Enzymes: Question 4
Syllabus 5.1
Two research groups each extract a protease enzyme that digests protein. Enzyme G is extracted from bacteria that live in the human gut. Enzyme H is extracted from bacteria that live in a geothermal hot spring, where the water is naturally very hot. Each group measures the rate of protein digestion produced by its enzyme at a range of temperatures, using the same concentration of enzyme and the same protein substrate every time.
| Temperature / °C | 10 | 20 | 37 | 50 | 65 | 80 | 95 |
|---|---|---|---|---|---|---|---|
| Rate for enzyme G / units per minute | 2 | 5 | 12 | 4 | 0 | 0 | 0 |
| Rate for enzyme H / units per minute | 0 | 1 | 3 | 6 | 10 | 13 | 2 |
(a) State the optimum temperature shown by these results for enzyme G and for enzyme H. [2]
(b) Explain, in terms of kinetic energy and the frequency of effective collisions between enzyme and substrate molecules, why the rate for enzyme G increases between 10°C and 37°C. [3]
(c) Explain why the rate for enzyme G has fallen to zero by 65°C, while enzyme H still has a high rate of reaction at this temperature. [3]
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Worked solution
Part (a): Reading the optimum temperatures from the table
The optimum temperature for an enzyme is the temperature at which its rate of reaction is greatest.
- Enzyme G’s highest recorded rate is 12 units per minute, at 37°C.
- Enzyme H’s highest recorded rate is 13 units per minute, at 80°C.
Part (b): Explaining the rise in enzyme G’s rate up to 37°C
As the temperature increases from 10°C towards 37°C, both the enzyme molecules and the substrate molecules gain more kinetic energy. Molecules with more kinetic energy move faster and collide with each other more often.
This means that the active sites of enzyme G collide with substrate molecules more frequently, and a greater proportion of these collisions have enough energy and the correct orientation to be effective collisions, collisions that result in an enzyme-substrate complex forming. As more enzyme-substrate complexes form each second, more product is made per minute, so the rate of reaction increases.
Part (c): Explaining the different behaviour of enzyme G and enzyme H at 65°C
For enzyme G, 65°C is far above its optimum temperature of 37°C. At this high temperature, the bonds holding the enzyme’s normal three-dimensional shape together are broken, and its active site is denatured. Permanently changed so it is no longer complementary to the shape of the substrate. Since the substrate can no longer bind, no enzyme-substrate complexes form, and the rate of reaction falls to zero.
Enzyme H, in contrast, comes from bacteria that live in a naturally hot environment, so it is adapted to withstand much higher temperatures before its active site is disrupted. At 65°C, enzyme H is still well below its own optimum of 80°C, so its active site retains the correct complementary shape and it continues to catalyse the reaction at a high rate. (Enzyme H’s own rate does eventually fall, to just 2 units per minute at 95°C, showing that it too will denature, but only at a much higher temperature than enzyme G.)
Final answers
- (a) Enzyme G: optimum 37°C. Enzyme H: optimum 80°C.
- (b) Higher temperature gives molecules more kinetic energy, increasing the frequency of effective collisions between enzyme G’s active site and the substrate, so more enzyme-substrate complexes form and the rate increases.
- (c) At 65°C, enzyme G is denatured (its active site’s shape is permanently changed), so its rate falls to zero; enzyme H is adapted to much higher temperatures, so its active site is still the correct shape at 65°C and it keeps working.