enzyme

/en"zuym/, n. Biochem.

any of various proteins, as pepsin, originating from living cells and capable of producing certain chemical changes in organic substances by catalytic action, as in digestion. Cf. -ase.

[1880-85; < MGk énzymos leavened (Gk en- EN-² + zým(e) leaven + -os adj. suffix)]

* * *

Substance that acts as a catalyst in living organisms, regulating the rate at which life's chemical reactions proceed without being altered in the process.

Enzymes reduce the activation energy needed to start these reactions; without them, most such reactions would not take place at a useful rate. Because enzymes are not consumed, only tiny amounts of them are needed. Enzymes catalyze all aspects of cell metabolism, including the digestion of food, in which large nutrient molecules (including proteins, carbohydrates, and fats) are broken down into smaller molecules; the conservation and transformation of chemical energy; and the construction of cellular materials and components. Almost all enzymes are proteins; many depend on a nonprotein cofactor, either a loosely associated organic compound (e.g., a vitamin; see coenzyme) or a tightly bound metal ion (e.g., iron, zinc) or organic (often metal-containing) group. The enzyme-cofactor combination provides an active configuration, usually including an active site into which the substance (substrate) involved in the reaction can fit. Many enzymes are specific to one substrate. If a competing molecule blocks the active site or changes its shape, the enzyme's activity is inhibited. If the enzyme's configuration is destroyed (see denaturation), its activity is lost. Enzymes are classified by the type of reaction they catalyze: (1) oxidation-reduction, (2) transfer of a chemical group, (3) hydrolysis, (4) removal or addition of a chemical group, (5) isomerization (see isomer; isomerism), and (6) binding together of substrate units (polymerization). Most enzyme names end in -ase. Enzymes are chiral catalysts, producing mostly or only one of the possible stereoisomeric products (see optical activity). The fermentation of wine, leavening of bread, curdling of milk into cheese, and brewing of beer are all enzymatic reactions. The uses of enzymes in medicine include killing disease-causing microorganisms, promoting wound healing, and diagnosing certain diseases.

* * *

▪ biochemistry

Introduction

 a substance that acts as a catalyst in living organisms, regulating the rate at which chemical reactions proceed without itself being altered in the process.

      A brief treatment of enzymes follows. For full treatment, see protein: Enzymes (protein).

      The biological processes that occur within all living organisms are chemical reactions (chemical reaction), and most are regulated by enzymes. Without enzymes, many of these reactions would not take place at a perceptible rate. Enzymes catalyze all aspects of cell metabolism. This includes the digestion of food, in which large nutrient molecules (such as proteins, carbohydrates, and fats) are broken down into smaller molecules; the conservation and transformation of chemical energy; and the construction of cellular macromolecules from smaller precursors. Many inherited human diseases, such as albinism, result from a deficiency of a particular enzyme.

      Enzymes also have valuable industrial and medical applications. The fermenting of wine, leavening of bread, curdling of cheese, and brewing of beer have been practiced from earliest times, but not until the 19th century were these reactions understood to be the result of the catalytic activity of enzymes. Since then, enzymes have assumed an increasing importance in industrial processes that involve organic chemical reactions. The uses of enzymes in medicine include killing disease-causing microorganisms, promoting wound healing, and diagnosing certain diseases.

Chemical nature.

      All enzymes were once thought to be proteins, but since the 1980s the catalytic ability of certain nucleic acids, called messenger RNAs, has been demonstrated, refuting this axiom. Because so little is yet known about the enzymatic functioning of RNA, this discussion will focus primarily on protein enzymes.

      A large protein enzyme molecule is composed of one or more amino acid chains called polypeptide chains. The amino acid sequence determines the characteristic folding patterns of the protein's structure, which is essential to enzyme specificity. If the enzyme is subjected to changes, such as fluctuations in temperature or pH, the protein structure may lose its integrity (denature) and its enzymatic ability. Denaturation is sometimes, but not always, reversible.

      Bound to some enzymes is an additional chemical component called a cofactor, which is a direct participant in the catalytic event and thus is required for enzymatic activity. A cofactor may be either a coenzyme—an organic molecule, such as a vitamin—or an inorganic metal ion; (ion) some enzymes require both. A cofactor may be either tightly or loosely bound to the enzyme. If tightly connected, the cofactor is referred to as a prosthetic group.

Nomenclature.

      An enzyme will interact with only one type of substance or group of substances, called the substrate, to catalyze a certain kind of reaction. Because of this specificity, enzymes often have been named by adding the suffix “-ase” to the substrate's name (as in urease, which catalyzes the breakdown of urea). Not all enzymes have been named in this manner, however, and to ease the confusion surrounding enzyme nomenclature, a classification system has been developed based on the type of reaction the enzyme catalyzes. There are six principal categories and their reactions: (1) oxidoreductases (oxidoreductase), which are involved in electron transfer; (2) transferases (transferase), which transfer a chemical group from one substance to another; (3) hydrolases (hydrolase), which cleave the substrate by uptake of a water molecule (hydrolysis); (4) lyases (lyase), which form double bonds by adding or removing a chemical group; (5) isomerases (isomerase), which transfer a group within a molecule to form an isomer; and (6) ligases (ligase), or synthetases, which couple the formation of various chemical bonds to the breakdown of a pyrophosphate bond in adenosine triphosphate or a similar nucleotide.

Mechanism of enzyme action.

      In most chemical reactions, an energy barrier exists that must be overcome for the reaction to occur. This barrier prevents complex molecules such as proteins and nucleic acids from spontaneously degrading, and so is necessary for the preservation of life. When metabolic changes are required in a cell, however, certain of these complex molecules must be broken down, and this energy barrier must be surmounted. Heat could provide the additional needed energy (called activation energy), but the rise in temperature would kill the cell. The alternative is to lower the activation energy level through the use of a catalyst. This is the role that enzymes play. They react with the substrate to form an intermediate complex—a “transition state”—that requires less energy for the reaction to proceed. The unstable intermediate compound quickly breaks down to form reaction products, and the unchanged enzyme is free to react with other substrate molecules.

      Only a certain region of the enzyme, called the active site, binds to the substrate. The active site is a groove or pocket formed by the folding pattern of the protein. This three-dimensional structure, together with the chemical and electrical properties of the amino acids and cofactors within the active site, permits only a particular substrate to bind to the site, thus determining the enzyme's specificity.

      Enzyme synthesis and activity also are influenced by genetic control and distribution in a cell. Some enzymes are not produced by certain cells, and others are formed only when required. Enzymes are not always found uniformly within a cell; often they are compartmentalized in the nucleus, on the cell membrane, or in subcellular structures. The rates of enzyme synthesis and activity are further influenced by hormones, neurosecretions, and other chemicals that affect the cell's internal environment.

Factors affecting enzyme activity.

      Because enzymes are not consumed in the reactions they catalyze and can be used over and over again, only a very small quantity of an enzyme is needed to catalyze a reaction. A typical enzyme molecule can convert 1,000 substrate molecules per second. The rate of an enzymatic reaction increases with increased substrate concentration, reaching maximum velocity when all active sites of the enzyme molecules are engaged. The enzyme is then said to be saturated, the rate of the reaction being determined by the speed at which the active sites can convert substrate to product.

      Enzyme activity can be inhibited in various ways. Competitive inhibition occurs when molecules very similar to the substrate molecules bind to the active site and prevent binding of the actual substrate. Penicillin is a competitive inhibitor that blocks the active site of an enzyme that many bacteria use to construct their cell walls.

      Noncompetitive inhibition occurs when an inhibitor binds to the enzyme at a location other than the active site. In some cases of noncompetitive inhibition, the inhibitor is thought to bind to the enzyme in such a way as to physically block the normal active site. In other instances, the binding of the inhibitor is believed to change the shape of the enzyme molecule, thereby deforming its active site and preventing it from reacting with its substrate. This latter type of noncompetitive inhibition is called allosteric inhibition; (allosteric control) the place where the inhibitor binds to the enzyme is called the allosteric site. Frequently, an end-product of a metabolic pathway serves as an allosteric inhibitor on an earlier enzyme of the pathway. This inhibition of an enzyme by a product of its pathway is a form of negative feedback.

      Allosteric control can involve stimulation of enzyme action as well as inhibition. An activator molecule can be bound to an allosteric site and induce a reaction at the active site by changing its shape to fit a substrate that could not induce the change by itself. Common activators include hormones and the products of earlier enzymatic reactions. Allosteric stimulation and inhibition allow production of energy and materials by the cell when they are needed and inhibit production when the supply is adequate.

* * *