Download Chapter 1 • Lesson 4 Objectives 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 threedimensional 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?