Enzymes. Enzymes accelerate chemical reactions as the engine accelerates this drag race.

Transcription

1 Chapter 30 Enzymes Enzymes accelerate chemical reactions as the engine accelerates this drag race. Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena

2 Course 30.1 Molecular Accelerators 30.2 Rates of Chemical Reactions 30.3 Enzyme Kinetics 30.4 Industrial-Strength Enzymes 30.5 Enzyme Active Site 30.6 Temperature and ph Effects on Enzyme Catalysis 30.7 Enzyme Regulation Chapter 30 Summary 2

3 Molecular Accelerators Enzymes are molecules that catalyze biochemical reactions and a large majority of these catalysts are proteins. Enzymes catalyze nearly all the myriad reactions that occur in living cells. Enzymes are essential to life. In the absence of enzymes biochemical reactions occur too slowly to maintain life. The typical biochemical reaction occurs more than a million times faster when catalyzed by an enzyme. 3

4 Molecular Accelerators In biological cells enzymes lower the activation energy and enable reactions to occur rapidly. Here is the reaction-energy profile of sucrose with oxygen that is catalyzed (in a cell) and uncatalyzed (in a sugar bowl). 4

5 Molecular Accelerators Each organism contains thousands of enzymes. Some are simple proteins consisting only of amino acid units. Others are conjugated and consist of a protein part, or apoenzyme, and a nonprotein part, or coenzyme. Both parts are essential. A functioning enzyme that consists of both the protein and nonprotein parts is called a holoenzyme. 5

6 Molecular Accelerators For some enzymes an inorganic component such as a metal ion (e.g., Ca 2+, Mg 2+ or Zn 2+ ) is required. This inorganic component is an activator. From the standpoint of function, an activator is analogous to a coenzyme, but inorganic components are not called coenzymes. 6

7 Molecular Accelerators Another remarkable property of enzymes is their specificity. That means that a certain enzyme catalyzes the reaction of a specific type of substance. For example each of these similar reactions requires a specific enzyme. 7

8 Molecular Accelerators The substance acted on by an enzyme is called the substrate. Enzymes are named by adding the suffix -ase to the root of the substrate name. Note the derivations of maltase, sucrase, and lactase from maltose, sucrose, and lactose. 8

9 Your Turn! An enzyme that converts the amino acid serine to phosphoserine would be called which of the following. Serinase Serine decarboxylase Serine dehydrogenase Serine kinase Serine phosphatase 9

10 Your Turn! Serine phosphatase The names of enzymes generally refer to the types of reactions they catalyze and the substrates they act on. So, of the names listed, your best guess would be that the enzyme serine phosphatase catalyzes the reaction that converts serine to phosphoserine. 10

11 Your Turn! What can be deduced about the reaction catalyzed by the enzyme lysine decarboxylase? The enzyme adds a CO 2 group to lysine The enzyme removes a CO 2 group from lysine The enzyme oxidizes lysine The enzyme reduces lysine The enzyme hydrolyzes lysine 11

12 Your Turn! The enzyme removes a CO 2 group from lysine The name of the enzyme is lysine decarboxylate. The lysine part of the name indicates that the enzyme acts on lysine. The prefix de- in the name of an enzyme generally means that something is being removed during the chemical reaction. The carbox refers to carbon dioxide (CO 2 ). So, a decarboxylase is an enzyme that removes carbon dioxide. 12

13 Molecular Accelerators In the International Union of Biochemistry (IUB) System, enzymes are assigned to one of six classes based on the reactions they catalyze. 1. Oxidoreductases: Enzymes that catalyze the oxidation reduction reactions between two substrates. 2. Transferases: Enzymes that catalyze the transfer of a functional group between two substrates. 3. Hydrolases: Enzymes that catalyze the hydrolysis of esters, carbohydrates, and proteins (polypeptides). 13

14 Molecular Accelerators 4. Lyases: Enzymes that catalyze the removal of groups from substrates by mechanisms other than hydrolysis. 5. Isomerases: Enzymes that catalyze the interconversion of stereoisomers and structural isomers. 6. Ligases: Enzymes that catalyze the linking of two compounds by breaking a phosphate anhydride bond in adenosine triphosphate (ATP). 14

15 Your Turn! What class of enzyme would catalyze the following reaction? COO - COO - O H C OH H C O P O - H C H H C H O - O O O P O - H O- 3-phosphoglycerate 2-phosphoglycerate 15

16 Your Turn! The two molecules have the same molecular formula and are structural isomers of each other. The enzyme that would catalyze this reaction is an isomerase. COO - COO - O H C OH H C O P O - H C H H C H O - O O O P O - H O- 3-phosphoglycerate 2-phosphoglycerate 16

17 Rates of Chemical Reactions Enzymes catalyze biochemical reactions and thus increase the rates of these chemical reactions. Every chemical reaction starts with at least one reactant and finishes with a minimum of one product. As the reaction proceeds, the reactant concentration decreases and the product concentration increases... 17

18 Rates of Chemical Reactions We can plot these changes as a function of time as shown for the hypothetical conversion of reactant A into product B. 18

19 Rates of Chemical Reactions A reaction rate is defined as a change in concentration with time. This is the rate at which the reactants of a chemical reaction disappear and the products form. 19

20 Your Turn! Calculate the reaction rate of the appearance of product D with the following reaction data. C D Reaction Time (hr) D Concentration (M)

21 Your Turn! Reaction Time (hr) D Concentration (M) Total change in concentration of D (0.0 M to 6.0 M) Rate of appearance of D = = Total change in time (0.0 hr to 4.0 hr) 6.0 M 4.0 hr = 1.5 M of D hr 21

22 Rates of Chemical Reactions The reactant must pass through a high-energy transition state to be converted into a product. This transition state is an unstable structure with characteristics of both the reactant and the product. The energy necessary to move a reactant to the transition state is called the activation energy. The larger this energy barrier is the slower the reaction rate will be. 22

23 Rates of Chemical Reactions This is an energy profile for the reaction between water and carbon dioxide showing the transition state. 23

24 Rates of Chemical Reactions There are three common ways to increase a reaction rate. Increasing the reactant concentration Increasing the reaction temperature Adding a catalyst 24

25 Your Turn! For the reaction of carbon dioxide with water predict which set of conditions 1) or 2) will yield a faster reaction. 1) CO 2 pressure = 100 torr, T = 37 C, Activation energy = 31 kcal/mol. versus 2) CO 2 pressure = 150 torr, T = 37 C, Activation energy = 31 kcal/mol. 25

26 Your Turn! 1) CO 2 pressure = 100 torr, T = 37 C, Activation energy = 31 kcal/mol. versus 2) CO 2 pressure = 150 torr, T = 37 C, Activation energy = 31 kcal/mol. Condition 2) will yield a faster reaction because the CO 2 pressure (concentration) is greater while the other conditions remain constant. 26

27 Your Turn! For the reaction of carbon dioxide with water predict which set of conditions 1) or 2) will yield a faster reaction. 1) CO 2 pressure = 100 torr, T = 37 C, Activation energy = 31 kcal/mol. versus 2) CO 2 pressure = 100 torr, T = 37 C, Activation energy = 28 kcal/mol. 27

28 Your Turn! 1) CO 2 pressure = 100 torr, T = 37 C, Activation energy = 31 kcal/mol. versus 2) CO 2 pressure = 100 torr, T = 37 C, Activation energy = 28 kcal/mol. Condition 2) will yield a faster reaction because the activation energy is lower while other conditions remain constant. 28

29 Enzyme Kinetics Two German researchers, Leonor Michaelis and Maud Menten measured enzyme-catalyzed reaction rates as a function of substrate (reactant) concentration. They observed that most enzyme-catalyzed reactions show an increasing rate with increasing substrate concentration, but only to a specific maximum velocity, V max. 29

30 Enzyme Kinetics A Michaelis Menten plot showing the rate of an enzymecatalyzed reaction as a function of substrate concentration is shown here. Notice that the enzyme has limited enzyme capacity. The rate of catalysis doesn t continue to increase with substrate concentration but reaches a maximum (V max ). 30

31 Enzyme Kinetics Michaelis Menten plots for two glucose metabolic enzymes. Kexokinase has a stronger attraction for glucose than glucokinase. 31

32 Enzyme Kinetics An enzyme s catalytic speed is also matched to an organism s metabolic needs. This catalytic speed is commonly measured as a turnover number which is the number of molecules an enzyme can convert, or turn over, in a given time span. Turnover number is a convenient way to compare enzymes to each other or to the effect of reaction conditions. 32

33 Your Turn! The enzyme lactase can break down 1197 molecules of lactose in 7 minutes. Calculate the turnover number per minute. 33

34 Your Turn! The enzyme lactase can break down 1197 molecules of lactose in 7 minutes. Calculate the turnover number per minute lactose molecules 7 minutes = 171 min 34

35 Industrial-Strength Enzymes Not only are enzymes important in biology, they are increasingly important in industry. About 75% of industrial enzymes have a digestive or breakdown function. They are hydrolases but are known by common names... 35

36 Industrial-Strength Enzymes Proteases (proteolytic enzymes) break down proteins and comprise about 40% of all industrial enzymes. Lipases digest lipids. Cellulases, amylases, xylanases, lactases, and pectinases break down cellulose, amylose, xylans, lactose, and pectin, respectively. 36

37 Industrial-Strength Enzymes Enzymes have long been used in food processing. For example. Amylases and other carbohydrate hydrolases act to soften the dough. About 25% of all industrial enzymes are used to convert cornstarch into syrups. The plant products used in animal feed are also commonly treated with industrial enzymes to make them 37 more nutritious.

38 Industrial-Strength Enzymes Industrial enzymes offer solutions to environmental pollution problems for some industries. A number of industries use cellulases instead of strong base to give a soft appearance to denim. 38

39 Industrial-Strength Enzymes Industrial enzymes are used in consumer goods like detergent additives. About 37% of all industrial enzymes are found in laundry detergents. Proteases digest clothing stains like grass, blood, and sweat. Lipases hydrolyze the fats. Amylases digest starchy residues. 39

40 Industrial-Strength Enzymes Enzymes are used in medicine primarily because of their specificity. Several proteases are used to dissolve blood clots in patients with such diseases as lung embolism (clot in the lung), stroke (clot in the brain), and heart attack (clot in the heart). 40

41 Your Turn! Explain why cellulases are not used to soften nylon fabrics. 41

42 Your Turn! Explain why cellulases are not used to soften nylon fabrics. Cellulases soften fabrics that contain cellulose such as cotton fabrics. Nylon doesn t contain cellulose. Nylon is a man-made polyamide. 42

43 Enzyme Active Site Catalysis takes place on a small portion of the enzyme structure called the enzyme active site. A space-filling model of the enzyme hexokinase is below. The substrate glucose enters the site and binds. The enzyme changes its shape before the reaction takes place. 43

44 Enzyme Active Site An enzyme must attract and bind the substrate. Once the substrate is bound, a chemical reaction is catalyzed. This two-step process is as follows. Enzyme (E) and substrate (S) combine to form an enzyme substrate intermediate (E S). The intermediate decomposes to give the product (P) and the enzyme. 44

45 Enzyme Active Site Each different enzyme has its own unique active site whose shape determines which substrates can bind. Enzymes are said to be stereospecific. Each enzyme catalyzes reactions for only a limited number of different reactant structures. Enzyme-substrate interaction is explained by two hypotheses. The lock-and-key hypothesis The induced-fit hypothesis 45

46 Enzyme Active Site The lock-and-key hypothesis envisions the substrate as a key that fits into the appropriate active site (the lock). The induced-fit model proposes that the active site adjusts its structure before the reaction can take place. Both of these hypotheses are demonstrated by the figure on the next slide... 46

47 Enzyme Active Site The correct substrate fits the active site (lock-and-key hypothesis). This substrate also causes an enzyme conformation change that positions a catalytic group (*) to cleave the appropriate bond (induced-fit model). 47

48 Enzyme Active Site Three additional ideas about enzyme-substrate interaction and catalysis are as follows. Proximity catalysis refers to an enzyme bringing the reactants close together. The productive binding hypothesis explains how the enzyme works to make sure that the correct bonds are broken and formed during the reaction. 48

49 Enzyme Active Site The strain hypothesis explains how the enzyme forces the substrate to change shape which allows catalysis to occur as shown in this figure. 49

50 Temperature and ph Effects on Enzyme Catalysis Chapter Essentially any change that affects protein structure also affects an enzyme s catalytic function. Catalytic activity will be lost when an enzyme is denatured. Strong acids and bases, organic solvents, mechanical action, and high temperature decrease an enzymecatalyzed rate of reaction. Even slight changes in the ph can have profound effects on enzyme catalysis. 50

51 Temperature and ph Effects on Enzyme Catalysis Chapter Enzymes have optimal operating temperatures and phs. These graphs show how enzyme catalysis is affected by changes in temperature and ph for enzymes that operate most effectively at physiological conditions. 51

52 Your Turn! If you plot the reaction rate of an enzyme-catalyzed reaction on the vertical axis and the temperature of the reaction on the horizontal axis you will almost always get a straight line. True False 52

53 Your Turn! False Enzymes generally act optimally within a small temperature range. So, if you plot the reaction rate of an enzymecatalyzed reaction on the vertical axis and the temperature of the reaction on the horizontal axis you will almost never get a straight line. The type of graph you would get is shown here. 53

54 Enzyme Regulation Enzyme catalysis is carefully controlled in cells. Cells use a variety of mechanisms to change the rates of enzymecatalyzed reactions to meet metabolic needs. Sometimes a new group of atoms covalently bond to the enzyme in a process called covalent modification. In other cases, another molecule is noncovalently bound to the enzyme to affect catalytic activity. The protein structural change that results can cause a decrease in enzyme activity, enzyme inhibition, or an 54 increase in activity, enzyme activation.

55 Enzyme Regulation The product binding to the enzyme and inhibiting catalysis is called product inhibition. Feedback inhibition and feedforward activation are two common forms of enzyme control. Feedback inhibition affects enzymes at the beginning of the reaction assembly line. In feedback inhibition the final product inhibits the enzyme. 55

56 Enzyme Regulation Feedforward activation controls enzymes at the end of the molecular assembly line. Here an excess of starting materials will feedforward and activate enzymes. 56

57 Your Turn! In the reaction scheme below product D inhibits enzyme E 1. What type of enzyme regulation is this an example of? E 1 E 2 E 3 A B C D 57

58 Your Turn! This is an example of feedback inhibition because the product inhibits the overall reaction. E 1 E 2 E 3 A B C D 58

59 Chapter 30 Summary Enzymes are proteins that catalyze biochemical reactions. Some enzymes are conjugated proteins. The protein part is the apoenzyme and the nonprotein part is the coenzyme. The conjugated protein is termed a holoenzyme. The substance acted on by an enzyme is called the substrate. 59

60 Chapter 30 Summary An enzyme is commonly studied by measuring a reaction rate which is the change in concentration of reactants or products with time. The transitions state in a reaction is the highest energy point during a reaction. Three common ways to increase a reaction rate are to increase the reactant concentration, increase the reaction temperature, or add a catalyst. 60

61 Chapter 30 Summary Michaelis Menten plots show the rates of enzymecatalyzed reactions. The turnover number, the number of substrates an enzyme can react with in a given time span, is a measure of an enzyme s catalytic ability. Catalysis takes place on a small portion of the enzyme surface called the active site. 61

62 Chapter 30 Summary The lock-and-key hypothesis and the induced-fit hypothesis describe how substrates and enzymes interact. Many enzymes use proximity catalysis, productive binding, or strain to affect catalysis. Enzyme structure and function are effected by ph and temperature. Enzymes are regulated by enzyme inhibition and enzyme activation. 62