Interpreting and evaluating experimental results
Enzymes from hot springs and soil
- Units 1 and 3
- 9 points
- About 25 minutes
You can use a calculator on this question, just like on exam day.
A long question built around a real-world experiment and its data in a table or graph. You describe the biology behind it, pick out the experiment's variables and controls, analyze the data or do a calculation, and make and justify a prediction. On the exam: Question 1 of 6 in Section II (90 minutes, 50% of the exam score); about 25 minutes suggested.
The question and its sources
Enzymes from organisms that live in very hot places often keep working at temperatures that denature the same kind of enzyme from other organisms. Researchers compared two forms of an enzyme that breaks a sugar into two smaller molecules: Enzyme S, from a soil bacterium, and Enzyme H, from a bacterium that lives in a hot spring.
Experiment 1
Researchers prepared reaction tubes with the same volume of buffer at pH 7.0, the same starting concentration of substrate (the sugar), and the same concentration of either Enzyme S or Enzyme H. Tubes were held at one of seven temperatures from 30°C to 90°C. For each enzyme and temperature, the researchers measured the initial rate of product formation in five replicate tubes.
At every temperature, the researchers also measured product formation in tubes that contained substrate and buffer but no enzyme. No product was detected in any of these tubes.
Source: Hypothetical study
Table 1. Initial rate of product formation (μmol/min), mean ± 2SE
| Temperature (°C) | Enzyme S | Enzyme H |
|---|---|---|
| 30 | 4.1 ± 0.3 | 1.2 ± 0.2 |
| 40 | 6.0 ± 0.4 | 2.0 ± 0.2 |
| 50 | 3.2 ± 0.4 | 3.4 ± 0.3 |
| 60 | 0.8 ± 0.2 | 5.1 ± 0.4 |
| 70 | 0.1 ± 0.1 | 7.3 ± 0.5 |
| 80 | 0.0 ± 0.0 | 8.0 ± 0.5 |
| 90 | 0.0 ± 0.0 | 3.9 ± 0.6 |
Source: Hypothetical data
Experiment 2
In a second experiment, the researchers measured the total amount of product in a single tube of Enzyme H at 80°C every minute for 5 minutes.
In a separate check, Enzyme H that had been held at 80°C for 5 minutes without substrate, then given fresh substrate at 80°C, had the same initial rate as Enzyme H that had not been preheated.
Source: Hypothetical study
Table 2. Total product formed by Enzyme H at 80°C
| Time (min) | Total product (μmol) |
|---|---|
| 0 | 0.0 |
| 1 | 8.1 |
| 2 | 16.0 |
| 3 | 24.2 |
| 4 | 31.8 |
| 5 | 38.0 |
Source: Hypothetical data
Suggested time: 25 minutes
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Part (a)
1 pointDescribe how an enzyme increases the rate of a chemical reaction.
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Part (b)
3 points(i) Identify the dependent variable in Experiment 1. (ii) Justify the researchers' inclusion of the tubes that contained substrate and buffer but no enzyme at each temperature. (iii) Based on Table 1, describe the effect of increasing temperature from 40°C to 70°C on the rate of product formation by Enzyme S.
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Part (c)
3 points(i) Using the data in Table 2, calculate the average rate of product formation by Enzyme H from 0 to 4 minutes. Include units. (ii) Based on Table 1, determine whether the difference between the rates of Enzyme S and Enzyme H at 50°C is statistically significant. Support your answer with the data. (iii) The rate of product formation in Table 2 is lower between 4 and 5 minutes than in earlier minutes. Explain this decrease.
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Part (d)
2 pointsResearchers find that Enzyme H has several disulfide bonds and ionic bonds between R groups that Enzyme S lacks. They make a mutant form of Enzyme H in which the amino acids that form these bonds are replaced by amino acids that cannot form them. (i) Predict the rate of product formation by the mutant enzyme at 80°C compared with the rate of normal Enzyme H at 80°C. (ii) Justify your prediction.
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