The enzyme carboxypeptidase, on the other hand, is far less specific. It catalyzes the removal of nearly any amino acid from the carboxyl end of any peptide or protein. Enzyme specificity results from the uniqueness of the active site in each different enzyme because of the identity, charge, and spatial orientation of the functional groups located there.
It regulates cell chemistry so that the proper reactions occur in the proper place at the proper time. Clearly, it is crucial to the proper functioning of the living cell.
Distinguish between the lock-and-key model and induced-fit model of enzyme action. Which enzyme has greater specificity—urease or carboxypeptidase?
The lock-and-key model portrays an enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site.
The induced fit model portrays the enzyme structure as more flexible and is complementary to the substrate only after the substrate is bound. Urease has the greater specificity because it can bind only to a single substrate. Carboxypeptidase, on the other hand, can catalyze the removal of nearly any amino acid from the carboxyl end of a peptide or protein. What type of interaction would occur between each group present on a substrate molecule and a functional group of the active site in an enzyme?
For each functional group in Exercise 1, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified. For each functional group in Exercise 2, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified.
The amino acid has a polar side chain capable of engaging in hydrogen bonding; serine answers will vary. The amino acid has a negatively charged side chain; aspartic acid answers will vary.
The amino acid has a polar side chain capable of engaging in hydrogen bonding; asparagine answers will vary. The amino acid has a nonpolar side chain; isoleucine answers will vary. The single most important property of enzymes is the ability to increase the rates of reactions occurring in living organisms, a property known as catalytic activity.
Because most enzymes are proteins, their activity is affected by factors that disrupt protein structure, as well as by factors that affect catalysts in general. Factors that disrupt protein structure, as we saw in Section 9. The activity of an enzyme can be measured by monitoring either the rate at which a substrate disappears or the rate at which a product forms.
In the presence of a given amount of enzyme, the rate of an enzymatic reaction increases as the substrate concentration increases until a limiting rate is reached, after which further increase in the substrate concentration produces no significant change in the reaction rate part a of Figure 9.
At this point, so much substrate is present that essentially all of the enzyme active sites have substrate bound to them. In other words, the enzyme molecules are saturated with substrate. The excess substrate molecules cannot react until the substrate already bound to the enzymes has reacted and been released or been released without reacting.
Ten taxis enzyme molecules are waiting at a taxi stand to take people substrate on a minute trip to a concert hall, one passenger at a time. If only 5 people are present at the stand, the rate of their arrival at the concert hall is 5 people in 10 minutes. If the number of people at the stand is increased to 10, the rate increases to 10 arrivals in 10 minutes.
With 20 people at the stand, the rate would still be 10 arrivals in 10 minutes. The rate would simply be higher 20 or 30 people in 10 minutes before it leveled off. When the concentration of the enzyme is significantly lower than the concentration of the substrate as when the number of taxis is far lower than the number of waiting passengers , the rate of an enzyme-catalyzed reaction is directly dependent on the enzyme concentration part b of Figure 9. This is true for any catalyst; the reaction rate increases as the concentration of the catalyst is increased.
To some extent, this rule holds for all enzymatic reactions. After a certain point, however, an increase in temperature causes a decrease in the reaction rate, due to denaturation of the protein structure and disruption of the active site part a of Figure 9. This fact has several practical applications. We sterilize objects by placing them in boiling water, which denatures the enzymes of any bacteria that may be in or on them.
We preserve our food by refrigerating or freezing it, which slows enzyme activity. Because most enzymes are proteins, they are sensitive to changes in the hydrogen ion concentration or pH. Ionizable side groups located in the active site must have a certain charge for the enzyme to bind its substrate.
An enzyme exhibits maximum activity over the narrow pH range in which a molecule exists in its properly charged form. The median value of this pH range is called the optimum pH of the enzyme part b of Figure 9.
With the notable exception of gastric juice the fluids secreted in the stomach , most body fluids have pH values between 6 and 8. Not surprisingly, most enzymes exhibit optimal activity in this pH range. However, a few enzymes have optimum pH values outside this range. For example, the optimum pH for pepsin, an enzyme that is active in the stomach, is 2.
The concentration of substrate X is low. What happens to the rate of the enzyme-catalyzed reaction if the concentration of X is doubled? What effect does an increase in the enzyme concentration have on the rate of an enzyme-catalyzed reaction?
If the concentration of the substrate is low, increasing its concentration will increase the rate of the reaction. An increase in the amount of enzyme will increase the rate of the reaction provided sufficient substrate is present. In non-enzyme-catalyzed reactions, the reaction rate increases as the concentration of reactant is increased.
In an enzyme-catalyzed reaction, the reaction rate initially increases as the substrate concentration is increased but then begins to level off, so that the increase in reaction rate becomes less and less as the substrate concentration increases. Explain this difference. An enzyme has an optimum pH of 7. What is most likely to happen to the activity of the enzyme if the pH drops to 6. What is most likely to happen to the activity of the enzyme if the pH increases to 8. In an enzyme-catalyzed reaction, the substrate binds to the enzyme to form an enzyme-substrate complex.
If more substrate is present than enzyme, all of the enzyme binding sites will have substrate bound, and further increases in substrate concentration cannot increase the rate. The activity will decrease; a pH of 6. In Section 9. These are nonspecific factors that would inactivate any enzyme. The activity of enzymes can also be regulated by more specific inhibitors.
Many compounds are poisons because they bind covalently to particular enzymes or kinds of enzymes and inactivate them Table 9. An irreversible inhibitor inactivates an enzyme by bonding covalently to a particular group at the active site.
The inhibitor-enzyme bond is so strong that the inhibition cannot be reversed by the addition of excess substrate. The nerve gases, especially DIFP, irreversibly inhibit biological systems by forming an enzyme-inhibitor complex with a specific OH group of serine situated at the active sites of certain enzymes.
The peptidases trypsin and chymotrypsin contain serine groups at the active site and are inhibited by DIFP. A reversible inhibitor inactivates an enzyme through noncovalent, more easily reversed, interactions. Unlike an irreversible inhibitor, a reversible inhibitor can dissociate from the enzyme. Reversible inhibitors include competitive inhibitors and noncompetitive inhibitors.
There are additional types of reversible inhibitors. A competitive inhibitor is any compound that bears a structural resemblance to a particular substrate and thus competes with that substrate for binding at the active site of an enzyme.
The inhibitor is not acted on by the enzyme but does prevent the substrate from approaching the active site. If the inhibitor is present in relatively large quantities, it will initially block most of the active sites. But because the binding is reversible, some substrate molecules will eventually bind to the active site and be converted to product. Increasing the substrate concentration promotes displacement of the inhibitor from the active site.
Competitive inhibition can be completely reversed by adding substrate so that it reaches a much higher concentration than that of the inhibitor.
Studies of competitive inhibition have provided helpful information about certain enzyme-substrate complexes and the interactions of specific groups at the active sites. As a result, pharmaceutical companies have synthesized drugs that competitively inhibit metabolic processes in bacteria and certain cancer cells. Many drugs are competitive inhibitors of specific enzymes.
A classic example of competitive inhibition is the effect of malonate on the enzyme activity of succinate dehydrogenase Figure 9. Malonate and succinate are the anions of dicarboxylic acids and contain three and four carbon atoms, respectively.
The malonate molecule binds to the active site because the spacing of its carboxyl groups is not greatly different from that of succinate.
This reaction will also be discussed in connection with the Krebs cycle and energy production. A dehydrogenation reaction occurs, and the product—fumarate—is released from the enzyme. In this case, however, no subsequent reaction occurs while malonate remains bound to the enzyme.
Chemotherapy is the strategic use of chemicals that is, drugs to destroy infectious microorganisms or cancer cells without causing excessive damage to the other, healthy cells of the host. From bacteria to humans, the metabolic pathways of all living organisms are quite similar, so the search for safe and effective chemotherapeutic agents is a formidable task.
Many well-established chemotherapeutic drugs function by inhibiting a critical enzyme in the cells of the invading organism. An antibiotic is a compound that kills bacteria; it may come from a natural source such as molds or be synthesized with a structure analogous to a naturally occurring antibacterial compound. Antibiotics constitute no well-defined class of chemically related substances, but many of them work by effectively inhibiting a variety of enzymes essential to bacterial growth.
Penicillin, one of the most widely used antibiotics in the world, was fortuitously discovered by Alexander Fleming in , when he noticed antibacterial properties in a mold growing on a bacterial culture plate. In , Ernst Chain and Howard Florey began an intensive effort to isolate penicillin from the mold and study its properties. The large quantities of penicillin needed for this research became available through development of a corn-based nutrient medium that the mold loved and through the discovery of a higher-yielding strain of mold at a United States Department of Agriculture research center near Peoria, Illinois.
Even so, it was not until that large quantities of penicillin were being produced and made available for the treatment of bacterial infections.
Penicillin functions by interfering with the synthesis of cell walls of reproducing bacteria. It does so by inhibiting an enzyme—transpeptidase—that catalyzes the last step in bacterial cell-wall biosynthesis. The defective walls cause bacterial cells to burst. Human cells are not affected because they have cell membranes, not cell walls. Several naturally occurring penicillins have been isolated. They are distinguished by different R groups connected to a common structure: a four-member cyclic amide called a lactam ring fused to a five-member ring.
The addition of appropriate organic compounds to the culture medium leads to the production of the different kinds of penicillin. They are effective in the treatment of diphtheria, gonorrhea, pneumonia, syphilis, many pus infections, and certain types of boils.
Penicillin G was the earliest penicillin to be used on a wide scale. However, it cannot be administered orally because it is quite unstable; the acidic pH of the stomach converts it to an inactive derivative. The major oral penicillins—penicillin V, ampicillin, and amoxicillin—on the other hand, are acid stable. Some strains of bacteria become resistant to penicillin through a mutation that allows them to synthesize an enzyme—penicillinase—that breaks the antibiotic down by cleavage of the amide linkage in the lactam ring.
To combat these strains, scientists have synthesized penicillin analogs such as methicillin that are not inactivated by penicillinase.
Their allergic reaction can be so severe that a fatal coma may occur if penicillin is inadvertently administered to them.
Fortunately, several other antibiotics have been discovered. Most, including aureomycin and streptomycin, are the products of microbial synthesis. Others, such as the semisynthetic penicillins and tetracyclines, are made by chemical modifications of antibiotics; and some, like chloramphenicol, are manufactured entirely by chemical synthesis. They are as effective as penicillin in destroying infectious microorganisms.
Many of these antibiotics exert their effects by blocking protein synthesis in microorganisms. Initially, antibiotics were considered miracle drugs, substantially reducing the number of deaths from blood poisoning, pneumonia, and other infectious diseases.
Some seven decades ago, a person with a major infection almost always died. Today, such deaths are rare. Seven decades ago, pneumonia was a dreaded killer of people of all ages. Today, it kills only the very old or those ill from other causes. Antibiotics have indeed worked miracles in our time, but even miracle drugs have limitations. Not long after the drugs were first used, disease organisms began to develop strains resistant to them.
In a race to stay ahead of resistant bacterial strains, scientists continue to seek new antibiotics. The penicillins have now been partially displaced by related compounds, such as the cephalosporins and vancomycin. Unfortunately, some strains of bacteria have already shown resistance to these antibiotics. Some reversible inhibitors are noncompetitive.
A noncompetitive inhibitor can combine with either the free enzyme or the enzyme-substrate complex because its binding site on the enzyme is distinct from the active site.
Binding of this kind of inhibitor alters the three-dimensional conformation of the enzyme, changing the configuration of the active site with one of two results. Either the enzyme-substrate complex does not form at its normal rate, or, once formed, it does not yield products at the normal rate. Because the inhibitor does not structurally resemble the substrate, the addition of excess substrate does not reverse the inhibitory effect. Feedback inhibition is a normal biochemical process that makes use of noncompetitive inhibitors to control some enzymatic activity.
In this process, the final product inhibits the enzyme that catalyzes the first step in a series of reactions. Feedback inhibition is used to regulate the synthesis of many amino acids. For example, bacteria synthesize isoleucine from threonine in a series of five enzyme-catalyzed steps. As the concentration of isoleucine increases, some of it binds as a noncompetitive inhibitor to the first enzyme of the series threonine deaminase , thus bringing about a decrease in the amount of isoleucine being formed Figure 9.
Threonine deaminase is the first enzyme in the conversion of threonine to isoleucine. Isoleucine inhibits threonine deaminase through feedback inhibition. What are the characteristics of an irreversible inhibitor? In what ways does a competitive inhibitor differ from a noncompetitive inhibitor? It inactivates an enzyme by bonding covalently to a particular group at the active site. A competitive inhibitor structurally resembles the substrate for a given enzyme and competes with the substrate for binding at the active site of the enzyme.
A noncompetitive inhibitor binds at a site distinct from the active site and can bind to either the free enzyme or the enzyme-substrate complex. What amino acid is present in the active site of all enzymes that are irreversibly inhibited by nerve gases such as DIFP? Would you expect oxaloacetate to be a competitive or noncompetitive inhibitor? Many enzymes are simple proteins consisting entirely of one or more amino acid chains.
There are two types of cofactors: inorganic ions [e. Most coenzymes are vitamins or are derived from vitamins. Vitamins are organic compounds that are essential in very small trace amounts for the maintenance of normal metabolism. They generally cannot be synthesized at adequate levels by the body and must be obtained from the diet. The absence or shortage of a vitamin may result in a vitamin-deficiency disease.
In the first half of the 20th century, a major focus of biochemistry was the identification, isolation, and characterization of vitamins. Despite accumulating evidence that people needed more than just carbohydrates, fats, and proteins in their diets for normal growth and health, it was not until the early s that research established the need for trace nutrients in the diet.
Because organisms differ in their synthetic abilities, a substance that is a vitamin for one species may not be so for another. Over the past years, scientists have identified and isolated 13 vitamins required in the human diet and have divided them into two broad categories: the fat-soluble vitamins , which include vitamins A, D, E, and K, and the water-soluble vitamins , which are the B complex vitamins and vitamin C.
All fat-soluble vitamins contain a high proportion of hydrocarbon structural components. There are one or two oxygen atoms present, but the compounds as a whole are nonpolar. In contrast, water-soluble vitamins contain large numbers of electronegative oxygen and nitrogen atoms, which can engage in hydrogen bonding with water.
Most water-soluble vitamins act as coenzymes or are required for the synthesis of coenzymes. The fat-soluble vitamins are important for a variety of physiological functions.
The key vitamins and their functions are found in Table 9. Antioxidants prevent damage from free radicals, which are molecules that are highly reactive because they have unpaired electrons. Free radicals are formed not only through metabolic reactions involving oxygen but also by such environmental factors as radiation and pollution. Free radicals react most commonly react with lipoproteins and unsaturated fatty acids in cell membranes, removing an electron from those molecules and thus generating a new free radical.
The process becomes a chain reaction that finally leads to the oxidative degradation of the affected compounds. The second type of reversible inhibition is known as non-competitive inhibition. Here a molecule binds to the enzyme at a location other than the active site and either slows the rate of reaction, as shown on the right, or completely distorts the active site and renders the enzyme inactive. Inhibitors that bind to the enzyme and drastically alter the shape of the active site are known as allosteric inhibitors as pictured on the right.
Irreversible forms of inhibition include denaturing, which is mentioned above, and the irreversible binding of poisons to the enzyme. Binding of poisons, such as nerve gas , impairs the function of the enzyme. Aspirin reacts chemically with a serine amino acid in the active site. It covalently attaches an acetyl group to this amino acid and permanently changes the active site. Covalent bonds are strong and form permanent links. Another medication called Ibuprofen also inhibits prostaglandin synthase.
Unlike Aspirin, Ibuprofen is reversible. Reversible inhibitors are at equilibrium with the enzyme as shown by the equation below. How do allosteric inhibitors differ from other non-competitive inhibitors? Ibuprofen is not as effective as Aspirin in pain relief. Explain why.
Why does heating denature the protein but does not break the amino acids apart? A specific chemical substrate matches this site like a jigsaw puzzle piece and makes the enzyme specific to its substrate.
Increasing the environmental temperature generally increases reaction rates because the molecules are moving more quickly and are more likely to come into contact with each other. However, increasing or decreasing the temperature outside of an optimal range can affect chemical bonds within the enzyme and change its shape. If the enzyme changes shape, the active site may no longer bind to the appropriate substrate and the rate of reaction will decrease.
Dramatic changes to the temperature and pH will eventually cause enzymes to denature. This model asserted that the enzyme and substrate fit together perfectly in one instantaneous step. However, current research supports a more refined view called induced fit.
Induced Fit : According to the induced fit model, both enzyme and substrate undergo dynamic conformational changes upon binding. The enzyme contorts the substrate into its transition state, thereby increasing the rate of the reaction. When an enzyme binds its substrate, it forms an enzyme-substrate complex.
This complex lowers the activation energy of the reaction and promotes its rapid progression by providing certain ions or chemical groups that actually form covalent bonds with molecules as a necessary step of the reaction process.
Enzymes also promote chemical reactions by bringing substrates together in an optimal orientation, lining up the atoms and bonds of one molecule with the atoms and bonds of the other molecule. This can contort the substrate molecules and facilitate bond-breaking. The active site of an enzyme also creates an ideal environment, such as a slightly acidic or non-polar environment, for the reaction to occur.
The enzyme will always return to its original state at the completion of the reaction. One of the important properties of enzymes is that they remain ultimately unchanged by the reactions they catalyze.
After an enzyme is done catalyzing a reaction, it releases its products substrates. Privacy Policy. Skip to main content. Search for:.
Control of Metabolism Through Enzyme Regulation Cells regulate their biochemical processes by inhibiting or activating enzymes. Learning Objectives Explain the effect of an enzyme on chemical equilibrium. In noncompetitive inhibition also known as allosteric inhibition , an inhibitor binds to an allosteric site; the substrate can still bind to the enzyme, but the enzyme is no longer in optimal position to catalyze the reaction.
Feedback inhibition involves the use of a reaction product to regulate its own further production. Inorganic cofactors and organic coenzymes promote optimal enzyme orientation and function.
Vitamins act as coenzymes or precursors to coenzymes and are necessary for enzymes to function. Key Terms coenzyme : An organic molecule that is necessary for an enzyme to function. Enzyme Active Site and Substrate Specificity Enzymes catalyze chemical reactions by lowering activation energy barriers and converting substrate molecules to products.
0コメント