Explain why enzymes are proteins and describe how the amino acid sequence of an enzyme determines its function.

AQA A-Level Biology (7402) — 3.1.4 Proteins and Enzymes · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Enzymes are large molecules made from chains of amino acids. Each enzyme has a unique three-dimensional shape that is essential for its role in the body.

Model answer (5 marks)

Enzymes are proteins, i.e. they are polymers of amino acids.

The linear sequence of amino acids determines the way the chain folds into a three‑dimensional structure. This folding creates a unique active site.

The active site has a specific shape that only fits one particular substrate – the lock‑and‑key model.

Because only that substrate can bind, the enzyme can catalyse only one specific reaction, giving it high specificity.

Examiner tips

Use the word "protein" first to answer the first part. Show the chain‑folding → active site → lock‑and‑key sequence. Mention that specificity comes from the unique active site shape.

Common mistakes

Saying enzymes are just any proteins without linking to amino‑acid chains. Forgetting to explain that the sequence determines the 3D shape. Using vague terms like "unique" without describing the lock‑and‑key mechanism.

Mark scheme (5 marks)

Enzymes are proteins (made from amino acids)
The sequence / order of amino acids determines the shape of the enzyme
Each enzyme has a unique / specific active site shape
Only the complementary / matching substrate can fit into the active site (lock and key)
This means each enzyme can only catalyse one specific reaction (is specific to one substrate)

Key terms in this question

protein