5.1.3Enzymes

Enzyme Action and Specificity — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 6

📚 Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Hub / Topical Revision

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Part 1 of 6Enzymes as biological catalysts

Cells carry out thousands of chemical reactions, together called metabolism.

Enzymes

Most of these would be far too slow at body temperature without help. Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up or changed.

Because they are not used up, a small amount of enzyme can convert a large amount of substance.

Different enzymes are needed for different reactions, and each enzyme is made in cells according to the instructions in a gene.

Part 2 of 6Structure and the active site

An enzyme is a long chain of amino acids folded into a precise three-dimensional shape.

Structure and the active site

Active site

On the surface is a small region called the active site, which has a specific shape.

Substrate

The substance that the enzyme acts on is called the substrate, and the substances formed are the products.

The active site is just the right shape to fit one type of substrate.

Specific

This is why each enzyme is specific: amylase digests starch but cannot digest protein, and protease digests protein but not starch.

The active site is formed by only a few amino acids, but the whole protein chain is needed to hold it in the correct shape.

Structure and the active site

This is why even a small change in the gene for an enzyme, a mutation that alters the amino acid sequence, can change the shape of the active site and stop the enzyme working.

Genetic diseases such as phenylketonuria arise in this way, when an enzyme that breaks down one amino acid is missing or faulty.

Part 3 of 6The lock and key model

In the lock and key model, the substrate is like a key and the active site like a lock. The steps are:

Enzyme-substrate complex

The substrate collides with the enzyme and binds to the active site, forming an enzyme-substrate complex.

The reaction occurs: for a breakdown reaction the substrate is split into smaller molecules, and for a synthesis reaction two substrates are joined.

The products leave the active site, and the enzyme is free to bind another substrate molecule.

Enzymes lower the energy needed for the reaction to start, so reactions occur quickly at the temperature of the body. Examples include amylase (starch to maltose), protease (proteins to amino acids) and lipase (fats to fatty acids and glycerol). Some enzymes work inside cells and others, such as digestive enzymes, work outside the cells in the gut.

Part 4 of 6Why shape matters: denaturation

The shape of an enzyme depends on bonds between parts of the protein chain.

If the temperature becomes too high or the pH moves far from the optimum, these bonds break, and the active site changes shape.

The substrate no longer fits, so the enzyme stops working.

Denatured

The enzyme is said to be denatured, and the change is usually permanent.

This is why a fever that is too high is dangerous: body enzymes may denature.

Term	Meaning
Substrate	The molecule on which the enzyme acts
Active site	The part of the enzyme where the substrate binds
Product	The molecule made in the reaction
Enzyme-substrate complex	The enzyme and substrate temporarily joined
Denaturation	Permanent change in shape of the active site

Part 5 of 6Examples in the human body

Digestion is the best-known use of enzymes.

Large insoluble food molecules are broken into small soluble ones that can be absorbed into the blood.

Amylase in saliva and the pancreas breaks down starch; proteases in the stomach and small intestine break down proteins; lipases from the pancreas break down lipids.

Enzymes are also used inside cells: catalase breaks down toxic hydrogen peroxide into water and oxygen,

and enzymes in respiration release energy from glucose.

Enzymes make it possible for organisms to build large molecules, for example when glucose molecules are joined to make starch or glycogen.

Enzymes are used in industry, for example in biological washing powders and in making sugar syrup.

Enzymes vary in where they worksome, such as the digestive enzymes, are released into the gut, while others stay inside cells.

Examples in the human body

The same enzyme from different species may have slightly different optimum conditions because each is adapted to the body temperature of its organism.

Cold-water fish enzymes work at low temperatures, and enzymes in the human body are tuned to 37 degrees Celsius.

In every case the principle is the same: a substrate fits a specific active site.

Part 6 of 6Exam-style questions

Describe the lock and key model of enzyme action. [3]3 marks
Explain why amylase cannot digest protein. [2]2 marks
A student says that enzymes are used up in reactions. Explain why this is incorrect. [2]2 marks

Key terms

Term	Meaning
Enzyme	A protein that acts as a biological catalyst.
Catalyst	A substance that speeds up a reaction and is not used up.
Active site	The region of an enzyme where the substrate binds.
Substrate	The substance acted on by an enzyme.
Denature	To permanently change the shape of a protein so that it no longer works.
Specific	Able to act on only one type of substrate.
Metabolism	All the chemical reactions in a living organism.

Avoid these mistakes

Saying enzymes are alive. They are protein molecules, so they cannot be killed, only denatured.

Writing that high temperature "kills" the enzyme. Use "denatures" and refer to the active site changing shape.

Stating that the substrate fits the active site because they are the same shape. They have complementary shapes.

Thinking an enzyme is used up in the reaction.

Last updated: 20 October 2026