Lock and Key Enzyme Model | IGCSE Biology 0610

Enzyme Action: The Lock and Key Hypothesis

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Enzymes are biological catalysts: proteins that speed up metabolic reactions without being used up. Each enzyme has a specifically shaped active site that only fits one substrate, like a lock fits one key. The substrate binds to form an enzyme-substrate complex, the reaction occurs, and products are released.

Enzymes control virtually every chemical reaction in living organisms. The lock and key model is the starting point for understanding enzyme specificity and how factors like temperature and pH affect enzyme activity. This guide covers the model, key terminology, and how examiners test your understanding.

Key Enzyme Concepts

Biological catalyst: speeds up a reaction without being changed or used up. The enzyme can be reused.
Active site: a region on the enzyme surface with a specific shape complementary to the substrate.
Substrate: the molecule that binds to the active site and is acted upon by the enzyme.
Enzyme-substrate complex: the temporary structure formed when the substrate binds to the active site.
Products: the molecules released after the reaction. They have a different shape from the substrate and leave the active site.
Specificity: each enzyme catalyses only one reaction because only one substrate fits its active site.

Sequence: Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Products

The Lock and Key Model Explained

The lock and key hypothesis compares the enzyme to a lock and the substrate to a key:

Just as only one key fits a particular lock, only one substrate fits a particular enzyme's active site.
The shape of the active site is complementary to the shape of the substrate: they fit together precisely.
If the substrate does not have the correct shape, it cannot bind to the active site and no reaction occurs.

This explains enzyme specificity: amylase only breaks down starch (not protein), because the active site of amylase is only complementary to starch molecules.

Supplement: induced fit model: A more accurate model suggests the active site changes shape slightly to fit the substrate more closely when it binds, like a glove moulding around a hand. However, the lock and key model is sufficient for most IGCSE questions.

Types of Enzyme Reactions

Enzymes can catalyse two types of reactions:

Catabolic reactions (breaking down): large molecules are broken into smaller molecules. Examples:
- Amylase breaks starch → maltose
- Protease breaks proteins → amino acids
- Lipase breaks lipids → glycerol + fatty acids
Anabolic reactions (building up): small molecules are joined to form larger molecules. Examples:
- Amino acids → proteins (in protein synthesis)
- Glucose → starch or glycogen (in storage)

In digestion, enzymes are catabolic: they hydrolyse large, insoluble food molecules into small, soluble molecules that can be absorbed.

Try a focused question (3 marks)

Explain why amylase can break down starch but cannot break down protein. (3 marks)

Show the answer and marking guidance

The active site of amylase has a specific shape that is complementary to starch.
Protein has a different shape and cannot fit into the active site of amylase.
An enzyme-substrate complex cannot form, so no reaction occurs / enzyme specificity.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying the substrate and active site have the "same" shape: they are COMPLEMENTARY (they fit together like puzzle pieces).
Saying enzymes are "used up" in the reaction: they are catalysts and are reused.
Confusing "denatured" with "dead": enzymes are proteins, not living things.