Laboratory Experiment 2: Enzyme Catalysis for AP Biology
This experiment draws on information on chemistry. The experiment is designed to practice the calculation of the rate of enzyme-catalyzed reactions through the measurement of the products produced. In this particular experiment, the enzyme catalase is used to convert hydrogen peroxide to water and oxygen, and the products are measured to assist in the determination of the rate of reaction. If you do not feel comfortable with your knowledge of enzyme-substrate interactions, refer to concepts below before continuing this section.
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The Nitty Gritty about Experiment 2
The reaction of interest in this experiment is:
2H2O2 → 2H2O + O2
This reaction does indeed occur without the assistance of catalase, but it occurs at a slow rate. When our friend catalase is added to the mix, the reaction occurs at a much faster clip. Take a look at the enzymatic activity curve in Figure 19.1. Notice the constant rate of reaction in the first 6 minutes of the experiment.
However, after the sixth minute, the rate slows, as if the enzyme has become tired. This is because as the reaction proceeds, the number of substrate molecules remaining declines, and this means that fewer enzyme–substrate interactions can occur.
When calculating the rate of reaction, it is the constant linear portion of the curve that matters. That is the accepted rate value for the enzyme. Do not attempt to factor in the slowing portion of the curve.
In this particular experiment, catalase is added to a beaker that holds H2O2 and is allowed to react for a certain period of time. After the reaction stops, the amount of H2O2 remaining in the beaker is measured. The information is then plotted on a curve similar to that in Figure 19.1 to determine the rate of reaction (the slope of the straight portion of the graph).
At one point during the experiment, acid is added to the beaker to stop the reaction. Why does this halt the reaction? Because it alters the pH, which has negative effects on the active sites of many enzymes, adversely affecting their ability to interact with substrates. Likewise, temperature has a negative effect on the rate of enzymatic activity because if the temperature is too low, the kinetic energy of the system will be such that very few collisions between enzymes and substrates may occur. If the temperature is too high, the enzyme itself might actually be denatured and break apart.
Things to Take Away from Experiment 2
There are a few points to be gleaned from this experiment that reinforce concepts mentioned both in this chapter and in Chapter 5:
- Reaction rate can be affected by four major factors: pH, temperature, substrate concentration, and enzyme concentration.
- The "rate of reaction" can be found by measuring either the appearance of product or the disappearance of reactant. Either measure can provide insight into the effectiveness of an enzyme's presence.
- When calculating the rate of reaction, if you are examining a graph, remember that the rate is actually the portion of the graph with a constant slope.
- To design an experiment to test the rate of reaction of an enzyme compared to the speed of the normal reaction, first run the reaction without the enzyme, then run the reaction with the enzyme and compare the two rates of reaction.
- To determine the ideal temperature (or pH) at which an enzyme functions, run the enzyme reaction at a series of different temperatures (or pH values) and measure the various reaction rates to compare the effects of temperature (or pH) on a particular enzyme. (Remember: Do not change both pH and temperature at the same time! Change them separately.)
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