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Enzyme Catalysis Model
Introduction
Imagining how molecules interact and what happens during a chemical reaction can be difficult. Sometimes a model can help to visualize the principles involved. In this activity, a model will be constructed to illustrate enzyme activity.
Concepts
Catalyst - Competitive inhibitor
- Enzyme/substrate
- Noncompetitive inhibitor
Background
Chemical reactions in most organisms take place within a fairly narrow range of temperatures. These temperatures are not high enough to supply the activation energy needed to start a reaction. Living cells do not have matches or electricity to provide activation energy. How do organisms carry out the myriad of chemical reactions needed for survival?
All living cells contain specialized proteins called enzymes that lower the activation energy required to make a reaction proceed. In this way, enzymes greatly speed up chemical reactions that would otherwise occur too slowly to sustain life. The reactions do not consume the enzymes and, therefore, the enzymes can be “recycled” and reused many times. Thus, enzymes fit the definition of a catalyst, they speed up a chemical reaction without being consumed as a result of the reaction. Enzymes are thought of as “life’s catalysts.”
Enzymes are very large protein molecules that catalyze reactions by utilizing a very small area of their structure called the active site. The active site is a specific spot on the protein where only certain molecules can “bind” or “associate” with each other. The active site can attract and hold only very specific molecules “matching” the enzyme active site. The molecules on which the enzyme can act are called the substrate. Each enzyme can catalyze only one or a very few specific chemical reactions because only a few molecules are sufficiently alike to fit the active site of the enzyme.
To act as a catalyst, an enzyme must temporarily interact with the substrate molecules. An “enzyme-substrate complex” is formed momentarily. The enzyme thus brings the reacting molecules together and by its unique structure lowers the activation energy necessary to speed up the reaction. Once the reaction occurs, the newly formed product molecules do not remain bound to the enzyme—they break away, leaving the enzyme the same as before the reaction. The enzyme molecule is then ready to attract new specific substrate molecules to continue catalyzing many repeats of the reaction. Enzymes can build molecules (anabolism) or they can break molecules apart (catabolism). Figure 1 shows a schematic representation of an anabolic reaction.
{10351_Background_Figure_1_Visualization of enzyme–substrate complex}
Most enzymes are subject to inhibition by specific agents that interfere with the binding of a substrate at the active site or with the conversion of the enzyme–substrate complex into products. In some cases, an inhibitor molecule greatly resembles the substrate structurally and is able to bind at the same site as the substrate. When this happens less active sites are available for substrate binding and the total reaction is slowed because of the competition for the active sites. This effect is logically called competitive inhibition since the substrate and inhibitor molecules are competing for the same active sites on the enzyme molecules.
There are other kinds of inhibitors that do not compete for the active site of the enzyme but instead work by interfering with the reaction of the enzyme–substrate complex. These inhibitors are logically called noncompetitive inhibitors. Figure 2 shows a pictorial representation of the two types of inhibition: A is competitive inhibition, and B is noncompetitive inhibition.
{10351_Background_Figure_2_Enzyme inhibition}
Materials
Pencil and paper
Polyfoam pieces, various colors, 4
Scissors
Zipper-lock bag
Safety Precautions
This lab is considered non-hazardous. Follow all standard laboratory safety guidelines.
Procedure
- Your goal is to make a working model to show how an enzyme functions as a catalyst. Your model should contain: (a) an enzyme (color 1), (b) the substrate (colors 2 and 3), (c) a competitive inhibitor (color 4) and (d) a noncompetitive inhibitor (color 4).
- Note the size of the polyfoam pieces you have received. Draw plans for your enzyme model on paper before you start to cut your foam pieces.
- Do not make your model the same shape as those drawn in the Background section. Be original and use the flexibility of the foam to illustrate the properties of the enzyme.
- When all components are drawn to scale, transfer your patterns to the pieces of foam.
- Cut the pieces carefully and slowly with a scissors. Try to make the pieces fit as precisely as possible.
- Use your completed model to teach someone in your class: (a) how an enzyme works, (b) how a competitive inhibitor works and (c) how a noncompetitive inhibitor might work.
- Exchange your model with another group. Examine the new model. Identify the enzyme, the active site, the substrate, the competitive inhibitor and the noncompetitive inhibitor.
- Answer the questions on the Enzyme Catalysis Worksheet.
Post-Lab Questions
- How is your model like the other models in your class? How is it different? How do the models compare to real enzymes? (Are real enzymes alike or different?)
- In your model, how did the competitive inhibitor work? How was the active site affected?
- In your model, how did the noncompetitive inhibitor work? How was the active site affected?
- On the back of this worksheet draw a sketch of at least two enzyme models from your class. On each model label the enzyme, the active site, competitive inhibitor, noncompetitive inhibitor and substrate.
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