At GCSE
At GCSE you use the lock-and-key model and describe the effect of temperature, pH and substrate concentration on rate. The required practical (effect of pH on amylase) is a frequent source of 6-mark answers.
GCSEA-LevelAQAEdexcelOCR
Enzymes revision — GCSE & A-Level Biology
Enzymes are biological catalysts — globular proteins that lower the activation energy of specific reactions. They appear in almost every Biology paper because they connect protein structure, metabolism, digestion and disease.
At GCSE you need the lock-and-key model and the effects of temperature, pH and substrate concentration. At A-Level the induced-fit model replaces lock-and-key, and you must also explain competitive vs non-competitive inhibition, including real examples like statins and penicillin.
At A-Level
At A-Level the induced-fit model replaces lock-and-key, and you must distinguish competitive and non-competitive inhibition with named examples (statins, penicillin, cyanide). Questions on initial rate, Vmax and the meaning of Km, plus immobilised enzymes in biotechnology, are common.
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Example flashcards
Q: What does it mean to say an enzyme has been denatured?
A: The tertiary structure has changed so that the active site is no longer complementary to the substrate — the enzyme can no longer catalyse the reaction.
Q: Compare the lock-and-key and induced-fit models.
A: Lock-and-key assumes a rigid active site; induced fit recognises that the active site changes shape slightly when the substrate binds, putting strain on substrate bonds.
Q: How does a competitive inhibitor reduce the rate of reaction?
A: It has a shape similar to the substrate and binds reversibly to the active site, blocking substrate access. Increasing substrate concentration overcomes the effect.
Q: Why does enzyme activity fall above the optimum temperature?
A: Increased kinetic energy breaks hydrogen and ionic bonds in the tertiary structure, distorting the active site and denaturing the enzyme.
Quick summary
Enzymes is a high-yield Biology topic for GCSE and A-Level students (AQA, Edexcel, OCR). At A-Level the induced-fit model replaces lock-and-key, and you must distinguish competitive and non-competitive inhibition with named examples (statins, penicillin, cyanide). Examiners reward precise definitions and applied explanations — focus on the core ideas and the small set of terms that come up every series.
Key terms
- Active site
- Substrate
- Lock and key
- Induced fit
- Denaturation
- Optimum temperature
- Competitive inhibitor
- Non-competitive inhibitor
- Activation energy
Enzymes FAQs
Are all enzymes proteins?+
Almost all are globular proteins, although a small number of RNA molecules (ribozymes) also have catalytic activity. For GCSE and A-Level, assume enzymes are proteins.
Why is enzyme activity affected by pH?+
pH changes the charges on the amino acid side chains in the active site, breaking ionic and hydrogen bonds and altering its shape so the substrate no longer fits.
What is the difference between Vmax and Km?+
Vmax is the maximum rate when every active site is saturated. Km (the Michaelis constant) is the substrate concentration that gives half Vmax — a measure of enzyme-substrate affinity.
Are inhibitors always harmful?+
No — many drugs are enzyme inhibitors. Penicillin inhibits a bacterial cell-wall enzyme; statins inhibit HMG-CoA reductase to lower cholesterol.
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