Explain how changes in pH affect the rate of an enzyme-catalysed reaction, with reference to the molecular basis of enzyme structure and active site function.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The active site has a specific 3‑D shape determined by the amino‑acid sequence and the interactions of their R‑groups, many of which are charged.
Changes in pH alter the ionisation state of acidic (–COOH) and basic (–NH3⁺) side chains. This disrupts ionic bonds and hydrogen bonds that stabilise the enzyme’s tertiary structure.
If the tertiary structure is altered, the active site is distorted or lost, so the substrate can no longer bind with the correct complementarity. The enzyme–substrate complex is less stable and the reaction rate falls.
At very low or very high pH the denaturation is irreversible; each enzyme has an optimum pH at which the active‑site conformation is best maintained and the rate is maximal.
Changes in pH alter the ionisation state of acidic (–COOH) and basic (–NH3⁺) side chains. This disrupts ionic bonds and hydrogen bonds that stabilise the enzyme’s tertiary structure.
If the tertiary structure is altered, the active site is distorted or lost, so the substrate can no longer bind with the correct complementarity. The enzyme–substrate complex is less stable and the reaction rate falls.
At very low or very high pH the denaturation is irreversible; each enzyme has an optimum pH at which the active‑site conformation is best maintained and the rate is maximal.
Examiner tips
- Use the word ‘active site’ and link pH to ionisation of side chains. Show that loss of structure → loss of substrate binding → lower rate. Mention irreversible denaturation and optimum pH.
- Use correct terminology: tertiary structure, ionic/hydrogen bonds, substrate‑enzyme complementarity.
Common mistakes
- Confusing pH with pKa or saying pH only changes the substrate. Saying the enzyme is permanently inactivated at all non‑optimal pH. Using vague terms like ‘shape’ without linking to side‑chain ionisation.
Mark scheme (4 marks)
- The active site of an enzyme has a specific three-dimensional shape determined by the sequence and interactions of amino acids, including R-group charges influenced by pH.
- Changes in pH alter the ionisation state of acidic and basic R groups, disrupting ionic bonds and hydrogen bonds that maintain the tertiary structure of the enzyme.
- Distortion or loss of the active site shape means substrate can no longer bind / enzyme–substrate complementarity is lost, reducing reaction rate.
- At extreme pH values denaturation is irreversible; each enzyme has an optimum pH at which the active site conformation is best maintained and reaction rate is maximal.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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