Activation Energy All chemical reactions require energy for them to start. This energy is called activation energy (E A

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1 Enzymes Enzymes are protein molecules that speed up the chemical reactions of organisms. Without enzymes, most of the reactions in a cell would proceed too slowly to maintain life. When an enzyme is used to speed up a reaction, the reactants are converted into products faster than they would be without the enzyme. At the end of the reaction, the enzyme remains undamaged, ready to catalyze another reaction. Activation Energy All chemical reactions require energy for them to start. This energy is called activation energy (E A ). Enzymes speed up reactions by lowering the activation energy needed to start a reaction (Figure 1). The most common source of activation energy is heat. An increase in temperature will increase the speed of most reactions. owever, proteins are denatured at high temperatures and lose their function. This could be devastating for a cell. Living cells cannot rely on high levels of heat as a source of activation energy; instead, cells make and use enzymes that allow reactions to proceed at speeds that keep the cell alive activation energy the energy required for a chemical reaction to occur E A with enzyme E A without enzyme Energy reactants products Progress of the Reaction Figure 1 An enzyme speeds up a reaction by lowering the activation energy (E A ). The names of enzymes usually end in -ase. Thus, amylase catalyzes the breakdown of amylose into maltose subunits, and maltase catalyzes the breakdown of maltose into individual glucose molecules. An enzymecatalyzed reaction is usually written with the name of the enzyme over the arrow, as follows: amylase amylose maltase individual maltose molecules maltose individual glucose molecules Cellular Biology 51

2 substrate the reactant that an enzyme acts on when it catalyzes a chemical reaction site the location where the substrate binds to an enzyme In order to catalyze a reaction, an enzyme attaches to the reactants. We call the reactants substrates. Substrates bind to a very small portion of an enzyme. The location where a substrate binds to the enzyme is called the site, and is usually a pocket or groove in the three-dimensional structure of the protein (Figure 2). induced-fit model a model of enzyme activity that describes an enzyme as a protein molecule that changes shape to better accommodate the substrate enzyme substrate complex an enzyme with its substrate attached to the site Figure 2 The binding of a substrate to the site of an enzyme Figure 3 The enzyme maltase catalyzes the breakdown of maltose into two separate glucose molecules by breaking the bond between the individual sugar molecules. The site is ready to receive the substrate. The substrate enters the site, and the enzyme s shape changes to better fit the substrate (induced fit). (c) The bond between the two glucose molecules is broken while the substrate is in the site. (d) Breaking the bond changes the shape of the protein again; the protein loses its attraction for the product molecules and releases them. The site now becomes available for another maltose to attach. The recycling of enzyme molecules causes cells to catalyze many reactions with relatively small numbers of enzymes. Enzymes are very specific for the types of substrates they attach. In most cases, they will bind to only one type of molecule. This means that a different enzyme is needed for every reaction. The substrate and the site must possess well-matched shapes for binding to occur. As the substrate enters the site, the protein changes shape to better fit the substrate. This is known as the induced-fit model of enzyme activity. The attachment of the substrate to the enzyme s site creates the enzyme substrate complex (Figure 3). enzyme's site (d) maltase (enzyme) maltose (substrate) (c) 52 Unit 1

3 Section 1.12 TRY TIS activity Materials: coloured paper clips, 5-cm 21.5-cm strip of paper 1. Prepare the strip of paper as in Figure Fold the strip of paper as in Figure Place paper-clip substrate 1 on site 1, spanning the backtwo layers of the paper enzyme. Place paperclip substrate 2 on site 2, spanning the front two layers of the paper enzyme, as in Figure 4(c). 4. Briskly pull the two tabs apart to activate the paper enzyme, as illustrated in Figure 4(d). Explain how the action of the paper enzyme relates to a real enzyme-catalyzed reaction. Try to produce a triclipide or a tetraclipide with one pull of the tabs. (c) Create a different enzyme simulation. Synthesizing a Paper-Clip Polymer with a Paper Enzyme (d) 3 cm site 1 site cm site 2 site 1 and the Environment Temperature and p have an effect on an enzyme s activity. As with all other reactions, enzyme-catalyzed reactions increase in speed with an increase in temperature. owever, as the temperature increases beyond a particular point, proteins begin to denature and the enzyme becomes less effective. Every enzyme has an optimal temperature at which it works best (Figure 5). fold site 2 fold site 2 8 cm 5.5 cm 8 cm (c) Figure 4 site 1 3 cm site 2 5 cm typical human enzyme enzyme of heattolerant organism optimal p for enzyme A optimal p for enzyme B Temperature ( C) p Figure 5 Enzymes have an optimal temperature at which they workbest. A typical human enzyme (red line) works best at approximately 37 C; the heat-tolerant organism enzyme works best at approximately 76 C. Enzymes have optimal p levels. Enzyme A (pepsin) works best at p 2; enzyme B (trypsin) works best at p 8. Cellular Biology 53

4 When temperatures are lower than the optimal temperature, the motion of the enzyme and the substrates is sluggish and reaction speed is reduced. Most human enzymes work best at around 37 C, normal body temperature. Enzymes also have an optimal p in which they work best (Figure 5, on the previous page). The digestive enzyme pepsin works best in the acidic environment of the stomach, p 2. The digestive enzyme trypsin has an optimal p of 8 and works best in the basic environment of the small intestine. Figure 6 Enzymes from yeast cells are used in baking, brewing, winemaking, and cheese manufacturing. Figure 7 Glucose is used as a sweetener in many food products. Figure 8 Enzymes are commonly added to detergents to help remove food stains from fabrics. This warning label shows that an ingredient in this detergent is enzymes. Industrial Uses of Enzymes In addition to the vital role they play in living cells, enzymes have many practical uses. In brewing, baking, and winemaking (Figure 6), enzymes produced by yeast cells catalyze the conversion of glucose (in flour starch or fruit juices) to ethanol and carbon dioxide gas. Ethanol and other compounds give beer and wine their characteristic flavours. In baked goods, the ethanol evaporates away during the heating process, and the carbon dioxide gas creates bubbles that give breads and cakes their spongy texture. Some of the largest industrial users of enzymes are companies that convert starch from corn, wheat, and barley into glucose. Glucose is used as a sweetener in many foods and beverages, such as candy, biscuits, jams and jellies, sweetened fruit juices, and vitamin preparations (Figure 7). In the production of glucose, large quantities of starch are broken down by the enzymes amylase and maltase, which are produced by certain bacteria and moulds (fungi). The glucose may be used directly as a sweetener or may be converted to fructose (a much sweeter sugar) by the action of the enzyme glucose isomerase that is produced by certain species of bacteria. Enzymes are also used in the cleaning industry. Stains commonly found on clothing, including blood, grass, milk, and perspiration, are usually mixtures of proteins, carbohydrates, and lipids. Although it is possible to remove these stains with soaps and detergents alone, detergent companies sometimes add a number of different amylases, proteases, and lipases to their products to help remove the tough protein, carbohydrate, and lipid stains from fabrics (Figure 8). Enzymes also allow stains to be removed at lower temperatures, and with less mechanical agitation in a washing machine. Table 1 lists some additional industrial uses of enzymes. Table 1 Some Industrial Uses of Enzymes Product or process ethanol fuel dairy cloth garments leather paper Enzyme function Enzymes convert starch into glucose and glucose into ethanol. Enzymes breakdown the lactose in milkinto glucose and galactose to produce lactose-free milkfor people allergic to lactose. Cellulases are used to soften cotton garments such as blue jeans. Enzymes are used to remove fat and hair from hides. Enzymes are used to de-inkpaper in recycling programs. 54 Unit 1

5 Section 1.12 Section 1.12 Questions Understanding Concepts 1. ow does an enzyme affect the activation energy of the chemical reaction it catalyzes? Why can a living cell not use large amounts of heat to speed up its chemical reactions? 2. Use diagrams to illustrate the induced-fit model of enzyme activity. Applying Inquiry Skills 3. An enzyme called polyphenol oxidase causes the sliced surface of some fruits to become brown when exposed to air. This reaction is called the browning reaction, and it occurs when you cut an apple or potato and leave it exposed for a while. Describe an experiment you could conduct to determine how temperature affects the browning reaction. 4. The enzyme amylase catalyzes the breakdown of amylose to maltose. The graph in Figure 9 was obtained in an experiment in which amylose was exposed to amylase in environments of various p p Provide a hypothesis for the low enzyme activity at p 10. What is the optimal p for amylase activity? Explain. Making Connections 5. Describe two industrial uses of enzymes, one in the food-making sector, the other in the cleaning sector. Why would a large diaper-cleaning company be interested in a new cleaning powder containing enzymes even though the new powder is slightly more expensive than regular detergent powder? G 6. Papain and bromelain are the two most commonly used enzymes in commercial meat tenderizers. Conduct research to answer the following questions about these enzymes: What are meat tenderizers? What are they used for? What type of enzymes are papain and bromelain? (What is their substrate?) (c) What is the source of papain and bromelain found in commercial meat tenderizer preparations? (d) The antemortem (before death) method of tenderizing meat involves the physical injection of a solution of papain or bromelain into the living animal. The enzyme tenderizes muscle tissue while the animal is alive. Discuss this method with fellow classmates and write a brief position paper on the ethics of this procedure. G Figure 9 Cellular Biology 55