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Enzyme

Enzyme

An enzyme is a biological catalyst. A catalyst is a chemical compound that speeds up the rate of some chemical reaction. When that chemical reaction occurs in a living organism, the catalyst is known as an enzyme.

Catalyzed and uncatalyzed reactions

Figure 1 shows how an enzyme (or any other catalyst) affects the rate of a chemical reaction. Consider the reaction in which a complex carbohydrate, such as starch, is broken down in the body to produce the simpler sugars known as sucrose. We can express this reaction by the following chemical equation:

starch sucrose

The compound present at the beginning of the reaction (starch) is known as the reactant. The compound that is formed as a result of the reaction (sucrose) is known as the product.

In most instances, energy has to be supplied to the reactant or reactants in order for a reaction to occur. For example, if you heat a suspension of starch in water, the starch begins to break down to form sucrose.

The line labeled "Uncatalyzed reaction" in Figure 1 represents changes in energy that take place in the reaction without a catalyst. Notice that the amount of energy needed to make the reaction happen increases from its beginning point to a maximum point, and then drops to a minimum point. The graph shows that an amount of energy equal to the value Ea has to be added to make the reaction happen.

The second line in Figure 1 shows energy changes that take place with a catalyst. Energy still has to be added to the reactant to get the reaction started, but the amount of energy is much less. In Figure 1, that amount of energy is indicated by the symbol Eb. Notice that Eb is much less than Ea. The difference in those two values is the savings in energy provided by using a catalyst in the reaction.

Enzymes in biological reactions

Living organisms could not survive without enzymes. During each second in the life of a plant or animal, thousands of different chemical reactions are taking place. Every one of those reactions requires the input of energy, as shown in Figure 1. Every one of those reactions could be made to occur by adding heat, electricity, or some other form of energybut not within a living organism. Imagine what would happen if the only way we had of digesting starch was to heat it to boiling inside our stomach!

Every one of those thousands of chemical reactions taking place inside plants and animals, then, is made possible by some specific enzyme. The presence of the enzyme means that the reaction can occur at some reasonable temperature, such as the temperature of a human body or the cells of a plant.

Words to Know

Amino acid: An organic compound that contains two special groups of atoms known as the amino group and the carboxylic acid group.

Catalyst: Any chemical compound that speeds up the rate of a chemical reaction.

Chemical reaction: Any change in which at least one new substance is formed.

Lock-and-key model: One of the ways in which enzymes bring about chemical reactions.

Product: A compound that is formed as the result of a chemical reaction.

Protein: A complex chemical compound that consists of many amino acids attached to each other which are essential to the structure and functioning of all living cells.

Reactant: A compound present at the beginning of a chemical reaction.

Substrate: The substance on which an enzyme operates in a chemical reaction.

Structure of enzymes

All enzymes are proteins. Proteins are complex organic compounds that consist of simpler compounds attached to each other. The simpler compounds of which proteins are made are amino acids. An amino acid gets its name from the fact that it contains two special groups of atoms, an amino (NH2) group and a carboxylic acid (COOH) group.

Amino acids are of particular importance because they can react with each other to form long chains. If you mix two amino acids with each other under the proper circumstances, the amino group on one amino acid will react with the carboxylic acid group on the second amino acid. If you add a third amino acid to the mixture, its amino or carboxylic acid group will combine with the product formed from the first two amino acids, and so on.

A protein, then, is a very long chain of amino acids strung together somewhat like a long piece of woolen thread.

Except that proteins are really more complex than that. The long protein does not remain in a neat threadlike shape for long. As soon as it is formed, it begins to twist and turn on itself until it looks more like a tangled mass of wool. It looks something like a skein of woolen thread would look if the family cat had a chance to play with it.

A protein molecule, then, has a complicated three-dimensional shape, with nooks and crannies and projections all over its surfaces. You could make your own model of a protein molecule by taking a Slinky toy and turning and twisting the coil into an irregular sphere.

Enzyme function

Enzymes can act as catalysts because of their three-dimensional shapes. Figure 2 shows one way that enzymes act as catalysts. The lower half of the drawing in Figure 2 represents the three-dimensional structure of an enzyme molecule. Notice the two gapsone with a rectangular shape and one with a triangular shapein the upper face of the molecule.

A molecule with this shape has the ability to combine with other molecules that have a complementary shape. In Figure 2, a second molecule of this kind, labeled "Substrate," is shown. The term substrate is used for molecules that can be broken apart by catalysts.

Notice that the shape of the substrate molecule in Figure 2 perfectly matches the shape of the enzyme molecule. The two molecules can fit together exactly, like a key fitting into a lock.

Here is how we think many kinds of enzyme-catalyzed reactions take place: a substrate molecule, such as starch, is ready to be broken apart in a living body. The energy needed to break apart the substrate is quite large, larger than is available in the body. The substrate remains in its complete form.

An enzyme with the correct molecular shape arrives on the scene and attaches itself to the substrate molecule, as in Figure 2. Chemical bonds form between the substrate and enzyme molecules. These bonds cause bonds within the substrate molecule to become weaker. The bonds may actually break, causing the substrate molecule to fall apart into two parts.

After a brief period of time, the bonds between the substrate and the enzyme molecules break. The two pieces move away from each other. By this time, however, the substrate molecule itself has also broken apart. The enzyme has made possible the breakdown of the substrate without the addition of a lot of energy. The products of the reaction are now free to move elsewhere in the organism, while the enzyme is ready to find another substrate molecule of the same kind and repeat the process.

[See also Catalyst; Reaction, chemical ]

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Enzyme

Enzyme

Enzymes have been called the "agents of life" because all life processes are dependent on them. Enzymes are protein molecules that act as catalysts (they speed up chemical reactions without undergoing any change themselves). They can build up or break down other molecules and are responsible for regulating the many chemical reactions that occur in plants and animals. If enzymes were absent from the human body, most of its metabolic reactions would occur at a rate, too slow to support life.

Enzymes accelerate reactions by at least a million times. Molecules in the cells of solid tissues and in circulating blood are constantly being split apart and welded together again by enzymatic action. It has been estimated that a single cell, roughly one-billionth the size of a drop of water, contains about 3,000 different enzymes.

Regulatory Functions

In addition to speeding up reactions, enzymes also have regulatory functions. It is essential that chemical reactions inside cells are controlled so that they do not make too little or too much of a particular product. Many of the processes, or pathways, in cells must be coordinated, and this is a function enzymes regulate. Enzymes are thus central not only to individual reactions within a cell but also to the life of the cell as a whole.

Enzymes are critical to the proper functioning of everything from breathing to thinking to blood circulation to digestion. They can be broken down into two major groups, metabolic enzymes and digestive enzymes. Metabolic enzymes are produced by the body to regulate functions in the blood, tissues, and organs. Digestive enzymes are produced to break down food and absorb nutrients.

Enzymes and Digestion

Prior to the eighteenth century, the process of digestion was believed to be solely a mechanical process, similar to a meat grinder. In 1752, however, French scientist Rene-Antoine Reaumur fed his pet falcon pieces of meat enclosed in a metal tube with holes in it. He wanted to protect the meat from the mechanical effects of the bird's stomach friction. When he removed the tube a few hours later, the meat had been digested, but the tube was still intact. It was evident that the digestion had resulted from chemical, not mechanical, action. In the 1780s Italian biologist Lazzaro Spallanzani also proved that meat could be digested by gastric juices extracted from falcons. His was probably the first experiment in which a vital reaction occurred outside the living organism.

John R. Young, who graduated from the University of Pennsylvania in 1803, added to the increasing knowledge about digestion. In his graduation essay, he described his experiments on frogs and snakes and on himself. He was the first researcher to reveal that gastric juice contains a strong acid. Young believed that the strong acidity of gastric juice was responsible for its digestive action. In 1835, however, German physiologist Theodor Schwann discovered that gastric juice also contained a non-acid digestive substance. He called the substance pepsin (from the Greek for "to digest"), which was later shown to be an enzyme.

Fermentation

The oldest known enzyme reaction is alcoholic fermentation, which was thought to be a spontaneous reaction until Louis Pasteur (18221895) proved otherwise in 1857. Pasteur found that fermentation was caused by yeast cells digesting sugar for their own nourishment. In 1878 German physiologist Wilhelm Kuhne (1837-1900) coined the term "enzyme," meaning "in leaven," to describe this process. The word enzyme was used later to refer to substances such as pepsin, and the word ferment was used to refer to chemical activity produced by living organisms.

In 1897 another German scientist, Eduard Buchner, discovered by accident that fermentation actually does not require the presence of living yeast cells. Buchner made an extract of yeast cells by grinding them and filtering off the remaining cell debris. Then he added a preservativesugarto the resulting cell-free solution to preserve it for future study. He observed that fermentation, the formation of alcohol from sugar, occurred. Buchner then realized that living cells were not required for carrying out metabolic processes such as fermentation. Instead, there must be some small entities capable of converting sugar to alcohol. These entities were enzymes. Buchner's accidental discovery won him the 1907 Nobel Prize in chemistry.

After Buchner's discovery, most scientists assumed that fermentation and other metabolic reactions were caused by enzymes. All attempts to isolate and determine the chemical nature of enzymes were unsuccessful, however, until 1926. That year American biochemist James Sumner of Cornell University isolated the enzyme urease from the jackbean after nine years of research. The enzymes pepsin and trypsin were isolated four years later by the American biochemist John H. Northrop. It was later shown that enzymes are proteins. In more recent research, ribonuclease, a three-dimensional enzyme, was discovered in 1938 by the American bacteriologist Ren6 Dubos. The enzyme was synthesized by American researchers in 1969.

Enzymes in Medicine

Some diseases can be treated by using substances that inhibit (curb) enzymes. Inhibitors can be used to attack enzymes that are critical to the survival of an organism when such undesirable organisms as disease-causing bacteria or parasites pose a threat to health. Neostigmine, used to treat myasthenia gravis (a disease that causes severe muscle weakness), strongly inhibits the enzyme cholinesterase. L-asparaginase is believed to be a potent weapon for treating leukemia. And a class of enzymes called dextrinases are believed to be effective in preventing tooth decay.

Research is also being conducted into malfunctioning of enzymes, which may be linked to such blood disorders as diabetes and anemia. Geneticists have also discovered that in some hereditary diseases, such as phenylketonuria and galactosemia, the affected individuals are actually missing certain enzymes. Some of these enzyme-deficiency diseases can now be effectively treated, and many researchers are concentrating on the search for more of these disorders, which may ultimately revolutionize the practice of medicine.

[See also Genetic engineering ]

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enzyme

enzyme, biological catalyst. The term enzyme comes from zymosis, the Greek word for fermentation, a process accomplished by yeast cells and long known to the brewing industry, which occupied the attention of many 19th-century chemists.

Louis Pasteur recognized in 1860 that enzymes were essential to fermentation but assumed that their catalytic action was inextricably linked with the structure and life of the yeast cell. Not until 1897 was it shown by German chemist Edward Büchner that cell-free extracts of yeast could ferment sugars to alcohol and carbon dioxide; Büchner denoted his preparation zymase. This important achievement was the first indication that enzymes could function independently of the cell.

The first enzyme molecule to be isolated in pure crystalline form was urease, prepared from the jack bean in 1926 by American biochemist J. B. Sumner, who suggested, contrary to prevailing opinion, that the molecule was a protein. In the period from 1930 to 1936, pepsin, chymotrypsin, and trypsin were successfully crystallized; it was confirmed that the crystals were protein, and the protein nature of enzymes was thereby firmly established.

Enzymatic Action

Like all catalysts, enzymes accelerate the rates of reactions while experiencing no permanent chemical modification as a result of their participation. Enzymes can accelerate, often by several orders of magnitude, reactions that under the mild conditions of cellular concentrations, temperature, pH, and pressure would proceed imperceptibly (or not at all) in the absence of the enzyme. The efficiency of an enzyme's activity is often measured by the turnover rate, which measures the number of molecules of compound upon which the enzyme works per molecule of enzyme per second. Carbonic anhydrase, which removes carbon dioxide from the blood by binding it to water, has a turnover rate of 106. That means that one molecule of the enzyme can cause a million molecules of carbon dioxide to react in one second.

Most enzymatic reactions occur within a relatively narrow temperature range (usually from about 30°C to 40°C), a feature that reflects their complexity as biological molecules. Each enzyme has an optimal range of pH for activity; for example, pepsin in the stomach has maximal reactivity under the extremely acid conditions of pH 1–3. Effective catalysis also depends crucially upon maintenance of the molecule's elaborate three-dimensional structure. Loss of structural integrity, which may result from such factors as changes in pH or high temperatures, almost always leads to a loss of enzymatic activity. An enzyme that has been so altered is said to be denatured (see denaturation).

Consonant with their role as biological catalysts, enzymes show considerable selectivity for the molecules upon which they act (called substrates). Most enzymes will react with only a small group of closely related chemical compounds; many demonstrate absolute specificity, having only one substrate molecule which is appropriate for reaction.

Numerous enzymes require for efficient catalytic function the presence of additional atoms of small nonprotein molecules. These include coenzyme molecules, many of which only transiently associate with the enzyme. Nonprotein components tightly bound to the protein are called prosthetic groups. The region on the enzyme molecule in close proximity to where the catalytic event takes place is known as the active site. Prosthetic groups necessary for catalysis are usually located there, and it is the place where the substrate (and coenzymes, if any) bind just before reaction takes place.

The side-chain groups of amino acid residues making up the enzyme molecule at or near the active site participate in the catalytic event. For example, in the enzyme trysin, its complex tertiary structure brings together a histidine residue from one section of the molecule with glycine and serine residues from another. The side chains of the residues in this particular geometry produce the active site that accounts for the enzyme's reactivity.

Identification and Classification

More than 1,500 different enzymes have now been identified, and many have been isolated in pure form. Hundreds have been crystallized, and the amino acid sequences and three-dimensional structure of a significant number have been fully determined through the technique of X-ray crystallography. The knowledge gained has led to great progress in understanding the mechanisms of enzyme chemistry. Biochemists categorize enzymes into six main classes and a number of subclasses, depending upon the type of reaction involved. The 124-amino acid structure of ribonuclease was determined in 1967, and two years later the enzyme was synthesized independently at two laboratories in the United States.

Enzyme Deficiency

A variety of metabolic diseases are now known to be caused by deficiencies or malfunctions of enzymes. Albinism, for example, is often caused by the absence of tyrosinase, an enzyme essential for the production of cellular pigments. The hereditary lack of phenylalanine hydroxylase results in the disease phenylketonuria (PKU) which, if untreated, leads to severe mental retardation in children.

Bibliography

See J. E. and E. T. Bell, Proteins and Enzymes (1988).

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enzyme

enzyme A protein that acts as a catalyst in biochemical reactions. Each enzyme is specific to a particular reaction or group of similar reactions. Many require the association of certain nonprotein cofactors in order to function. The molecule undergoing reaction (the substrate) binds to a specific active site on the enzyme molecule to form a short-lived intermediate (see enzyme–substrate complex): this greatly increases (by a factor of up to 1020) the rate at which the reaction proceeds to form the product. Enzyme activity is influenced by substrate concentration and by temperature and pH, which must lie within a certain range. Other molecules may compete for the active site, causing inhibition of the enzyme or even irreversible destruction of its catalytic properties.

Enzyme production is governed by a cell's genes. Enzyme activity is further controlled by pH changes, alterations in the concentrations of essential cofactors, feedback inhibition by the products of the reaction, and activation by another enzyme, either from a less active form or an inactive precursor (zymogen). Such changes may themselves be under the control of hormones or the nervous system. See also enzyme kinetics.

Enzymes are classified into six major groups, according to the type of reaction they catalyse: (1) oxidoreductases; (2) transferases; (3) hydrolases; (4) lyases; (5) isomerases; (6) ligases. The names of most individual enzymes also end in -ase, which is added to the names of the substrates on which they act. Thus lactase is the enzyme that acts to break down lactose; it is classified as a hydrolase.

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enzyme

enzyme A protein that catalyses a metabolic reaction, so increasing its rate. Enzymes are specific for both the compounds acted on (the substrates) and the reactions carried out. Because of this, enzymes extracted from plants, animals, or micro‐organisms, or those produced by genetic manipulation are widely used in the chemical, pharmaceutical, and food industries (e.g. chymosin in cheese making, maltase in beer production, for synthesis of vitamin C and citric acid).

Because they are proteins, enzymes are permanently inactivated by heat, strong acid or alkali, and other conditions which cause denaturation of proteins.

Many enzymes contain non‐protein components which are essential for their function. These are known as prosthetic groups, coenzymes, or cofactors, and may be metal ions, metal ions in organic combination (e.g. haem in haemoglobin and cytochromes) or a variety of organic compounds, many of which are derived from vitamins. The (inactive) protein without its prosthetic group is known as the apo‐enzyme, and the active assembly of protein plus prosthetic group is the holo‐enzyme. See also enzyme activation assays.

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DAVID A. BENDER. "enzyme." A Dictionary of Food and Nutrition. 2005. Encyclopedia.com. 27 Sep. 2016 <http://www.encyclopedia.com>.

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enzyme

enzyme (Gk. zymosis, ‘fermentation’) Protein that functions as a catalyst in biochemical reactions. They remain chemically unaltered in these reactions and so are effective in tiny quantities. The fermentation properties of yeast cells, for example, have long been utilized in the brewing trade. Chemical reactions can occur several thousand or million times faster with enzymes than without. They operate within a narrow temperature range, usually 30°C to 40°C (86°F to 104°F) and have optimal pH ranges. Many enzymes have to be bound to non-protein molecules to function. These molecules include trace elements (such as metals) and co-enzymes (such as vitamins).

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enzyme

enzyme (en-zym) n. a protein that, in small amounts, speeds up the rate of a biological reaction without itself being used up in the reaction (i.e. it acts as a catalyst). Enzymes are essential for the normal functioning and development of the body. Failure in the production or activity of a single enzyme may result in metabolic disorders; such disorders are often inherited and some have serious effects.
enzymatic adj.

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enzyme

en·zyme / ˈenzīm/ • n. Biochem. a substance produced by a living organism that acts as a catalyst to bring about a specific biochemical reaction. DERIVATIVES: en·zy·mat·ic / ˌenzəˈmatik/ adj. en·zy·mic / enˈzīmik; -ˈzimik/ adj.

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enzyme

enzyme A molecule, wholly or largely protein, produced by a living cell, which acts as a biological catalyst. Enzymes are present in all living organisms, and through their high degree of specificity exert close control over cellular metabolism.

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enzyme

enzyme A molecule, wholly or largely protein, produced by a living cell, that acts as a biological catalyst. Enzymes are present in all living organisms, and through their high degree of specificity exert close control over cellular metabolism.

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MICHAEL ALLABY. "enzyme." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. 27 Sep. 2016 <http://www.encyclopedia.com>.

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enzyme

enzyme A molecule, wholly or largely protein, produced by a living cell, that acts as a biological catalyst. Enzymes are present in all living organisms, and through their high degree of specificity exert close control over cellular metabolism.

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enzyme

enzyme XIX. — G. enzym, f. modGr. énzumos leavened, f. Gr. en IN + zūmē leaven.

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T. F. HOAD. "enzyme." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 27 Sep. 2016 <http://www.encyclopedia.com>.

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enzyme

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