Enzyme

Resources

Enzymes are biological catalysts. A catalyst is an agent which increases the rate of chemical reactions (the speed at which the reactions occur) without being used up or altered in the reaction. Enzymes are also proteins. They are able to function as catalysts because they have sites to which portions of other molecules can bind. The binding is typically relatively weak, so that molecules are not irreversibly bound. These weak binding interactions allow enzymes to accelerate specific reaction rates over and over again.

Louis Pasteur, a scientist who made a number of fundamentally important discoveries to science in general and microbiology in particular was among the first to study enzyme action. He incorrectly hypothesized that the conversion of sugar into alcohol by yeast was catalyzed by “ferments” that could not be separated from living cells. In 1897 the German biochemist Eduard Buchner (1860-1917) isolated the enzymes which catalyze alcoholic fermentation from living yeast cells, represented in the equation:

glucose → ethanol + carbon dioxide

The early twentieth century saw dramatic advancement in enzyme studies. Emil Fischer (1852-1919) recognized the importance of substrate shape for binding by enzymes. Leonor Michaelis (1875-1949) and Maud Menten introduced a mathematical approach for quantifying enzyme-catalyzed reactions. James Sumner (1887-1955) and John Northrop (1891-1987) were among the first to produce highly ordered enzyme crystals and firmly establish the protein nature of these biological catalysts. In 1937 Hans Krebs (1900-1981) postulated how a series of enzymatic reactions were coordinated in the citric acid cycle for the production of ATP from glucose metabolites. Today, enzymology is a central part of biochemical study.

Protein structural studies are a very active area of biochemical research, and it is known that the biological function (activity) of an enzyme is related to its structure. There are 20 common amino acids that make up the building blocks of all known enzymes. They have similar structures, differing mainly in their substituents. The organic substituent of an amino acid is called the R group.

The biologically common amino acids are designated as L-amino acids (see Figure 1). This is called Fischer projection. The amino acids are covalently joined via peptide bonds. Enzymes have many peptide linkages and range in molecular mass from 12,000 to greater than one million (see Figure 2).

Enzymes may also consist of more than a single polypeptide chain. Each polypeptide chain is called a subunit, and may have a separate catalytic function. Some enzymes have non-protein groups that are necessary for enzymatic activity. Metal ions and organic molecules called coenzymes are components of many enzymes. Coenzymes, which are tightly or covalently attached to enzymes, are termed prosthetic groups. Prosthetic groups contain critical chemical groups which allow the overall catalytic event to occur.

Enzymes bind their reactants (substrates) at special folds and clefts in their structures called active sites. Because active sites have chemical groups precisely located and orientated for binding the substrate, they generally display a high degree of substrate specificity. The active site of an enzyme consists of two key regions. The catalytic site, which interacts with the substrate during the reaction, and the binding site, the chemical groups of the enzyme which bind the substrate to allow chemical interaction at the catalytic site.

Although the details of enzyme active sites differ between different enzymes, there are common motifs. The active site represents only a small fraction of the total protein volume. The reason enzymes are so large is that many interactions are necessary for reaction. The crevice of the active site creates a microenvironment that is critical for catalysis. Environmental factors include polarity, hydrophobicity, and precise arrangement of atoms and chemical groups. In 1890 Emil Fischer compared the enzyme-substrate relationship to the fit between and lock and a key (see Figure 3). Fischer postulated that the active site and the enzyme have complimentary three-dimensional shapes. This model was extended by Daniel Koshland Jr. in 1958 by his so-called induced fit model, which reasoned that actives sites are complimentary to substrate shape only after the substrate is bound.

An example of enzyme action is a simple, uncatalyzed chemical reaction reactant → product B. As the concentration of reactant A is increased, the rate of product B formation increases. Rate of reaction is defined as the number of molecules of B formed per unit time. In the presence of a catalyst, the reaction rate is accelerated. For reactant to be converted to product, a thermodynamic energy barrier must be overcome. This energy barrier is known as the activation energy (E_a). A catalyst speeds up a chemical process by lowering the activation energy that the reactant must reach before being converted to product. It does this by allowing a different chemical mechanism or pathway which has a lower activation energy (see Figure 4).

Enzymes have high catalytic power, high substrate specificity, and are generally most active in aqueous solvents at mild temperature and physiological pH.

There are thousands of known enzymes, but nearly all can be categorized according to their biological activities into six major classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Most enzymes catalyze the transfer of electrons, atoms, or groups of atoms, and are assigned names according to the type of reaction.

Thousands of enzymes have been discovered and purified to date. The structure, chemical function, and mechanism of hundreds of enzymes has given biochemists a solid understanding of how they work. In the 1930s, J.B.S. Haldane (1892-1964) described the principle that interactions between the enzyme and its substrate can be used to distort the shape of the substrate and induce a chemical reaction. The energy released and used to lower activation energies from the enzyme-substrate interaction is called the binding energy. The chemical form of the substrate at the top of the energy barrier is called a transition state. The maximum number of weak enzyme-substrate interactions is attained when the substrate reaches its transition state. Enzymes stabilize the transition state, allowing the reaction to proceed in the forward direction. The rate-determining process of a simple enzyme catalyzed reaction is the breakdown of the activated complex between the transition state and the enzyme. In 1889 the Swedish chemist Svante Arrhenius (1859-1927) showed the dependence of the rate of reaction on the magnitude of the energy barrier (activation energy). Reactions that consist of several chemical steps will have multiple activated complexes. The over-all rate of the reaction will be determined by the slowest activated complex decomposition. This is called the rate-limiting step.

In addition to enhancing the reaction rate, enzymes have the ability to discriminate among competing substrates. Enzyme specificity arises mainly from three sources. The first is the exclusion of other molecules from the active site because of the presence of incorrect functional groups, or the absence of necessary binding groups. Secondly, many weak interactions exist between the substrate and the enzyme. It is known that the requirement for many interactions is one reason enzymes are very large compared to the substrate. The noncovalent interactions that bind molecules to enzymes are similar to the intramolecular forces that control enzyme conformation. Induced conformational changes shift important chemical groups on the enzyme into close proximity for catalysis. Finally, stereospecificity arises from the fact that enzymes are built from only L-amino acids. They have active sites which are asymmetric and will bind only certain substrates.

Enzyme activity is controlled by many factors, including environment, enzyme inhibitors, and regulatory binding sites on the enzyme. Concerning the environment, enzymes generally have an optimum

KEY TERMS

Activation energy— The amount of energy required to convert the substrate to the transition state.

Active site— The fold or cleft of an enzyme which binds substrate and catalytically transforms it to product.

Hydrophobic— Nonpolar, insoluble in water.

Inducedfit— Enzyme conformational change caused by substrate binding and resulting in catalytic activity.

Lock-and-key— Geometrically complementary shapes of the enzyme (lock) and the substrate (key) resulting in specificity.

Peptide bond— Chemical covalent bond formed between two amino acids in a polypeptide.

Polypeptide— A long chain of amino acids linked by peptide bonds.

Rate of reaction— The quantity of substrate molecules converted to product per unit time.

Transition state— The activated form of the reactant occupying the highest point on the reaction coordinate.

Weak interactions— Noncovalent interactions between the substrate and the enzyme, allowing enzyme conformational change and catalysis.

pH range in which they are most active. The pH of the environment will affect the ionization state of catalytic groups at the active site and the ionization of the substrate. Electrostatic interactions are therefore controlled by pH. pH may also control the conformation of the enzyme through ionizable amino acids which are located distant from the active site, but are nevertheless critical for the three-dimensional shape of the macromolecule.

Enzyme inhibitors diminish the activity of an enzyme by altering the way substrates bind. Inhibitor structure may be similar to the substrate, but they react very slowly or not at all. Chemically, inhibitors may be large organic molecules, small molecules or ions. They are important because they can be useful for chemotherapeutic treatment of disease, and for providing experimental insights into the mechanism of enzyme action. Inhibitors work in either a reversible or an irreversible manner. Irreversible inhibition is characterized by extremely tight or covalent interaction with the enzyme, resulting in very slow dissociation and inactivation. Reversible inhibition is displayed by rapid dissociation of the inhibitor from the enzyme. There are three main classes of reversible inhibition, competitive inhibition, noncompetitive inhibition, and mixed inhibition. They are distinguishable by how they bind to enzymes and response to increasing substrate concentration. Enzyme inhibition is studied by examining reaction rates and how rates change in response to changes in experimental parameters.

Regulatory enzymes are characterized by increased or decreased activity in response to chemical signals. Metabolic pathways are regulated by controlling the activity of one or more enzymatic steps along that path. Regulatory control allows cells to meet changing demands for energy and biomolecules.

Enzymatic activity is regulated in four general ways. The first is termed allosteric control. Allosteric enzymes are those that have distinct binding sites for effector molecules, which control their rates of reaction. The second reglator of enzymatic activity involves proteins designated control proteins, which participate in cellular regulatory processes. For example, calmodulin is a Ca²⁺ binding protein which binds with high affinity and modulates the activity of many Ca²⁺-regulated enzymes. In this way, Ca²⁺ acts as to allow events like feedback control in metabolic pathways (see Figure 5). A third enzyme regulatory mechanism involves the reversible chemical modification that can be carried out with a variety of chemical groups. A common example is the transfer of phosphate groups, catalyzed by enzymes known as kinases (see Figure 6). Finally, proteolytic activation (activation of enzyme activity that occurs when part of the protein is removed) can regulate the activity of some enzymes. One example are zymogens, which are converted to an active form following the irreversible removal of a portion of the protein. Proteolytic enzymes are controlled by this latter mechanism.

See also Catalyst and catalysis; Krebs cycle.

Resources

BOOKS

Aehle, Wolfgang. Enzymes in Industry: Production and Applications. New York: Wiley, 2004.

Defelice, Karen. Enzymes for Digestive Health and Nutritional Wealth: The Practical Guide for Digestive Enzymes. Johnston IA: Thundersnow Interactive, 2003.

Silverman, Richard E. Organic Chemistry of Enzyme-Catalyzed Reactions, Revised 2nd. ed. New York: Academic Press, 2002.

Leonard D. Holmes

Enzyme

Enzymes are biological catalysts, agents which increase the rate of chemical reactions without being used up in the reaction. They are proteins which possess special binding sites for a certain biochemicals. Weak binding interactions with the biochemical allow enzymes to accelerate specific reaction rates millions of times. Enzyme kinetics is the study of enzyme reactions and mechanisms. Enzyme inhibitor studies have allowed researchers to develop therapies for treatment of disease .

Historical background of enzyme research

Louis Pasteur was among the first to study enzyme action. He incorrectly hypothesized that the conversion of sugar into alcohol by yeast was catalyzed by "ferments" that could not be separated from living cells. In 1897 the German biochemist Eduard Buchner (1860-1917) isolated the enzymes which catalyze alcoholic fermentation from living yeast cells, represented in the equation:

The early twentieth century saw dramatic advancement in enzyme studies. Emil Fischer (1852-1919) recognized the importance of substrate shape for binding by enzymes. Leonor Michaelis (1875-1949) and Maud Menten introduced a mathematical approach for quantifying enzyme-catalyzed reactions. James Sumner (1887-1955) and John Northrop (1891-1987) were among the first to produce highly ordered enzyme crystals and firmly establish the protein nature of these biological catalysts. In 1937 Hans Krebs (1900-1981) postulated how a series of enzymatic reactions were coordinated in the citric acid cycle for the production of ATP from glucose metabolites. Today, enzymology is a central part of biochemical study.

Enzyme structure

Protein structural studies are a very active area of biochemical research, and it is known that the biological function (activity) of an enzyme is related to its structure. There are 20 common amino acids which make up the building blocks of all known enzymes. They have similar structures, differing mainly in their substituents. The organic substituent of an amino acid is called the R group. The structure of the amino acid alanine is depicted in Figure 1.

The biologically common amino acids are designated as L-amino acids because the amino group is on the left of the a alpha-carbon when the molecule is drawn as indicated. This is called Fischer projection. The amino acids are covalently joined via peptide bonds. A series of three amino acids is shown in Figure 2, illustrating the structure of a tripeptide. Enzymes have many peptide linkages and range in molecular mass from 12,000 to greater than one million.

Enzymes may also consist of more than a single polypeptide chain. Each polypeptide chain is called a subunit, and may have a separate catalytic function. Some enzymes have non-protein groups which are necessary for enzymatic activity. Metal ions and organic molecules called coenzymes are components of many enzymes. Coenzymes which are tightly or covalently attached to enzymes are termed prosthetic groups. Prosthetic groups contain critical chemical groups which allow the overall catalytic event to occur.

Enzymes bind their reactants (substrates) at special folds and clefts in their structures called "active sites." Because active sites have chemical groups precisely located and orientated for binding the substrate, they generally display a high degree of substrate specificity. The active site of an enzyme consists of two key regions. The catalytic site, which interacts with the substrate during the reaction, and the binding site, the chemical groups of the enzyme which bind the substrate to allow chemical interaction at the catalytic site.

Although the details of enzyme active sites differ between different enzymes, there are common motifs. The active site represents only a small fraction of the total protein volume . The reason enzymes are so large is that many interactions are necessary for reaction. The crevice of the active site creates a microenvironment which is critical for catalysis. Environmental factors include polarity, hydrophobicity, and precise arrangement of atoms and chemical groups. In 1890 Emil Fischer compared the enzyme-substrate relationship to a "lockand-key." Fischer postulated that the active site and the enzyme have complimentary three dimensional shapes. This model was extended by Daniel Koshland Jr. in 1958 by his "induced fit" model, which reasoned that actives sites are complimentary to substrate shape only after the substrate is bound (Fig. 3).

Enzyme function

Consider the simple, uncatalyzed chemical reaction reactant A product B.

As the concentration of reactant A is increased, the rate of product B formation increases. Rate of reaction is defined as the number of molecules of B formed per unit time . In the presence of a catalyst, the reaction rate is accelerated. For reactant to be converted to product, a thermodynamic energy barrier must be overcome. This energy barrier is known as the activation energy (Ea ). A catalyst speeds up a chemical process by lowering the activation energy which the reactant must reach before being converted to product. It does this by allowing a different chemical mechanism or pathway which has a lower activation energy (Fig. 4).

Enzymes have high catalytic power, high substrate specificity, and are generally most active in aqueous solvents at mild temperature and physiological pH . There are thousands of known enzymes, but nearly all can be categorized according to their biological activities into six major classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Most enzymes catalyze the transfer of electrons, atoms, or groups of atoms, and are assigned names according to the type of reaction.

Thousands of enzymes have been discovered and purified to date. The structure, chemical function, and mechanism of hundreds of enzymes has given biochemists a solid understanding of how they work. In the 1930s, J. B. S. Haldane (1892-1964) described the principle that interactions between the enzyme and its substrate can be used to distort the shape of the substrate and induce a chemical reaction. The energy released and used to lower activation energies from the enzyme-substrate interaction is called the binding energy. The chemical form of the substrate at the top of the energy barrier is called a transition state. The maximum number of weak enzyme-substrate interactions is attained when the substrate reaches its transition state. Enzymes stabilize the transition state, allowing the reaction to proceed in the forward direction. The rate-determining process of a simple enzyme catalyzed reaction is the breakdown of the activated complex between the transition state and the enzyme. In 1889 the Swedish chemist Svante Arrhenius (1859-1927) showed the dependence of the rate of reaction on the magnitude of the energy barrier (activation energy). Reactions which consist of several chemical steps will have multiple activated complexes. The over-all rate of the reaction will be determined by the slowest activated complex decomposition . This is called the rate-limiting step.

In addition to enhancing the reaction rate, enzymes have the ability to discriminate among competing substrates. Enzyme specificity arises mainly from three sources: (1) Exclusion of other molecules from the active site because of the presence of incorrect functional groups, or the absence of necessary binding groups. (2) Many weak interactions between the substrate and the enzyme. It is known that the requirement for many interactions is one reason enzymes are very large compared to the substrate. The noncovalent interactions which bind molecules to enzymes are similar to the intramolecular forces which control enzyme conformation. Induced conformational changes shift important chemical groups on the enzyme into close proximity for catalysis. (3) Stereospecificity arises from the fact that enzymes are built from only L-amino acids. They have active sites which are asymmetric and will bind only certain substrates.

Regulation of enzyme activity

Enzyme activity is controlled by many factors, including environment, enzyme inhibitors, and regulatory binding sites on the enzyme.

Environment

Enzymes generally have an optimum pH range in which they are most active. The pH of the environment will effect the ionization state of catalytic groups at the active site and the ionization of the substrate. Electrostatic interactions are therefore controlled by pH. pH may also control the conformation of the enzyme through ionizable amino acids which are located distant from the active site, but are nevertheless critical for the three-dimensional shape of the macromolecule.

Enzyme inhibitors

Inhibitors diminish the activity of an enzyme by altering the way substrates bind. Inhibitor structure may be similar to the substrate, but they react very slowly or not at all. Chemically, inhibitors may be large organic molecules, small molecules or ions. They are important because they can be useful for chemotherapeutic treatment of disease, and for providing experimental insights into the mechanism of enzyme action. Inhibitors work in either a reversible or an irreversible manner. Irreversible inhibition is characterized by extremely tight or covalent interaction with the enzyme, resulting in very slow dissociation and inactivation. Reversible inhibition is displayed by rapid dissociation of the inhibitor from the enzyme. There are three main classes of reversible inhibition, competitive inhibition, noncompetitive inhibition, and mixed inhibition. They are distinguishable by how they bind to enzymes and response to increasing substrate concentration. Enzyme inhibition is studied by examining reaction rates and how rates change in response to changes in experimental parameters.

Regulatory binding sites

Regulatory enzymes are characterized by increased or decreased activity in response to chemical signals. Metabolic pathways are regulated by controlling the activity of one or more enzymatic steps along that path. Regulatory control allows cells to meet changing demands for energy and biomolecules. The scheme shown in Figure 5 illustrates the concept of a regulatory feedback mechanism.

Enzymatic activity is regulated in four general ways:

1. Allosteric control. Allosteric enzymes have distinct binding sites for effector molecules which control their rates of reaction.
2. Control proteins participate in cellular regulatory processes. For example, calmodulin is a Ca2+ binding protein which binds with high affinity and modulates the activity of many Ca2+-regulated enzymes. In this way, Ca2+ acts as "second messenger" to allow crosstalk and feedback control in metabolic pathways.
3. Enzyme activity controlled by reversible covalent modification is carried out with a variety of chemical groups. A common example is the transfer of phosphate groups, catalyzed by enzymes known as kinases (Figure 6). Glucose metabolism is modulated by phosphate transfer.
4. Proteolytic activation. The above example is reversible in nature. Irreversible hydrolytic cleavage of peptide fragments from inactive forms of enzymes known as zymogens converts them into fully active forms. Proteolytic enzymes are controlled by this mechanism.

See also Catalyst and catalysis; Krebs cycle.

Resources

books

Branden, C., and J. Tooze. Introduction to Protein Structure. New York: Garland, 1991.

Lehninger, A.L., D.L. Nelson, and M.M. Cox. Principles ofBiochemistry. 2nd ed. New York: Worth, 1993.

periodicals

Kraut, J. "How Do Enzymes Work?" Science 242 (1988): 533-540.

Leonard D. Holmes

KEY TERMS

Activation energy

—The amount of energy required to convert the substrate to the transition state.

Active site

—The fold or cleft of an enzyme which binds substrate and catalytically transforms it to product.

Hydrophobic

—Nonpolar, insoluble in water.

Induced fit

—Enzyme conformational change caused by substrate binding and resulting in catalytic activity.

Lock-and-key

—Geometrically complementary shapes of the enzyme (lock) and the substrate (key) resulting in specificity.

Peptide bond

—Chemical covalent bond formed between two amino acids in a polypeptide.

Polypeptide

—A long chain of amino acids linked by peptide bonds.

Rate of reaction

—The quantity of substrate molecules converted to product per unit time.

Transition state

—The activated form of the reactant occupying the highest point on the reaction coordinate.

Weak interactions

—Noncovalent interactions between the substrate and the enzyme, allowing enzyme conformational change and catalysis.

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 E_a 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 E_b. Notice that E_b is much less than E_a. 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 energy—but 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 gaps—one with a rectangular shape and one with a triangular shape—in 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 ]

Enzyme

An enzyme is a protein that acts as a catalyst (a substance that speeds up a chemical reaction) and speeds up chemical reactions in living things. The chemistry of life would not be possible without enzymes since they allow reactions in organisms to happen very quickly and therefore support life. Each enzyme is highly specific and will only work in one particular reaction.

Like a catalyst, an enzyme is not consumed or used up during a reaction. Since it is not changed or affected in any way by the process it helps create, an enzyme is immediately ready to be used again for the same purpose. Enzymes are essential to living things because without them, most of the chemical reactions that take place inside an organism would happen too slowly to keep it alive. Temperature is a key factor in a chemical reaction, and the temperature of most living cells is too low to allow the necessary reactions to take place quickly enough. With the proper enzyme, temperature no longer is a limiting factor. Since without the right enzyme, the reaction might occur so slowly that the cell would die. Although enzymes are catalysts, they are far from being typical—while a catalyst can be any type of simple substance, an enzyme is a complex protein. Further, it is by far the most efficient catalyst, since it can, at times, increase reaction rates by factors of 1,000,000 or more.

Since enzymes are complex proteins, they have their own unique three-dimensional shape. It is this shape that makes a particular enzyme act in a certain way. Their shape is the determining factor in how they will act because of the nature of a chemical reaction. A chemical reaction results in the formation of a new compound from existing ones. The mechanism of a chemical reaction involves either the breaking or the forming of a chemical bond (the link between its atoms). Whether breaking or forming bonds, energy is always necessary, and it is here that enzymes play their part. Enzymes either add energy to make something happen or reduce the energy required for something to happen. Either way, they are able to create a reaction that would not have occurred without the enzyme.

Since each enzyme has a unique shape, it will only "fit" or work for one particular reaction. This means the more reactions an organism needs, the greater the number of different enzymes required. For example, in the process called cellular respiration (in which food is broken down to release energy), approximately thirty chemical reactions take place, and each is controlled by its own enzyme. It is estimated that the typical animal cell (roughly one-billionth the size of a drop of water) contains about three thousand different enzymes, all of which are programmed to work in a certain chemical reaction.

ANSELME PAYEN

French chemist Anselme Payen (1795–1871) investigated the chemical reactions carried out by plants and discovered diastase, the first enzyme known to science. He also introduced the filtering properties of activated charcoal and discovered cellulose, a basic constituent of plant cells.

Anselme Payen was born in Paris, France, the son of a lawyer who turned to a career in industry and started up several chemical production factories. At the age of twenty, Payen was put in charge of his father's borax production plant. Borax is a crystalline mineral often found in salt lakes that is refined and used in metallurgy (metal-making) and to make soaps, glass, and pottery glazes, among other things. Payen discovered a method of preparing borax from boric acid which was readily available from Italy. Since his production costs for this method were so low, he was able to undersell his competitors, the Dutch, who had a monopoly on borax. Five years later he turned his attention to his company's production of sugar from sugar beets. Seeking a way to remove color impurities from beet sugar, he invented a method of using activated charcoal to filter out, or catch, large molecules. As a result of Payen's process, the filtering properties of charcoal have since been put to many uses, the most notable of which was their use in gas masks during wartime.

In 1833, Payen made another major discovery. That year he separated a substance from malt extract (grain that was germinating or starting to sprout) that seemed to be responsible for speeding up the conversion of starch into sugar. Further tests suggested to Payen that this substance acted as yeast did, meaning that it acted as a catalyst. A catalyst is a chemical that usually speeds up the rate of a chemical reaction, but is not itself affected, or changed, in any way. Payen named this new substance "diastase" from a Greek word meaning "to separate," since in many ways it separated the individual building blocks of starch into its individual components of sugar. To his delight, he found that diastase even worked when it was taken out of the original malt extract that produced it. Diastase, therefore, could be called an organic catalyst, or an enzyme, since an enzyme is a substance that acts as a catalyst in biological systems. Thus, diastase was the first enzyme to be produced in concentrated form. Ever since, enzymes that have been discovered have been named with the "-ase" suffix, the pattern started by Payen.

Payen was the first to isolate cellulose, a carbohydrate (a compound consisting of only carbon, hydrogen, and oxygen). While studying different types of wood, he obtained a substance that was definitely not starch but that nonetheless could be broken down into units of sugar as starch could. Because he obtained it from the cell walls of plants, he named it "cellulose." Once more, Payen established a naming system, and ever since carbohydrates always end with "-ose," like glucose and sucrose. Much later, cellulose went on to be used as the building block for many other products. Treated with acids and other additives, it was the main ingredient in the manufacture of guncotton (an explosive), celluloid (film), cellophane, and rayon, among others. By 1835 Payen abandoned business altogether and became professor of industrial and agricultural chemistry in Paris. He died in Paris during the Franco-Prussian War after refusing to leave the city as the Prussian army advanced.

Enzymes work at the cellular level and can be considered a cell's chemical regulator or system of control. The needs of a living cell are constantly changing as it strives to adjust to the ever-changing demands of its environment. Therefore, the cell needs the flexible system of control that enzymes provide. Another advantage of an enzyme is that the reactions that it stimulates are reversible if the cell wants the opposite reaction.

The synthesis, or the making, of an enzyme is controlled by a specific gene. Usually it is a hormone that switches on the gene, which, in turn, signals that a certain enzyme should be produced. Since enzyme production can be turned on, it can also be turned off. Both of these on/off mechanisms are natural and happen all the time, but certain forms of artificial "inhibitors" can turn off production permanently. Many poisons and some drugs have this effect, and can lead to death. For example, certain nerve gases as well as the poison cyanide permanently inhibit or stop the important enzyme that allows the body to use oxygen. This results in a quick death. Besides the natural enzymes in the bodies of organisms, enzymes are put to use everyday in the production of beer, wine, cheese, and bread. Without the proper enzymes contained in yeast cells, none of these important food products could be made.

[See alsoProtein ]

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 (1822—1895) 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 preservative—sugar—to 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 ]