Chapter 5: Concept 5.5
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Objectives
Explain how enzymes affect activation energy.
Describe how an enzyme's shape is important to its function.
Key Terms
activation energy
catalyst
enzyme
substrate
active site
Each of your cells is like a miniature chemical factory capable of
performing thousands of different reactions. As a cell's needs change,
some of these reactions speed up, while others slow down. In this section
you'll read about a group of specialized proteins that coordinate the cell's
chemistry.
Enzymes and Activation Energy
To start a chemical reaction, it is first necessary to weaken chemical
bonds in the reactant molecules. This activation process requires that the
molecules absorb energy. For example, to burn a candle you need to
provide the initial energy with a match. This "start-up" energy is called
activation energy because it activates the reactants and triggers a
chemical reaction.
One way to provide activation energy is to heat up the mixture of
molecules. Hotter molecules may collide with enough energy to weaken
bonds, whereas cooler molecules collide with less energy. However,
heating up a cell would cause many unnecessary reactions to occur at
once, including reactions that destroy the cell's delicate structures.
Instead, cellular reactions depend on the assistance of catalysts,
compounds that speed up chemical reactions. The main catalysts of
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chemical reactions in organisms are specialized proteins called enzymes.
Enzymes provide a way for reactions to occur at the cell's normal
temperature. An enzyme doesn't supply activation energy to the reacting
molecules, but instead lowers the energy requirement barrier so that the
reaction can proceed at normal cell temperatures. Figure 5-15 shows an
analogy to help explain why reactions run faster if they require less
activation energy. The rock wall represents the activation energy barrier.
The frogs represent reactants and products of the reaction.
Figure 5-15
The activation energy barrier is like a wall between two
parts of a pond. If an enzyme lowers the wall, more frogs
have enough energy to reach the other side.
Each enzyme catalyzes a specific kind of chemical reaction. At any
moment in the cell's life, the specific enzymes that are present and active
determine which reactions occur.
How Enzymes Work
Just how does an enzyme catalyze only one type of reaction? The reason
is that the shape of each enzyme fits the shape of only particular reactant
molecules. A specific reactant acted upon by an enzyme is called the
enzyme's substrate. The substrate fits into a particular region of the
enzyme, called the active site.
The fit between substrate and enzyme is not rigid. As the substrate
enters, the active site changes shape slightly, fitting the substrate more
snugly. This places certain functional groups of the active site in position
to catalyze the reaction. The tighter grip may also bend the substrate,
weakening its bonds and making them easier to break.
Figure 5-16 follows the action of the enzyme sucrase, which catalyzes
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the hydrolysis of sucrose (the substrate). (Most enzymes have names that
end in -ase.) Sucrose is slightly distorted as it enters the active site. The
weakened bond reacts with water. The result is two products: a glucose
molecule and a fructose molecule. Once these products are released, the
enzyme's active site is ready to accept another molecule of sucrose. In
fact, this recycling ability is a key characteristic of enzymes.
Figure 5-16
A substrate binds to an enzyme at an active site. The
enzyme-substrate interaction lowers the activation
energy required for the reaction to proceed. In this
example, water is added to the weakened bond in
sucrose, breaking sucrose into glucose and fructose.
Another way that an enzyme can lower activation energy is by accepting
two reactant molecules (substrates) into adjacent sites. Holding the
reactants together enables them to react more easily. In this way,
enzymes can catalyze the formation of larger molecules from smaller
molecules.
As with any other protein, an enzyme's structure and shape are essential
to its function. And like other proteins, an enzyme's shape is sensitive to
changes in its surrounding environment. Therefore, factors such as pH
and temperature can greatly affect how well an enzyme works or if it can
work at all. This is one reason why cells (and hence organisms) can only
survive and function within certain ranges of conditions.
Concept Check 5.5
1. Explain the role of activation energy in a reaction. How does an
enzyme affect activation energy?
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2. Describe how a substrate interacts with an enzyme.
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