I. INTRODUCTION

Enzyme, any one of many specialized organic substances, composed of polymers of amino acids, that act as catalysts to regulate the speed of the many chemical reactions involved in the metabolism of living organisms. The name enzyme was suggested in 1867 by the German physiologist Wilhelm Kühne (1837-1900); it is derived from the Greek phrase en zyme, meaning "leavened" (an element that produces an altering or transforming influence). Those enzymes identified now number more than 700.

Enzymes are classified into several broad categories, such as hydrolytic, oxidizing, and reducing, depending on the type of reaction they control. Hydrolytic enzymes accelerate reactions in which a substance is broken down into simpler compounds through reaction with water molecules. Oxidizing enzymes, known as oxidases, accelerate oxidation reactions; reducing enzymes speed up reduction reactions, in which oxygen is removed. Many other enzymes catalyze other types of reactions.

Individual enzymes are named by adding ase to the name of the substrate with which they react. The enzyme that controls urea decomposition is called urease; those that control protein hydrolyses are known as proteinases. Some enzymes, such as the proteinases trypsin and pepsin, retain the names used before this nomenclature was adopted.

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II. PROPERTIES OF ENZYMES

As the Swedish chemist Jöns Jakob Berzelius suggested in 1823, enzymes are typical catalysts: they are capable of increasing the rate of reaction without being consumed in the process. See Catalysis.

Some enzymes, such as pepsin and trypsin, which bring about the digestion of meat, control many different reactions, whereas others, such as urease, are extremely specific and may accelerate only one reaction. Still others release energy to make the heart beat and the lungs expand and contract. Many facilitate the conversion of sugar and foods into the various substances the body requires for tissue-building, the replacement of blood cells, and the release of chemical energy to move muscles.

Pepsin, trypsin, and some other enzymes possess, in addition, the peculiar property known as autocatalysis, which permits them to cause their own formation from an inert precursor called zymogen. As a consequence, these enzymes may be reproduced in a test tube.

As a class, enzymes are extraordinarily efficient. Minute quantities of an enzyme can accomplish at low temperatures what would require violent reagents and high temperatures by ordinary chemical means. About 30 g (about 1 oz) of pure crystalline pepsin, for example, would be capable of digesting nearly 2 metric tons of egg white in a few hours.

The kinetics of enzyme reactions differ somewhat from those of simple inorganic reactions. Each enzyme is selectively specific for the substance in which it causes a reaction and is most effective at a temperature peculiar to it. Although an increase in temperature may accelerate a reaction, enzymes are unstable when heated. The catalytic activity of an enzyme is determined primarily by the enzyme's amino-acid sequence and by the tertiary structure—that is, the three-dimensional folded structure—of the macromolecule. Many enzymes require the presence of another ion or a molecule, called a cofactor, in order to function.

As a rule, enzymes do not attack living cells. As soon as a cell dies, however, it is rapidly digested by enzymes that break down protein. The resistance of the living cell is due to the enzyme's inability to pass through the membrane of the cell as long as the cell lives. When the cell dies, its membrane becomes permeable, and the enzyme can then enter the cell and destroy the protein within it. Some cells also contain enzyme inhibitors, known as antienzymes, which prevent the action of an enzyme upon a substrate.

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III. PRACTICAL USES OF ENZYMES

Alcoholic fermentation and other important industrial processes depend on the action of enzymes that are synthesized by the yeasts and bacteria used in the production process. A number of enzymes are used for medical purposes. Some have been useful in treating areas of local inflammation; trypsin is employed in removing foreign matter and dead tissue from wounds and burns.

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IV. HISTORICAL REVIEW

Alcoholic fermentation is undoubtedly the oldest known enzyme reaction. This and similar phenomena were believed to be spontaneous reactions until 1857, when the French chemist Louis Pasteur proved that fermentation occurs only in the presence of living cells (see Spontaneous Generation). Subsequently, however, the German chemist Eduard Buchner discovered (1897) that a cell-free extract of yeast can cause alcoholic fermentation. The ancient puzzle was then solved; the yeast cell produces the enzyme, and the enzyme brings about the fermentation. As early as 1783 the Italian biologist Lazzaro Spallanzani had observed that meat could be digested by gastric juices extracted from hawks. This experiment was probably the first in which a vital reaction was performed outside the living organism. After Buchner's discovery scientists assumed that fermentations and vital reactions in general were caused by enzymes. Nevertheless, all attempts to isolate and identify their chemical nature were unsuccessful. In 1926, however, the American biochemist James B. Sumner succeeded in isolating and crystallizing urease. Four years later pepsin and trypsin were isolated and crystallized by the American biochemist John H. Northrop. Enzymes were found to be proteins, and Northrop proved that the protein was actually the enzyme and not simply a carrier for another compound.

Research in enzyme chemistry in recent years has shed new light on some of the most basic functions of life. Ribonuclease, a simple three-dimensional enzyme discovered in 1938 by the American bacteriologist René Dubos and isolated in 1946 by the American chemist Moses Kunitz, was synthesized by American researchers in 1969. The synthesis involves hooking together 124 molecules in a very specific sequence to form the macromolecule. Such syntheses led to the probability of identifying those areas of the molecule that carry out its chemical functions, and opened up the possibility of creating specialized enzymes with properties not possessed by the natural substances. This potential has been greatly expanded in recent years by genetic engineering techniques that have made it possible to produce some enzymes in great quantity (see Biochemistry).

The medical uses of enzymes are illustrated by research into L-asparaginase, which is thought to be a potent weapon for treatment of leukemia; into dextrinases, which may prevent tooth decay; and into the malfunctions of enzymes that may be linked to such diseases as phenylketonuria, diabetes, and anemia and other blood disorders.

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