Chapter 1: Cell Biology Lesson 4

Chapter 1 • Lesson 4 Objectives 4.1.3 Enzymes Key Terms • reactant • product • catalyst • enzyme • substrate • activation energy Getting the Idea All the activities and processes that take place in cells—growth, reproduction, interaction with the environment, and responses to stimuli—are the result of chemical reactions. These reactions require energy and specific conditions in order to occur quickly and efficiently. Cells use a special class of proteins to make many of these reactions possible. Characteristics of Chemical Reactions A chemical reaction is a change in the arrangement of atoms or molecules that yields, or produces, different substances. All chemical reactions involve reactants and products. Reactants are the atoms, molecules, or compounds that enter into a chemical reaction. The products are the atoms, molecules, or compounds that result from the reaction. During a chemical reaction, the chemical bonds in one or more reactants are broken, and new bonds are formed in one or more products. The chemical reaction shown below summarizes photosynthesis: This equation shows how plants use the energy in sunlight to convert carbon dioxide and water into glucose and oxygen gas. In cellular respiration, the reaction is reversed: During cellular respiration, organisms break down glucose and oxygen to produce water, carbon dioxide, and energy in the form of ATP. You will learn more about these biochemical reactions in Lesson 7. Biochemical Catalysts The biochemical reactions in living things must occur at certain speeds, or rates, in order to be useful. The rate of a chemical reaction depends on several factors, such as temperature, concentration of the chemicals, and surface area. If all these factors are held constant, adding a catalyst can speed a chemical reaction. A catalyst is a substance that increases the rate of a chemical reaction without itself being changed by the reaction. Most catalysts of biochemical reactions are enzymes. Recall that an enzyme is a protein that serves as a catalyst, enabling or speeding chemical reactions in cells. It makes those reactions possible, or speeds them up, without being changed by the reaction. Without enzymes, many of the chemical reactions in organisms could not happen. Others would be much too slow for the organism to survive. Enzymes give reactants a site where they can come together to form a new compound. The reactants in enzyme-catalyzed reactions are called substrates. Like all proteins, enzymes have characteristic three-dimensional shapes. Recall from the last lesson that this shape depends on the amino acids that make up the protein and on how the chains of amino acids are folded. The distinct shape of the surface of an enzyme allows it to interact only with certain substrates. A single organism may have thousands of different enzymes. Each is specific to one reaction. The part of an enzyme to which a substrate binds is called the active site. The three-dimensional shapes of the substrate and the active site are complementary, or opposite. As a result, an enzyme and its substrate fit together much like a lock and key. The enzyme and substrates are held together by intermolecular forces. Together, they form an enzyme-substrate complex. The enzyme and substrates in the complex stay bound until the reaction is complete. When the reaction ends, the products separate from the active site. The enzyme, which is unchanged, is then available to repeat the process. It is not used up. Activation Energy For a chemical reaction to occur, reactants must come together with enough energy to break existing bonds and form new ones. Enzymes speed reactions by reducing the amount of energy needed. In technical terms, they reduce the activation energy of the reaction. The activation energy of a reaction is the minimum amount of energy needed for colliding molecules to react. A graph can be used to compare the energy levels of the reactants and products. The peak represents the activation energy for the reaction. The graph below compares the activation energy of a reaction with and without an enzyme as a catalyst. When the activation energy is lowered, more reactant particles have enough energy to react. Enzyme activity can be affected by any factors that influence chemical reactions. One important factor is temperature. Enzymes of different organisms tend to work best at different temperatures. The optimal temperature in humans, for example, is about 37°C, which is normal body temperature. The optimum temperature in most plants is about 25°C. Enzymes are generally destroyed at temperatures above 50°C. Another factor that affects reaction rates is pH. The pH of a solution describes how acidic or basic the solution is. Most enzymes function in narrow pH ranges, which depend on the enzyme and its job. For most reactions, the optimal pH is close to 7, which is neutral—that is, not acidic or basic. Low (acidic) or high (basic) pH values tend to inhibit enzyme activity. The concentration of enzyme and substrate molecules also affects enzyme activity. Consider a reaction for which the number of enzyme molecules is fixed. If there are more substrate molecules than enzyme molecules, the reaction rate will rise until all the enzyme molecules are attached to substrate molecules. The reaction rate will then level off. The same is true if there are more enzyme molecules than substrate molecules. The reaction rate will rise and then level off when all the substrate molecules are reacting. Important Enzyme-Catalyzed Reactions Enzymes are critical to proper cell function. The cells in your body contain thousands of enzymes that catalyze different chemical reactions. All these cells are alike in some ways. For example, they all have some of the same enzymes, such as those that catalyze the breakdown of glucose to release energy. However, some enzymes are found only in certain kinds of cells. Your nerve cells, for example, have enzymes that produce neurotransmitters, the chemicals that carry impulses from nerves to muscles. Muscle cells, by contrast, have receptor sites on their membranes that trigger enzyme actions in response to those neurotransmitters. Carbon dioxide in your blood combines with water to form carbonic acid. This reaction is so slow that if it were not catalyzed, carbon dioxide would build up in your bloodstream, which could be fatal. However, the blood contains carbonic anhydrase. This enzyme makes the reaction about 1 million times faster than it would be without the enzyme. As a result, carbon dioxide, which is toxic in high concentrations, is quickly removed from the blood. Cells in your pancreas produce an enzyme called lipase. Lipase speeds the digestion of lipids. Another important enzyme in cells is RNA polymerase. RNA is a nucleic acid needed for the production of proteins. RNA polymerase speeds the transcription stage of this process. You will read more about protein production and transcription in Lesson 21. An inability to make some enzymes can lead to disease or death. Tay-Sachs disease is a genetic disease that affects the human brain. In this disease, the cells do not produce an enzyme that helps break down lipids. Over time, the buildup of lipids in and around brain cells causes blindness, seizures, and usually death. A genetic disease called Cori disease causes a form of muscular dystrophy. Individuals with Cori disease lack the debrancher enzyme. This enzyme helps the body break down glycogen, which is stored in the muscles and liver. Lack of the enzyme interferes with the body's ability to break down glycogen for energy. Cori disease may first appear in childhood. In children, the disease causes weakness, slow growth, seizures, and often death. Focus on Inquiry You know that scientists learn from other scientists. They read published articles, news reports, reference books, and other materials. Scientists also need to understand and explain the research of other people. Many university science departments have journal clubs. The members read published papers and then present oral reports on the work described in the papers. Phenylketonuria (PKU) and Pompe disease are diseases caused by enzyme deficiencies. Do some library or Internet research to find out more about PKU, Pompe disease, or another enzyme deficiency disease. You should be able to describe the disease and answer the questions below. Disease researched: _____________________________ How does the disease affect the body? What enzyme deficiency causes the disease? What is that enzyme's job in the body? How is the disease treated?