, or jelly-like cellular material, of green plant cells. In 1998, researchers were able to determine the crystal structures of catechol oxidase extracted from sweet potatoes.

The enzyme is located in the cytoplasm of plant cells, including tea leaves. The flavonols, or catechins, are in the vacuoles of the cells. A membrane called the tonoplast separates the cytoplasm and vacuole and prevents the contents from mixing. Tea manufacturers roll the plant leaves, rupturing the tonoplast and allowing the flavonols and enzyme to mix. Oxygen causes oxidation of the mixture. The oxidation that makes black tea also occurs in the manufacturing process for coffee and cocoa.

Overview

An oxidation reaction is one in which an oxidizing agent takes electrons from something. Oxygen is the most common oxidizing agent, but catechol is the oxidizing agent that results in catechol oxidase. Most plants contain catechol. When a plant is damaged, catechol oxidase may be released into the plant structure. If catechol oxidase and catechol are mixed together, they form a compound called ortho-quinone, which acts as an antiseptic and protects the plant from attacks by bacteria and fungi. If oxygen is present in the plant cells due to the plant damage, it may interact with catechol oxidase to produce o-benzoquinone and melanin. The melanin—which is the same as that found in human skin that results in tanning after sun exposure—can form a sort of scab that protects the damaged area of the plant as it heals. This is the characteristic browning seen on apples, bananas, and other fruits and vegetables.

Enzymatic browning is a significant concern in the food industry. Fruit and vegetable spoilage caused by enzymatic browning may be responsible for up to half the losses in production. A number of methods may be used to inactivate the enzymes that result in browning. Substances can be applied to change the pH level. For example, citric or other acids will lower the pH level to 4.0 and inhibit enzyme activity. Other agents, such as enzyme inhibitors, may also be added to food. Irradiation, or cold pasteurization, uses ionized radiation to kill bacteria in many types of foods. It also slows enzyme activity. High pressure processing (HPP) increases atmospheric pressure to stop microbial and enzyme activity. Enzyme activity in some foods such as shrimp and lobster may be halted using supercritical carbon dioxide treatment. This is high-pressure fluid carbon dioxide that is applied to destroy microorganisms. It decreases pH levels and inactivates enzymes.

Dehydration removes water molecules from foods. This inhibits enzyme activity, which requires water. Drying of green tea leaves, for example, reduces oxidation. This method is used to prevent green tea from becoming black tea. Freeze-drying, which involves quickly freezing and vacuum sealing items to remove ice, also inhibits enzyme activity. Freeze-drying is commonly used to preserve many foods.

Refrigeration and chilling are commonly used to slow enzyme activity on produce, especially when transporting fresh fruits and vegetables from fields to stores. Freezing also slows but does not inactivate enzyme activity. When the food is thawed, the enzyme activity will begin again. Blanching is a food preparation method often used on produce before it is frozen. The cut up vegetables are briefly submerged in steam or boiling hot water, then quickly cooled, typically in a cold-water bath. The surface enzymes are inactivated, and the food will retain its color and texture.

In a number of instances, enzymatic activity is desired and encouraged. In addition to fermentation of green tea leaves to produce black tea, processers use enzymes to produce coffee and cocoa. The enzymes may also be useful in protecting plants from predation. Studies show that tomatoes, for example, use them to fight off disease. This opens the possibility of using the enzymes to protect plants.

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