Lesson 7 of 8 / Biological molecules
pH and enzyme activity
Why can an enzyme work well in the stomach but poorly in the mouth?
I can explain pH effects and plan a fair comparison of enzyme activity at different pH values.
I can investigate and explain temperature and pH effects on enzyme reactions.Syllabus
Syllabus K224 / K225, Topic 3(e). Investigate and explain temperature and pH effects on enzyme reactions.
About 5 min
Make a guess. You are not marked.
Pepsin digests protein in the stomach, where the pH is about 2. What happens to pepsin activity at pH 7?
Show the answer
It falls, because pH 7 is far from its optimum.
Pepsin is adapted to acid. Away from its optimum pH the shape of the active site changes, so fewer complexes form.
The key idea
Each 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
The shape of an enzyme's active site can be changed by pH. Away from its optimum pH, the substrate fits less well, so fewer enzyme-substrate complexes form and the rate falls. A very high or very low pH can denature the enzyme.
Different enzymes have different optimum pH values. The protease in the stomach works best in acid. Amylase in saliva works best near neutral. So not every enzyme works best at pH 7.
To test the effect of pH, use buffer solutions to set several different pH values. Keep the amounts of enzyme and substrate the same, and keep the temperature the same in a water bath. Measure the rate in one way every time, for example the time for starch to disappear.
Repeat each pH and compare the means. If the temperature also changed, you cannot say the difference was caused by pH. Test several pH values close to the fastest one before you name an optimum.
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.
Watch out for this
A student says: "Acid denatures every enzyme immediately." What is wrong with this?
Show the answer
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.
Check your understanding
Which method best isolates the effect of pH on amylase activity?
Show the answer
Use different pH buffers but keep temperature and enzyme and substrate amounts constant.
A consistent endpoint and repeated measurements then support a fair comparison of pH conditions.
Quick recall
Why can an enzyme work well in the stomach but poorly in the mouth?