Lesson 7 of 7 / Biological molecules
pH and enzyme activity
Why can an enzyme work well in the stomach but poorly in the mouth?
In this lesson: Explain pH effects and plan a fair comparison of enzyme activity at different pH values.
About 5 min
The key ideaEach enzyme has a suitable pH range. Moving away from its optimum can change the active site and reduce activity; extreme pH can denature it.
The pH scale describes how acidic or alkaline a solution is. Values below 7 are acidic, 7 is neutral, and values above 7 are alkaline.
Explore the idea
The best pH depends on the enzyme
pH 7: 10/10 in this model. The active site is suited to substrate binding under these conditions.
Original qualitative teaching curves, not experimental data or exact predictions. They illustrate different optima; real responses depend on the enzyme and conditions. Hold temperature and concentrations constant when investigating pH.
Explanation
An enzyme's active site depends on the structure of its protein. Changes in pH can affect that structure and the interactions needed for substrate binding. Away from the optimum pH, fewer productive enzyme-substrate complexes may form, so the rate decreases. Extreme pH can denature the enzyme.
The optimum depends on the enzyme. A stomach protease works best in acidic conditions, whereas salivary amylase works best near neutral conditions. A single rule such as "all enzymes need pH 7" cannot explain their different roles.
To investigate pH, prepare reaction mixtures with buffers at several chosen pH values. Use the same enzyme and substrate concentrations and volumes, and keep temperature constant in a water bath. Choose one consistent measure, such as starch disappearance for an amylase investigation.
Repeat at each pH and compare average rates. If pH and temperature both change, an observed difference cannot be attributed to pH alone. Include enough pH values around the apparent peak before suggesting an optimum; a sparse set only identifies the best of the conditions tested.
Step by step
- 1
Set pH with a buffer
Use a suitable buffer for each pH so that the intended condition is maintained.
- 2
Control other factors
Keep temperature, volumes, enzyme concentration, substrate concentration and measurement method the same.
- 3
Link results to the active site
Explain a reduced rate using poorer substrate binding or denaturation, rather than saying that acid always stops every enzyme.
Worked example
Spotting a confounded experiment
Amylase is tested at pH 5 at 20 degrees C and pH 7 at 40 degrees C. The second reaction is faster. Can the student conclude that pH alone caused the difference?
One way to explain it
No. Both pH and temperature changed, and both affect enzyme activity. Repeat with temperature held constant while changing pH, keeping the other reaction conditions and endpoint method the same.
Why this answer works
- Identify both changed variables.
- Explain why the result has more than one possible cause.
- Propose a comparison that isolates pH.
Is this true? "Acid denatures every enzyme immediately."
Enzymes have different pH ranges. Some stomach enzymes are adapted to acidic conditions. Explain the response of the particular enzyme using its optimum and active site.