K327 / K328 / 2027

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 K327 / K328, 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?

  1. It stays the same, because pH affects bacteria, not enzymes.
  2. It falls, because pH 7 is far from its optimum.
  3. It rises, because neutral pH 7 is the best pH for enzymes.
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

Illustrative salivary amylase pH responseThe illustrative peak is near neutral pH. At selected pH 7, the model activity is 10 out of 10 relative units.135791113Relative activitypH

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. 1

    Set pH with a buffer

    Use a suitable buffer for each pH so that the intended condition is maintained.

  2. 2

    Control other factors

    Keep temperature, volumes, enzyme concentration, substrate concentration and measurement method the same.

  3. 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?

  1. Test each pH at a different temperature, so that the full range of conditions is covered.
  2. Use different pH buffers but keep temperature and enzyme and substrate amounts constant.
  3. Change pH, temperature and the amount of enzyme together.
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

Card 1 of 3. Answer in your head, then check.

Why can an enzyme work well in the stomach but poorly in the mouth?

You can return to this lesson any time.