Enzymes as Catalysts
Jean Brainard, Ph.D.
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Printed: July 30, 2014
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
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Chapter 1. Enzymes as Catalysts
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Enzymes as Catalysts
Define enzyme.
Explain why chemical reactions in living cells require enzymes.
Describe how enzymes work.
Identify two human digestive enzymes.
An enzyme cleaner like the one pictured here contains proteins called enzymes. The enzymes attach themselves to
particular stains and help break them down. Different enzymes work on different types of stains, such as grease
stains or bloodstains. Removing stains isn’t the only use of enzymes. Enzymes are also essential to life.
Why Living Things Need Enzymes
Chemical reactions constantly occur inside the cells of living things. However, under the conditions inside cells,
most biochemical reactions would occur too slowly to maintain life. That’s where enzymes come in. Enzymes are
catalysts in living things. Like other catalysts, they speed up chemical reactions. Enzymes are proteins that are
synthesized in the cells that need them, based on instructions encoded in the cells’ DNA.
How Enzymes Work
Enzymes increase the rate of chemical reactions by reducing the amount of activation energy needed for reactants
to start reacting. One way this can happen is modeled in the Figure 1.1. Enzymes aren’t changed or used up in
the reactions they catalyze, so they can be used to speed up the same reaction over and over again. Each enzyme
is highly specific for the particular reaction is catalyzes, so enzymes are very effective. A reaction that would take
many years to occur without its enzyme might occur in a split second with the enzyme. Enzymes are also very
efficient, so waste products rarely form.
Q: This model of enzyme action is called the lock-and-key model. Explain why.
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FIGURE 1.1
A: The substrates (reactants) fit precisely into the active site of the enzyme like a key into a lock. Being brought
together in the enzyme in this way helps the reactants react more easily. After the product is formed, it is released
by the enzyme. The enzyme is now ready to pick up more reactants and catalyze another reaction. You can see an
animated version of this model at the following URL.
http://www.youtube.com/watch?v=V4OPO6JQLOE
MEDIA
Click image to the left for more content.
Digestive Enzymes
More than 1000 different enzymes are necessary for human life. Many enzymes are needed for the digestion of food.
Two examples are amylase and pepsin. Both are described in the Figure 1.2.
Summary
• Enzymes are catalysts in living things. They speed up biochemical reactions.
• Under the conditions inside living cells, biochemical reactions would occur too slowly to support life without
the help of enzymes.
• Enzymes increase the rate of biochemical reactions by reducing the amount of activation energy needed for
reactants to start reacting.
• More than 1000 different enzymes are necessary for human life, and many help digest food. Two examples
are amylase and pepsin.
Vocabulary
• enzyme: Biochemical catalyst that speeds up chemical reactions in living things.
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Chapter 1. Enzymes as Catalysts
FIGURE 1.2
Practice
Watch the video about enzymes at the following URL, and then answer the questions below.
http://www.youtube.com/watch?v=870MWm0peRI
MEDIA
Click image to the left for more content.
1. In the video, the presenter uses a pair of scissors as an analogy for an enzyme. Identify the active site of the
scissors enzyme, its substrate, and the chemical reaction it catalyzes.
2. What is the induced fit theory? How does it differ from the lock-and-key model described above?
3. Identify two factors that affect how well an enzyme works.
Review
1.
2.
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4.
What are enzymes?
Why are enzymes needed to catalyze chemical reactions in living things?
How do enzymes speed up biochemical reactions?
Describe two human digestive enzymes.
References
1. Zachary Wilson. The lock and key model illustrates how many enzymes function. CC BY-NC 3.0
2. Top: Flickr:LearningLark; Bottom: Laura Guerin (CK-12 Foundation). Amylase and pepsin are used in our
digestive system. Top: CC BY 2.0; Bottom: CC BY-NC 3.0
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