Enzyme Reading
Name: ________________________________
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.
Cellular reactions depend on the assistance of catalysts, compounds that speed up chemical reactions. The
main catalysts of 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.
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.
This figure follows the action of the enzyme sucrase,
which catalyzes 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.
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.
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.
Worksheet for Enzyme
1. Explain the role of activation energy in a reaction. How does an enzyme affect activation energy?
2. Describe how a substrate interacts with an enzyme.
Step 1.
1. Molecule C is a large protein (or several proteins together) that we call an
________________.
2. Molecules A and B are called substrate, and are usually _______________ or
building blocks of larger macromolecules.
3. Which of the four major biological macromolecules is C made of?
Step 2.
1. Is molecule C breaking apart (hydrolysis) or building (dehydration synthesis) a
macromolecule?
2. If a solution is too acidic or basic, molecule C can ________________ or change it
shape so that A and B will no longer fit.
Step 3.
1. What will happen to molecule C now that the reaction is complete?
2. What are two things that affect how fast molecule C works?
3. Molecule D is made of building blocks represented by A and B. If D represents a
polysaccharide, A and B would represent ___________________.
Label the substrate(s), product(s), enzyme, and active site in the picture below. Provide descriptions of the
process at 1, 2, 3, and 4.
Analyzing Graphs Use the graph to answer the questions below.
a. At which temperature does enzyme A perform best? _______________ Enzyme B?_________
b. Knowing that one of these enzymes is found in humans and the other in thermophilic (heat-loving) bacteria,
hypothesize which enzyme came from which organism.
c. Propose a hypothesis that explains why the rate of the reaction catalyzed by enzyme A slows down at
temperatures above 40°C.