Enzymes and Vitamins PDF

Enzymes and Vitamins 21 C...

Enzymes and Vitamins 21 CHAPTER O UT LI N E 21.1 General Characteristics of Enzymes 754 21.2 Enzyme Structure 755 21.3 Nomenclature and Classification of Enzymes 756 21.4 Models of Enzyme Action 760 21.5 Enzyme Specificity 762 21.6 Factors That Affect Enzyme Activity 763 © Ed Reschke/Peter Arnold/Photo Library Chemistry at a Glance Enzyme Activity 766 21.7 Extremozymes 766 21.8 Enzyme Inhibition 767 21.9 Regulation of Enzyme Activity 769 Chemistry at a Glance Enzyme Inhibition 770 21.10 Prescription Drugs That Inhibit Enzyme Activity 773 The microbial life present in hot springs and thermal vents possesses enzymes that are 21.11 Medical Uses of specially adapted to higher temperature conditions. Enzymes 776 21.12 General Characteristics of Vitamins 778 21.13 Water-Soluble Vitamins: T wo topics constitute the subject matter for this chapter: enzymes and vita- Vitamin C 780 mins. Most enzymes are specialized proteins that function as biochemical 21.14 Water-Soluble Vitamins catalysts. Without enzymes, most chemical reactions in biochemical sys- The B Vitamins 781 tems would occur too slowly to produce adequate amounts of the substances 21.15 Fat-Soluble Vitamins 787 needed for cells to function properly. In some cases, needed reactions would not occur at all without the presence of enzymes. Chemical Connections Vitamins are dietary organic compounds required in very small quantities 21-A Enzymatic Browning: Discoloration of Fruits and for normal cellular function. Many enzymes have vitamins as part of their Vegetables 760 structures, the presence of which is an absolute necessity for the enzymes to carry out their catalytic function. Hence the topic of vitamins is considered in 21-B H. pylori and Stomach Ulcers 764 conjunction with the topic of enzymes. 21-C Enzymes, Prescription Medications, and the “Grapefruit Effect” 777 21.1 General Characteristics of Enzymes An enzyme is a compound, usually a protein, that acts as a catalyst for a bio- chemical reaction. Each cell in the human body contains thousands of different enzymes because almost every reaction in a cell requires its own specific enzyme. Enzymes cause cellular reactions to occur millions of times faster than corre- Sign in to OWL at www.cengage.com/owl to view tutorials and simulations, develop sponding uncatalyzed reactions. As catalysts (Section 9.6), enzymes are not con- problem-solving skills, and complete online sumed during the reaction but merely help the reaction occur more rapidly. homework assigned by your professor. 754  21.2 Enzyme Structure 755 The word enzyme comes from the Greek words en, which means “in,” and zyme, which means “yeast.” Long before their chemical nature was understood, yeast enzymes were used in the production of bread and alcoholic beverages. The action of yeast on sugars produces the carbon dioxide gas that causes bread to rise © Steven Needham/Envision (Figure 21.1). Fermentation of sugars in fruit juices using the same yeast enzymes produces alcoholic beverages. Most enzymes are globular proteins (Section 20.16). Some are simple pro- teins, consisting entirely of amino acid chains. Others are conjugated proteins, containing additional chemical components (Section 21.2). Until the 1980s, it was thought that all enzymes were proteins. A few enzymes are now known that are made of ribonucleic acids (RNA; Section 22.7) and that catalyze cellular reac- tions involving nucleic acids. In this chapter, only enzymes that are proteins are Figure 21.1 Bread dough rises considered. as the result of carbon dioxide Enzymes undergo all the reactions of proteins, including denaturation production resulting from the (Section 20.15). Slight alterations in pH or temperature affect enzyme activity dra- action of yeast enzymes on sugars matically. Good cooks realize that overheating yeast kills the action of the yeast. present in the dough. A person suffering from a high fever (greater than 1068F) runs the risk of having cellular enzymes denatured. The biochemist must exercise extreme caution in hand- ling enzymes to avoid the loss of their activity. Even vigorous shaking of an enzyme solution can destroy enzyme activity. Enzymes differ from nonbiochemical (laboratory) catalysts not only in size, being much larger, but also in that their activity is usually regulated by other sub- stances present in the cell in which they are found. Most laboratory catalysts need to be removed from a reaction mixture to stop their catalytic action; this is not so with enzymes. In some cases, if a certain chemical is needed in the cell, the Enzymes, the most efficient cata- enzyme responsible for its production is activated by other cellular components. lysts known, increase the rates of When a sufficient quantity has been produced, the enzyme is then deactivated. In biochemical reactions by factors other situations, the cell may produce more or less enzyme as required. Because of up to 1020 over uncatalyzed reac- tions. Nonenzymatic catalysts, different enzymes are required for nearly all cellular reactions, certain necessary on the other hand, typically enhance reactions can be accelerated or decelerated without affecting the rest of the cel- the rate of a reaction by factors lular chemistry. of 102 to 104. 21.2 Enzyme Structure Enzymes can be divided into two general structural classes: simple enzymes and conjugated enzymes. A simple enzyme is an enzyme composed only of protein (amino acid chains). A conjugated enzyme is an enzyme that has a nonprotein part in addi- tion to a protein part. By itself, neither the protein part nor the nonprotein portion of a conjugated enzyme has catalytic properties. An apoenzyme is the protein part of a conjugated enzyme. A cofactor is the nonprotein part of a conjugated enzyme. The term holoenzyme is often used to designate a biologically active combined apo- enzyme-cofactor entity. A holoenzyme is the biochemically active conjugated enzyme produced from an apoenzyme and a cofactor. Apoenzyme 1 cofactor 5 holoenzyme Why do apoenzymes need cofactors? Cofactors provide additional chemically reactive functional groups besides those present in the amino acid side chains of apoenzymes. A cofactor is generally either a small organic molecule or an inorganic ion (usually a metal ion). A coenzyme is a small organic molecule that serves as a cofac- tor in a conjugated enzyme. Many vitamins (see Section 21.12) have coenzyme func- tions in the human body. Typical inorganic ion cofactors include Zn21, Mg21, Mn21, and Fe21. The nonmetallic Cl2 ion occasionally acts as a cofactor. Dietary minerals are an impor- tant source of inorganic ion cofactors. 756 Chapter 21 Enzymes and Vitamins 21.3 Nomenclature and Classification of Enzymes Enzymes are most commonly named by using a system that attempts to provide information about the function (rather than the structure) of the enzyme. The type of reaction catalyzed and the substrate identity are focal points for the nomencla- ture. A substrate is the reactant in an enzyme-catalyzed reaction. The substrate is the substance upon which the enzyme “acts.” Three important aspects of the enzyme-naming process are the following: 1. The suffix -ase identifies a substance as an enzyme. Thus urease, sucrase, and lipase are all enzyme designations. The suffix -in is still found in the names of some of the first enzymes studied, many of which are digestive enzymes. Such names include trypsin, chymotrypsin, and pepsin. 2. The type of reaction catalyzed by an enzyme is often noted with a prefix. An oxidase enzyme catalyzes an oxidation reaction, and a hydrolase enzyme catalyzes a hydrolysis reaction. 3. The identity of the substrate is often noted in addition to the type of reaction. Enzyme names of this type include glucose oxidase, pyruvate carboxylase, and suc- cinate dehydrogenase. Infrequently, the substrate but not the reaction type is given, as in the names urease and lactase. In such names, the reaction involved is hydro- lysis; urease catalyzes the hydrolysis of urea, lactase the hydrolysis of lactose. E XAM P L E 21.1 Predicting Enzyme Function from an Enzyme’s Name Predict the function of the following enzymes. a. Cellulase b. Sucrase c. L-Amino acid oxidase d. Aspartate aminotransferase Solution a. Cellulase catalyzes the hydrolysis of cellulose. b. Sucrase catalyzes the hydrolysis of the disaccharide sucrose. c. L-Amino acid oxidase catalyzes the oxidation of L-amino acids. d. Aspartate aminotransferase catalyzes the transfer of an amino group from aspartate to a different molecule. Practice Exercise 21.1 Predict the function of the following enzymes. a. Maltase b. Lactate dehydrogenase c. Fructose oxidase d. Maleate isomerase Answers: a. Hydrolysis of maltose; b. Removal of hydrogen from lactate ion; c. Oxidation of fructose; d. Rearrangement (isomerization) of maleate ion Recall from Section 14.9 that for Enzymes are grouped into six major classes on the basis of the types of reac- organic redox reactions, the follow- tions they catalyze. ing two operational rules are used instead of oxidation numbers 1. An oxidoreductase is an enzyme that catalyzes an oxidation–reduction reaction. (Section 9.2) to characterize oxida- Because oxidation and reduction are not independent processes but linked pro- tion and reduction processes: cesses that must occur together (Section 9.3), an oxidoreductase requires a coen- 1. An organic oxidation reaction is zyme that is oxidized or reduced as the substrate is reduced or oxidized. Lactate an oxidation that increases the dehydrogenase is an oxidoreductase that removes hydrogen atoms from a molecule. number of C!O bonds and/or decreases the number of C!H COO2 Lactate COO2 bonds. A dehydrogenase A 1 2. An organic reduction reaction is HO OC OH 1 NAD1 CP O 1 NADH 1 H A A a reduction that decreases the CH3 CH3 number of C!O bonds and/or Lactate Pyruvate increases the number of C!H Reduced Oxidized Oxidized Reduced bonds. substrate coenzyme product coenzyme  21.3 Nomenclature and Classification of Enzymes 757 2. A transferase is an enzyme that catalyzes the transfer of a functional group from one molecule to another. Two major subtypes of transferases are trans- aminases and kinases. A transaminase catalyzes the transfer of an amino group from one molecule to another. Kinases, which play a major role in metabolic energy-production reactions (see Section 24.2), catalyze the transfer of a phosphate group from adenosine triphosphate (ATP) to give adenosine diphosphate (ADP) and a phosphorylated product (a product containing an additional phosphate group). H H HO O O C OH P O O O C OH O O Hexokinase 1 ATP 88888888n ADP 1 OH OH HO OH HO OH OH OH Glucose Adenosine Adenosine Glucose 6-phosphate triphosphate diphosphate (3 phosphate (2 phosphate The symbol P is a shorthand groups present) groups present) notation for a PO32– unit. 3. A hydrolase is an enzyme that catalyzes a hydrolysis reaction in which the addition of a water molecule to a bond causes the bond to break. Hydrolysis reactions are central to the process of digestion. Carbohydrases effect the breaking of glycosidic bonds in oligo- and polysaccharides, proteases effect the breaking of peptide linkages in proteins, and lipases effect the breaking of ester linkages in triacylglycerols. CH2OH CH2OH CH2OH CH2OH O O O O Maltase 1 H2O 88888n 1 OH O OH OH OH HO OH HO OH HO OH OH OH OH OH Maltose Glucose Glucose 4. A lyase is an enzyme that catalyzes the addition of a group to a double bond or the removal of a group to form a double bond in a manner that does not involve hydrolysis or oxidation. A dehydratase effects the removal of the components of water from a double bond and a hydratase effects the addition of the com- ponents of water to a double bond. COO2 COO2 A A C O H 1 H2O Fumarase HO O C O H B A HO C HO C OH A A COO2 COO2 Fumarate L-Malate 5. An isomerase is an enzyme that catalyzes the isomerization (rearrangement of atoms) of a substrate in a reaction, converting it into a molecule isomeric with itself. There is only one reactant and one product in reactions where isomer- ases are operative. COO2 COO2 A Phosphoglyceromutase A H O C O OH H O C OOO P A A CH2O O P CH2OH 3-Phosphoglycerate 2-Phosphoglycerate 758 Chapter 21 Enzymes and Vitamins 6. A ligase is an enzyme that catalyzes the bonding together of two molecules into one with the participation of ATP. ATP involvement is required because such reactions are generally energetically unfavorable and they require the simultaneous input of energy obtained by a hydrolysis reaction in which ATP is converted to ADP (such energy release is considered in Section 23.3). COO2 COO2 A Pyruvate carboxylase A CO2 1 C O 1 ATP C O 1 ADP 1 P 1 H1 B B A A Phosphate CH3 CH2 Pyruvate A COO2 Oxaloacetate Within each of these six main classes of enzymes there are enzyme subclasses. Table 21.1 gives further information about enzyme subclass terminology, some of which was used in the preceding discussion of the main enzyme classes. For exam- ple, dehydratases and hydratases are subclass designations; both of these enzyme subtypes are lyases (Table 21.1). Table 21.1 Main Classes and Subclasses of Enzymes Selected Main Classes Subclasses Type of Reaction Catalyzed oxidoreductases oxidases oxidation of a substrate reductases reduction of a substrate dehydrogenases introduction of double bond (oxidation) by formal removal of two H atoms from substrate, the H being accepted by a coenzyme transferases transaminases transfer of an amino group between substrates kinases transfer of a phosphate group between substrates hydrolases lipases hydrolysis of ester linkages in lipids proteases hydrolysis of amide linkages in proteins nucleases hydrolysis of sugar–phosphate ester bonds in nucleic acids carbohydrases hydrolysis of glycosidic bonds in carbohydrates phosphatases hydrolysis of phosphate–ester bonds lyases dehydratases removal of H2O from a substrate decarboxylases removal of CO2 from a substrate deaminases removal of NH3 from a substrate hydratases addition of H2O to a substrate isomerases racemases conversion of D isomer to L isomer, or vice versa mutases transfer of a functional group from one position to another in the same molecule ligases synthetases formation of new bond between two substrates, with participation of ATP carboxylases formation of new bond between a substrate and CO2, with participation of ATP  21.3 Nomenclature and Classification of Enzymes 759 EX A M PLE 21.2 Classifying Enzymes by the Type of Chemical Reaction They Catalyze To what main enzyme class do the enzymes that catalyze the following chemical reactions belong? a. COO2 COO2 A A HO O C OH 1 NAD C O 1 NADH 1 H1 1 B A A CH2 CH2 A A COO2 COO2 b. O H O H 1 B A 1 B A H3NO CHOCONOCHOCOO2 1 H2O H3NO CHOCOO 1 H2N 2 O CHOCOO2 A A A 1 A R1 R2 R1 R2 Solution a. In this reaction, two H atoms are removed from the substrate and a carbon–oxygen double bond is formed. Loss of two hydrogen atoms by a molecule is an indication of oxidation. This is an oxidation–reduction reaction, and the enzyme needed to effect the change will be an oxidoreductase. COO2 COO2 A A HO C H 1 NAD1 C O 1 NADH 1 H1 A A B A A CH2 CH2 A A COO2 COO2 Malate Oxaloacetate b. In this reaction, the components of a molecule of H2O are added to the substrate (a dipeptide) with the resulting breaking of the peptide bond to produce two amino acids. This is an example of a hydrolysis reaction. An enzyme that effects such a change is called a hydrolase. O H O H 1 B A 1 B A H3NO CHOCONOCHOCOO2 1 H2O H3NO CHOCOO 1 H2N 2 O CHOCOO2 A A A 1 A R1 R2 R1 R2 Dipeptide Amino acid Amino acid Practice Exercise 21.2 To what main enzyme class do the enzymes that catalyze the following chemical reactions belong? a. O O P OOOCH2 CH2OH P OOOCH2 CH2OO P A A A A A 1 ATP ADP 1 A HO HO A OH A OH A A HO HO Fructose 6-phosphate Fructose 1,6-bisphosphate b. COO2 COO2 A A H C O P C O P 1 HOH A A A A A P HO C H C A A A A A H H H 2-Phosphoglycerate Phosphoenolpyruvate Answers: a. Transferase; b. Lyase 760 Chapter 21 Enzymes and Vitamins C HE MIC AL CONNECTIONS 21-A Enzymatic Browning: Discoloration of Fruits and Vegetables Everyone is familiar with the way fruits such as apples, pears, © Leonard Lessin/Peter Arnold, Inc./Photo Library peaches, apricots, and bananas, and vegetables such as po- tatoes, quickly turn brown when their tissue is exposed to oxygen. Such oxygen exposure occurs when the food is sliced or bitten into or when it has sustained bruises, cuts, or other injury to the peel. This “browning reaction” is related to the work of an enzyme called phenolase (or polyphenoloxidase), a conjugated enzyme in which copper is present. Phenolase is classified as an oxidoreductase. The sub- strates for phenolase are phenolic compounds present in the tissues of the fruits and vegetables. Phenolase hydroxylates phenol derivatives to catechol (Section 14.13) derivatives and then oxidizes the catechol derivatives to o-benzoquinone de- rivatives (see the chemical equations below for the structural relationships). The o-benzoquinone derivatives then enter At left, a freshly cut apple. Brownish oxidation products form into a number of other reactions, which produce the “un- in a few minutes (at right). wanted” brown discolorations. o-Benzoquinone derivative formation is enzyme- and oxygen-dependent. Once the qui- nones have formed, the subsequent reactions occur sponta- activity even more, and boiling temperatures destroy (dena- neously and no longer depend on the presence of phenolase ture) the enzyme. A long-used method for preventing brown- or oxygen. ing involves lemon juice. Phenolase works very slowly in the Enzymatic browning can be prevented or slowed in sev- acidic environment created by the lemon juice’s presence. In eral ways. Immersing the “injured” food (for example, apple addition, the vitamin C (ascorbic acid) present in lemon juice slices) in cold water slows the browning process. The lower functions as an antioxidant. It is more easily oxidized than temperature decreases enzyme activity, and the water limits the phenolic-derived compounds, and its oxidation products the enzyme’s access to oxygen. Refrigeration slows enzyme are colorless. OH OH O OH O O2 O2 Brownish oxidation Phenolase Phenolase products Phenol derivatives Catechol derivatives o-Benzoquinone derivatives A commonly observed enzyme-influenced phenomenon that occurs outside the human body is the discoloration (browning) that occurs when freshly cut fruit (apples, pears, etc.) and vegetables (potatoes) are exposed to air for a short period of time. The enzyme involved, which is present in the food, is an oxidoreductase enzyme called phenolase. The focus on relevancy feature Chemical Connections 21-A above gives further information about the effects of the action of phenolase on fruits and vegetables. 21.4 Models of Enzyme Action Explanations of how enzymes function as catalysts in biochemical systems are based on the concepts of an enzyme active site and enzyme–substrate complex formation. Enzyme Active Site Studies show that only a small portion of an enzyme molecule called the active site participates in the interaction with a substrate or substrates during a reaction. The active site is the relatively small part of an enzyme’s structure that is actually involved in catalysis.  21.4 Models of Enzyme Action 761 The active site in an enzyme is a three-dimensional entity formed by groups that Active site come from different parts of the protein chain(s); these groups are brought together by the folding and bending (secondary and tertiary structure; Sections 20.11 and 20.12) of the protein. The active site is usually a “crevicelike” location in the enzyme (Figure 21.2). Enzyme–Substrate Complex Catalysts offer an alternative pathway with lower activation energy through which a reaction can occur (Section 9.6). In enzyme-controlled reactions, this alternative pathway involves the formation of an enzyme–substrate complex as an intermediate species in the reaction. An enzyme–substrate complex is the inter- mediate reaction species that is formed when a substrate binds to the active site of an enzyme. Within the enzyme–substrate complex, the substrate encounters more Figure 21.2 The active site of favorable reaction conditions than if it were free. The result is faster formation an enzyme is usually a crevicelike of product. region formed as a result of the protein’s secondary and tertiary Lock-and-Key Model structural characteristics. To account for the highly specific way an enzyme recognizes a substrate and binds it to the active site, researchers have proposed several models. The simplest of these The lock-and-key model is more models is the lock-and-key model. than just a “shape fit.” In addition, In the lock-and-key model, the active site in the enzyme has a fixed, rigid geo- there are weak binding forces metrical conformation. Only substrates with a complementary geometry can be (R group interactions) between accommodated at such a site, much as a lock accepts only certain keys. Figure 21.3 parts. illustrates the lock-and-key concept of substrate–enzyme interaction. Products Figure 21.3 The lock-and-key model for enzyme activity. Only a substrate whose shape and Substrate chemical nature are complementary to those of the active site can interact with the enzyme. Enzyme Enzyme active site active site Induced-Fit Model The lock-and-key model explains the action of numerous enzymes. It is, how- ever, too restrictive for the action of many other enzymes. Experimental evidence indicates that many enzymes have flexibility in their shapes. They are not rigid and static; there is constant change in their shape. The induced-fit model is used for this type of situation. The induced-fit model allows for small changes in the shape or geometry of the active site of an enzyme to accommodate a substrate. A good analogy is the changes that occur in the shape of a glove when a hand is inserted into it. The induced fit is a result of the enzyme’s flexibility; it adapts to accept the incoming substrate. This model, illustrated in Figure 21.4, is a more thorough explanation Products Figure 21.4 The induced-fit model for enzyme activity. The enzyme active site, although not Substrate exactly complementary in shape to that of the substrate, is flexible enough that it can adapt to the Enzyme Enzyme shape of the substrate. active site active site 762 Chapter 21 Enzymes and Vitamins Figure 21.5 A schematic diagram representing amino acid R group interactions that bind a substrate Substrate to an enzyme active site. The R group interactions that bind the R group interactions that maintain substrate to the enzyme active site the three-dimensional structure of the enzyme (secondary and tertiary R group interactions that maintain structure) are also shown. the three-dimensional structure of the enzyme Noninteracting R groups that help determine the solubility of the enzyme for the active-site properties of an enzyme because it includes the specificity of the lock-and-key model coupled with the flexibility of the enzyme protein. The forces that draw the substrate into the active site are many of the same forces that maintain tertiary structure in the folding of peptide chains. Electrostatic interactions, hydrogen bonds, and hydrophobic interactions all help attract and bind substrate molecules. For example, a protonated (positively charged) amino group in a substrate could be attracted and held at the active site by a negatively charged aspartate or glutamate residue. Alternatively, cofactors such as positively charged metal ions often help bind substrate molecules. Figure 21.5 is a schematic representation of the amino acid R group interactions that bind a substrate to an enzyme active site. 21.5 Enzyme Specificity Enzymes exhibit different levels of selectivity, or specificity, for substrates. Enzyme specificity is the extent to which an enzyme’s activity is restricted to a specific sub- strate, a specific group of substrates, a specific type of chemical bond, or a specific type of chemical reaction. The degree of enzyme specificity is determined by the active site. Some active sites accommodate only one particular compound, whereas others can accommodate a “family” of closely related compounds. Types of enzyme specificity include: 1. Absolute specificity—the enzyme will catalyze only one reaction. This most restrictive of all specificities is not common. Catalase is an enzyme with absolute specificity. It catalyzes the conversion of hydrogen peroxide (H2O2) to O2 and H2O. Hydrogen peroxide is the only substrate it will accept. 2. Group specificity—the enzyme will act only on molecules that have a specific functional group, such as hydroxyl, amino, or phosphate groups. Carboxy- peptidase is group-specific; it cleaves amino acids, one at a time, from the carboxyl end of a peptide chain. 3. Linkage specificity—the enzyme will act on a particular type of chemical bond, irrespective of the rest of the molecular structure. Phosphatases hydrolyze phosphate-ester bonds in all types of phosphate esters. Linkage specificity is the most general of the common specificities. 4. Stereochemical specificity—the enzyme will act on a particular stereoisomer. Chirality is inherent in an enzyme active site because amino acids are chiral compounds. An L-amino acid oxidase will catalyze the oxidation of the L-form of an amino acid but not the D-form of the same amino acid.  21.6 Factors That Affect Enzyme Activity 763 21.6 Factors That Affect Enzyme Activity Enzyme activity is a measure of the rate at which an enzyme converts substrate to Increased number products in a biochemical reaction. Four factors affect enzyme activity: tempera- of enzyme– Optimum ture, pH, substrate concentration, and enzyme concentration. substrate temperature collisions Temperature Temperature is a measure of the kinetic energy (energy of motion) of molecules. Higher temperatures mean molecules are moving faster and colliding more fre- Reaction rate quently. This concept applies to collisions between substrate molecules and enzymes. As the temperature of an enzymatically catalyzed reaction increases, so does the rate (velocity) of the reaction. However, when the temperature increases beyond a certain point, the increased Denaturation energy begins to cause disruptions in the tertiary structure of the enzyme; denatur- due to excess ation is occurring. Change in tertiary structure at the active site impedes catalytic heat action, and the enzyme activity quickly decreases as the temperature climbs past this point (Figure 21.6). The temperature that produces maximum activity for an Temperature enzyme is known as the optimum temperature for that enzyme. Optimum tempera- ture is the temperature at which an enzyme exhibits maximum activity. Figure 21.6 A graph showing the For human enzymes, the optimum temperature is around 378C, normal body effect of temperature on the rate of temperature. A person who has a fever where body core temperature exceeds 408C an enzymatic reaction. can be in a life-threatening situation because such a temperature is sufficient to initi- ate enzyme denaturation. The loss of function of critical enzymes, particularly those of the central nervous system, can result in dysfunction sufficient to cause death. The “destroying” effect of temperature on bacterial enzymes is used in a hos- pital setting to sterilize medical instruments and laundry. In high-temperature, high-pressure vessels called autoclaves, super-heated steam is used to produce a temperature sufficient to denature bacterial enzymes. pH The pH of an enzyme’s environment can affect its activity. This is not surprising because the charge on acidic and basic amino acids (Section 20.2) located at the active site depends on pH. Small changes in pH (less than one unit) can result in enzyme denaturation (Section 20.16) and subsequent loss of catalytic activity. Most enzymes exhibit maximum activity over a very narrow pH range. Only within this narrow pH range do the enzyme’s amino acids exist in properly charged forms (Section 20.4). Optimum pH is the pH at which an enzyme exhibits maximum activity. Figure 21.7 shows the effect of pH on an enzyme’s activity. Biochemical Maximum rate buffers help maintain the optimum pH for an enzyme. Each enzyme has a characteristic optimum pH, which usually falls within the physiological pH range of 7.0–7.5. Notable exceptions to this generalization are the digestive enzymes pepsin and trypsin. Pepsin, which is active in the stomach, Reaction rate functions best at a pH of 2.0. On the other hand, trypsin, which operates in the small intestine, functions best at a pH of 8.0. The amino acid sequences present in pepsin and trypsin are those needed such that the R groups present can main- tain protein tertiary structure (Section 20.11) at low (2.0) and high (8.0) pH values, respectively. Optimum pH A variation from normal pH can also affect substrates, causing either proton- ation or deprotonation of groups on the substrate. The interaction between the altered substrate and the enzyme active site may be less efficient than normal—or 5.0 7.0 9.0 even impossible. The reason that pickles and other pickled foods do not readily undergo spoilage pH is due to the acidic conditions associated with their preparation. These acidic con- ditions significantly reduce the enzymatic activity of any microorganisms present. Figure 21.7 A graph showing An interesting bacterial adaptation to an acidic environment occurs within the effect of pH on the rate of an the human stomach. Bacteria are now known to be the cause of most stomach enzymatic reaction. 764 Chapter 21 Enzymes and Vitamins C HE MIC AL CONNECTIONS 21-B H. pylori and Stomach Ulcers Helicobacter pylori, commonly called H. pylori, is a bacte- Approximately two-thirds of the world’s population is in- rium that can function in the highly acidic environment of fected with H. pylori. In the United States 30% of the adult the stomach. The discovery in 1982 of the existence of this population is infected, with the infection most prevalent bacterium in the stomach was startling to the medical pro- among older adults. About 20% of people under the age of fession because conventional thought at the time was that 40 and half of those over 60 have it. Only one out of every bacteria could not survive at the stomach’s pH of about 1.4. six people infected with H. pylori ever suffer symptoms re- It is now known that H. pylori causes more than 90% lated to ulcers. Why H. pylori does not cause ulcers in every of duodenal ulcers and up to 80% of gastric ulcers. Before infected person is not known. this discovery, it was thought that most ulcers were caused H. pylori bacteria are most likely spread from person to by excess stomach acid eating the stomach lining. Con- person through fecal–oral or oral–oral routes. Possible envi- tributory causes were thought to be spicy food and stress. ronmental sources include contaminated water sources. The Conventional treatment involved acid-suppression or infection is more common in crowded living conditions with acid-neutralization medications. Now, treatment regimens poor sanitation. In countries with poor sanitation, 90% of involve antibiotics. The medical profession was slow to ac- the adult population can be infected. cept the concept of a bacterial cause for most ulcers, and it was not until the mid-1990s that antibiotic treatment became common. How the enzymes present in the H. pylori bacterium can function in the acidic environment of the stomach (where they should be denatured) is now known. Present on the sur- Eye of Science/Photo Researchers, Inc. face of the bacterium is the enzyme urease, an enzyme that converts urea to the basic substance ammonia. The ammonia then neutralizes acid present in its immediate vicinity; a pro- tective barrier is thus created. The urease itself is protected from denaturation by its complex quaternary structure. H. pylori causes ulcers by weakening the protective mucous coating of the stomach and duodenum, which allows acid to get through to the sensitive lining beneath. Both the acid and the bacteria irritate the lining and cause a sore—the ulcer. Ultimately the H. pylori themselves burrow into the lining to an acid-safe area within the lining. H. pylori bacteria. ulcers. How these ulcer-causing bacteria survive the highly acidic conditions Maximum reaction rate of the human stomach is considered in the focus on relevancy feature Chemical Reaction rate (velocity) Connections 21-B above. Substrate Concentration Rate approaches When the concentration of an enzyme is kept constant and the concentration of maximum substrate is increased, the enzyme activity pattern shown in Figure 21.8 is obtained. This activity pattern is called a saturation curve. Enzyme activity increases up to a certain substrate concentration and thereafter remains constant. What limits enzymatic activity to a certain maximum value? As substrate Substrate concentration concentration increases, the point is eventually reached where enzyme capabilities are used to their maximum extent. The rate remains constant from this point on Figure 21.8 A graph showing the (Figure 21.8). Each substrate must occupy an enzyme active site for a finite amount change in enzyme activity with a of time, and the products must leave the site before the cycle can be repeated. When change in substrate concentration each enzyme molecule is working at full capacity, the incoming substrate molecules at constant temperature, pH, and must “wait their turn” for an empty active site. At this point, the enzyme is said to enzyme concentration. Enzyme be under saturation conditions. activity remains constant after a The rate at which an enzyme accepts substrate molecules and releases prod- certain substrate concentration is uct molecules at substrate saturation is given by its turnover number. An enzyme’s reached. turnover number is the number of substrate molecules transformed per minute by one  21.6 Factors That Affect Enzyme Activity 765 Table 21.2 Turnover Numbers for Selected Enzymes Turnover Number Enzyme (per minute) Reaction Catalyzed carbonic anhydrase 36,000,000 CO2 1 H2O m H2CO3 catalase 5,600,000 2H2O2 m 2H2O 1 O2 cholinesterase 1,500,000 hydrolysis of acetylcholine penicillinase 120,000 hydrolysis of penicillin lactate dehydrogenase 60,000 conversion of pyruvate to lactate DNA polymerase I 900 addition of nucleotides to DNA chains molecule of enzyme under optimum conditions of temperature, pH, and saturation. Table 21.2 gives turnover numbers for selected enzymes. Some enzymes have a much faster mode of operation than others. Reaction rate Enzyme Concentration Because enzymes are not consumed in the reactions they catalyze, the cell usu- ally keeps the number of enzymes low compared with the number of substrate molecules. This is efficient; the cell avoids paying the energy costs of synthesiz- ing and maintaining a large work force of enzyme molecules. Thus, in general, the concentration of substrate in a reaction is much higher than that of the enzyme. Enzyme concentration If the amount of substrate present is kept constant and the enzyme concentra- tion is increased, the reaction rate increases because more substrate molecules can Figure 21.9 A graph showing be accommodated in a given amount of time. A plot of enzyme activity versus the change in reaction rate with a enzyme concentration, at a constant substrate concentration that is high relative to change in enzyme concentration enzyme concentration, is shown in Figure 21.9. The greater the enzyme concentra- for an enzymatic reaction. tion, the greater the reaction rate. Temperature, pH, and substrate The Chemistry at a Glance feature on the next page reviews what has been concentration are constant. The substrate concentration is high presented about enzyme activity. relative to enzyme concentration. EX A M PLE 21.3 Determining How Enzyme Activity Is Affected by Various Changes Describe the effect that each of the following changes would have on the rate of a bio- chemical reaction that involves the substrate urea and the liver enzyme urease. a. Increasing the urea concentration b. Increasing the urease concentration c. Increasing the temperature from its optimum value to a value 108C higher than this value d. Decreasing the pH by one unit from its optimum value Solution a. The enzyme activity rate will increase until all of the enzyme molecules are engaged with urea substrate. b. The enzyme activity rate will increase until all of the urea molecules are engaged with urease enzymes. c. At temperatures higher than the optimum temperature, enzyme activity will decrease from that at the optimum temperature. d. At pH values lower than the optimum pH value, enzyme activity will decrease from that at the optimum pH. (continued) 766 Chapter 21 Enzymes and Vitamins C H E MISTRY AT A G L A NC E Enzyme Activity THE MECHANISM OF ENZYME ACTION Formation of an enzyme–substrate complex as an intermediate species provides an alternative path- way, with lower activation energy, through which a reaction can occur. Lock-and-Key Model Induced-Fit Model The active site has a fixed geometric The active site has a flexible shape shape. Only a substrate with a matching that can change to accept a variety of shape can fit into it. related substrates. Enzymes vary in their degree of specificity for substrates. Substrate Enzyme Substrates active site Enzyme active site FACTORS THAT AFFECT THE RATE OF ENZYME ACTIVITY Concentration Concentration Temperature pH of Substrate of Enzyme Reaction rate increases with Maximum enzymatic activity is Reaction rate increases with Reaction rate increases with temperature until the point at possible only within a narrow substrate concentration until increasing enzyme which the protein is denatured pH range; outside this pH full saturation occurs; then concentration, assuming and activity drops sharply. range, the protein is denatured the rate levels off. enzyme concentration is much and activity drops sharply. lower than that of substrate. Practice Exercise 21.3 Describe the effect that each of the following changes would have on the rate of a bio- chemical reaction that involves the substrate sucrose and the intestinal enzyme sucrase. a. Decreasing the sucrase concentration b. Increasing the sucrose concentration c. Lowering the temperature to 108C d. Increasing the pH by one unit from its optimum value Answers: a. Decrease rate; b. Increase rate; c. Decrease rate; d. Decrease rate 21.7 Extremozymes An extremophile is a microorganism that thrives in extreme environments, environ- ments in which humans and most other forms of life could not survive. Extremo- phile environments include the hydrothermal areas of Yellowstone National  21.8 Enzyme Inhibition 767 Park (Figure 21.10) and hydrothermal vents on the ocean floor where tempera- tures and pressures can be extremely high. The ability of extremophiles to sur- vive under such harsh conditions is related to the amino acid sequences present in the enzymes (proteins) of extremophiles. These sequences are stable under the extraordinary conditions present. During the 1980s and 1990s research carried out on extremophiles found at many different locations resulted in the identification of numerous diverse types of such entities. Types identified include acidophiles (optimal growth at pH levels of 3.0 or below), alkaliphiles (optimal growth at pH levels of 9.0 or above), halophiles Science Source/Photo Researchers (a salinity that exceeds 0.2 M NaCl needed for growth), hypothermophiles (a tem- perature between 80°C and 122°C needed to thrive), piezophiles (a high hydrostatic pressure needed for growth), and cryophiles (a temperature of 15°C or lower needed for growth). The enzymes present in extremophiles are called extremozymes. An extremo- zyme is a microbial enzyme active at conditions that would inactivate human enzymes as well as enzymes present in other types of higher organisms. The study of extremozymes is an area of high interest and active research for industrial chemists. Enzymes are heavily used in industrial processes, a context Figure 21.10 Extremophiles that in which they offer advantages similar to those associated with enzyme function live in the harsh environment of in biochemical reactions within living cells. Because industrial processes usually deep-ocean thermal vents possess require higher temperatures and pressures than do physiological processes, enzymes that are adapted to such extremozymes have characteristics that have been found to be useful. The enzymes harsh conditions. present in some detergent formulations, which must function in hot water, are the result of research associated with high-temperature microbial enzymes. Similarly, cold-water-wash laundry detergents contain enzymes originally characterized in cold-environment microbial organisms. The development of commercially useful enzymes using extremophile sources The upper temperature limit for life involves the following general approach: now stands at 1218C as the result of the discovery, in 2004, of a new 1. Samples containing the extremophile are gathered from the extreme environ- “heat-loving” microbe. The microbe ment where it is found. was found in a water sample from 2. DNA material is extracted from the extremophile and processed. a hydrothermal vent deep in the 3. Macroscopic amounts of the DNA are produced using the polymerase chain Northeast Pacific Ocean. Its method of respiration involves reduction of reaction (Section 22.15). Fe(III) to Fe(II) to produce energy. 4. The macroscopic amount of DNA is analyzed to identify the genes present (Section 22.8) that are involved in extremozyme production. 5. Genetic engineering techniques (Section 22.14) are used to insert the extremo- zyme gene into bacteria, which then produce the extremozyme. 6. The process is then commercialized. Commercially produced enzymes are used in the petroleum industry during oil- well drilling operations. A thick mixture of enzymes, guar gum, sand, and water is forced into the bore hole as the drill works its way through rock. At the appropri- ate time, explosives are used to crack the rock and the enzyme-containing mixture enters the cracks produced. The enzymes present convert the gum mixture into a freely flowing liquid, through hydrolysis of the gum. The freely flowing liquid carries the oil and gas out of the rock. Because of the high temperature generated from the drilling operations and explosive use, the enzymes used must be stable at high temperatures; such enzymes must thus be extremozymes. 21.8 Enzyme Inhibition The rates of enzyme-catalyzed reactions can be decreased by a group of substances called inhibitors. An enzyme inhibitor is a substance that slows or stops the nor- mal catalytic function of an enzyme by binding to it. In this section, three modes by which inhibition takes place are considered: reversible competitive inhibition, reversible noncompetitive inhibition, and irreversible inhibition. 768 Chapter 21 Enzymes and Vitamins Reversible Competitive Inhibition The treatment for methanol In Section 21.5 it was noted that enzymes are quite specific about the molecules poisoning involves giving a patient they accept at their active sites. Molecular shape and charge distribution are key de- intravenous ethanol (Section 14.5). termining factors in whether an enzyme accepts a molecule. A competitive enzyme This action is based on the principle of competitive enzyme inhibition. inhibitor is a molecule that sufficiently resembles an enzyme substrate in shape and The same enzyme, alcohol dehydro- charge distribution that it can compete with the substrate for occupancy of the enzyme’s genase, detoxifies both methanol active site. and ethanol. Ethanol has 10 times When a competitive inhibitor binds to an enzyme active site, the inhibitor re- the affinity for the enzyme than mains unchanged (no reaction occurs), but its physical presence at the site prevents methanol has. Keeping the enzyme busy with ethanol as the substrate a normal substrate molecule from occupying the site. The result is a decrease in gives the body time to excrete the enzyme activity. methanol before it is oxidized to The formation of an enzyme–competitive inhibitor complex is a reversible the potentially deadly formaldehyde process because it is maintained by weak interactions (hydrogen bonds, etc.). With (Section 14.5). time (a fraction of a second), the complex breaks up. The empty active site is then available for a new occupant. Substrate and inhibitor again compete for the empty active site. Thus the active site of an enzyme binds either inhibitor or normal substrate on a random basis. If inhibitor concentration is greater than substrate concentration, the inhibitor dominates the occupancy process. The reverse is also true. Competitive inhibition can be reduced by simply increasing the concentration Competitive Normal substrate inhibitor of the substrate. Figure 21.11 compares the binding of a normal substrate and that of a com- petitive inhibitor at an enzyme’s active site. Note that the portions of these two molecules that bind to the active site have the same shape, but that the two mol- ecules differ in overall shape. It is because of this overall difference in shape that the Enzyme Enzyme substrate reacts at the active site but the inhibitor does not. Numerous drugs act by means of competitive inhibition. For example, anti- histamines are competitive inhibitors of histidine decarboxylation, the enzymatic a b reaction that converts histidine to histamine. Histamine causes the usual allergy Figure 21.11 A comparison of and cold symptoms: watery eyes and runny nose. an enzyme with a substrate at its active site (a) and an enzyme Reversible Noncompetitive Inhibition with a competitive inhibitor at its active site (b). A noncompetitive enzyme inhibitor is a molecule that decreases enzyme activity by binding to a site on an enzyme other than the active site. The substrate can still occupy the active site, but the presence of the inhibitor causes a change in the struc- ture of the enzyme sufficient to prevent the catalytic groups at the active site from properly effecting their catalyzing action. Figure 21.12 contrasts the processes of reversible competitive inhibition and reversible noncompetitive inhibition. Figure 21.12 The Competitive difference between a Normal substrate inhibitor Normal substrate reversible competitive inhibitor and a reversible noncompetitive inhibitor. Enzyme Enzyme Enzyme Noncompetitive inhibitor a An enzyme–substrate b A competitive inhibitor c A noncompetitive inhibitor binds to a site complex in absence of binds to the active site, other than the active site; the normal an inhibitor. which prevents the normal substrate still binds to the active site but substrate from binding to the enzyme cannot catalyze the reaction the site. due to the presence of the inhibitor.  21.9 Regulation of Enzyme Activity 769 Unlike the situation in competitive inhibition, increasing the concentration of substrate does not completely overcome the inhibitory effect in this case. However, lowering the concentration of a noncompetitive inhibitor sufficiently does free up many enzymes, which then return to normal activity. Examples of noncompetitive inhibitors include the heavy metal ions Pb21, Ag , and Hg21. The binding sites for these ions are sulfhydryl (9SH) groups 1 located away from the active site. Metal sulfide linkages are formed, an effect that disrupts secondary and tertiary structure. Irreversible Inhibition An irreversible enzyme inhibitor is a molecule that inactivates enzymes by forming a strong covalent bond to an amino acid side-chain group at the enzyme’s active site. In general, such inhibitors do not have structures similar to that of the enzyme’s normal substrate. The inhibitor–active site bond is sufficiently strong that addi- tion of excess substrate does not reverse the inhibition process. Thus the enzyme is permanently deactivated. The actions of chemical warfare agents (nerve gases) and organophosphate insecticides are based on irreversible inhibition. The Chemistry at a Glance feature on the next page summarizes what has been considered concerning enzyme inhibition. EX A M PLE 21.4 Identifying the Type of Enzyme Inhibition from Inhibitor Characteristics Identify the type of enzyme inhibition each of the following inhibitor characteristics is associated with. a. An inhibitor that decreases enzyme activity by binding to a site on the enzyme other than the active site b. An inhibitor that inactivates enzymes by forming a strong covalent bond at the enzyme active site Solution a. Inhibitor binding at a nonactive site location is a characteristic of a reversible noncompetitive inhibitor. b. Covalent bond formation at the active site, with amino acid residues located there, is a characteristic of an irreversible inhibitor. Practice Exercise 21.4 Identify the type of enzyme inhibition each of the following inhibitor characteristics is associated with. a. An inhibitor that has a shape and charge distribution similar to that of the enzyme’s normal substrate b. An inhibitor whose effect can be reduced by simply increasing the concentration of normal substrate present Answers: a. Reversible competitive inhibitor; b. Reversible competitive inhibitor 21.9 Regulation of Enzyme Activity Regulation of enzyme activity within a cell is a necessity for many reasons. Illustra- tive of this need are the following two situations, both of which involve the concept of energy conservation. 1. A cell that continually produces large amounts of an enzyme for which sub- strate concentration is always very low is wasting energy. The production of the enzyme needs to be “turned off.” 2. A product of an enzyme-catalyzed reaction that is present in plentiful (more than needed) amounts in a cell is a waste of energy if the enzyme continues to catalyze the reaction that produces the product. The enzyme needs to be “turned off.” 770 Chapter 21 Enzymes and Vitamins C H E MISTRY AT A G L A NC E Enzyme Inhibition ENZYME INHIBITORS Substances that bind to an enzyme and stop or slow its normal catalytic activity Competitive Enzyme Inhibitor Noncompetitive Enzyme Inhibitor Irreversible Enzyme Inhibitor A molecule closely resembling the A molecule that binds to a site on an A molecule that forms a covalent bond to substrate. Binds to the active site and enzyme that is not the active site. The a part of the active site, permanently temporarily prevents substrates from normal substrate still occupies the active preventing substrates from occupying it. occupying it, thus blocking the reaction. site but the enzyme cannot catalyze the reaction due to the presence of the inhibitor. Competitive Substrate Substrate Irreversible Substrate inhibitor Substrate inhibitor Enzyme Enzyme active site Noncompetitive inhibitor Many mechanisms exist by which enzymes within a cell can be “turned on” and “turned off.” In this section, three such mechanisms are considered: (1) feedback control associated with allosteric enzymes (2) proteolytic enzymes and zymogens and (3) covalent modification. Allosteric Enzymes Many, but not all, of the enzymes responsible for regulating cellular processes are allosteric enzymes. Characteristics of allosteric enzymes are as follows: 1. All allosteric enzymes have quaternary structure; that is, they are composed of two or more protein chains. 2. All allosteric enzymes have two kinds of binding sites: those for substrate and those for regulators. 3. Active and regulatory binding sites are distinct from each other in both loca- tion and shape. Often the regulatory site is on one protein chain and the active site is on another. 4. Binding of a molecule at the regulatory site causes changes in the overall three-dimensional structure of the enzyme, including structural changes at the active site. The term allosteric comes from the Thus an allosteric enzyme is an enzyme with two or more protein chains (quaternary Greek allo, which means “other,” structure) and two kinds of binding sites (substrate and regulator). and stereos, which means “site Substances that bind at regulatory sites of allosteric enzymes are called regula- or space.” tors. The binding of a positive regulator increases enzyme activity; the shape of the active site is changed such that it can more readily accept substrate. The binding of a negative regulator (a noncompetitive inhibitor; Section 21.8) decreases enzyme  21.9 Regulation of Enzyme Activity 771 Negative Allosteric Control Figure 21.13 The differing effects of positive and negative regulators Substrate on an allosteric enzyme. Active site Substrate cannot enter Allosteric site Negative allosteric regulator Positive Allosteric Control Unavailable Substrate can active site enter Positive allosteric regulator activity; changes to the active site are such that substrate is less readily accepted. Some regulators of allosteric Figure 21.13 contrasts the different effects on an allosteric enzyme that positive enzyme function are inhibitors and negative regulators produce. (negative regulators), and some increase enzyme activity (positive regulators). Feedback Control One of the mechanisms by which allosteric enzyme activity is regulated is feedback control. Feedback control is a process in which activation or inhibition of the first reaction in a reaction sequence is controlled by a product of the reaction sequence. As illustrative of the feedback control mechanism, consider a biochemical pro- Most biochemical processes within cess within a cell that occurs in several steps, each step catalyzed by a different enzyme. cells take place in several steps rather than in a single step. A dif- Enzyme 1 Enzyme 2 Enzyme 3 ferent enzyme is required for each A 788888n B 788888n C 788888n D step of the process. The product of each step is the substrate for the next enzyme. What will happen in this reaction series if the final product (D) is a negative regulator of the first enzyme (enzyme 1)? At low concentrations of D, the reaction sequence proceeds rapidly. At higher concentrations of D, the activity of enzyme 1 becomes inhibited (by feedback), and eventually the activity stops. At the stop- Feedback control is operative in ping point, there is sufficient D present in the cell to meet its needs. Later, when devices that maintain a constant the concentration of D decreases through use in other cell reactions, the activity of temperature, such as a furnace or enzyme 1 increases and more D is produced. an oven. If you set the control ther- mostat at 688F, the furnace or oven Feedback control produces heat until that tempera- g Inhibition of enzyme 1 by product D ture is reached, and then, through Enzyme 1 Enzyme 2 Enzyme 3 electronic feedback, the furnace or A 788888n B 788888n C 788888n D oven is shut off. 772 Chapter 21 Enzymes and Vitamins The general term allosteric control Feedback control is not the only mechanism by which an allosteric enzyme is often used to describe a process can be regulated; it is just one of the more common ways. Regulators of a par- in which a regulatory molecule that binds at one site in an enzyme influ- ticular allosteric enzyme may be products of entirely different pathways of reac- ences substrate binding at the tion within the cell, or they may even be compounds produced outside the cell active site in the enzyme. (hormones). Proteolytic Enzymes and Zymogens A second mechanism for regulating cellular enzyme activity is based on the pro- duction of enzymes in an inactive form. These inactive enzyme precursors are then “turned on” at the appropriate time. Such a mechanism for control is often encountered in the production of proteolytic enzymes. A proteolytic enzyme is an enzyme that catalyzes the breaking of peptide bonds that maintain the primary structure of a protein. Because they would otherwise destroy the tissues that produce them, proteolytic enzymes are generated in an inactive form and then later, when they are needed, are converted to their active form. Most digestive and blood-clotting enzymes are proteolytic enzymes. The inactive forms of pro- teolytic enzymes are called zymogens. A zymogen is the inactive precursor of a proteolytic enzyme. (An alternative, but less often used, name for a zymogen is proenzyme.) Activation of a zymogen requires an enzyme-controlled reaction that removes some part of the zymogen structure. Such modification changes the three-dimensional structure (secondary and tertiary structure) of the zymogen, which affects active The names of zymogens can be site conformation. For example, the zymogen pepsinogen is converted to the active recognized by the suffix -ogen or enzyme pepsin in the stomach, where it then functions as a digestive enzyme. Pepsin the prefix pre- or pro-. would digest the tissues of the stomach wall if it were prematurely generated in active form. Pepsinogen activation involves removal of a peptide fragment from its structure (Figure 21.14). Covalent Modification of Enzymes A third mechanism for regulation of enzyme activity within a cell, called cova- lent modification, involves adding or removing a group from an enzyme through the forming or breaking of a covalent bond. Covalent modification is a process in which enzyme activity is altered by covalently modifying the structure of the enzyme through attachment of a chemical group to or removal of a chemical group from a particular amino acid within the enzyme’s structure. The most commonly encountered type of covalent modification involves the processes by which a phosphate group is added to or removed from an enzyme. The source of the added phosphate group is often an ATP molecule. The process of Figure 21.14 Conversion of a Peptide fragment zymogen (the inactive form of a to be removed proteolytic enzyme) to a proteolytic enzyme (the active form of the S S enzyme) often involves removal of S S a peptide chain segment from the zymogen structure. S S S S Activation Zymogen Proteolytic enzyme