How enzymes work

Key concepts in biology · Enzyme action and activity · note 1 of 5

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In short

Enzymes are proteins that act as biological catalysts, speeding up reactions without being used up. Each enzyme has an active site with a particular shape. In the lock and key model, only a substrate with a shape complementary to the active site can fit, forming an enzyme-substrate complex. This is why each enzyme is specific to one type of reaction.

Enzymes are proteins that act as biological catalysts. A catalyst speeds up a reaction without being used up. Each enzyme has a region called the active site, which has a particular shape.

The molecule an enzyme acts on is called the substrate. The substrate has a shape that is complementary to the active site, so it fits in, like a key in a lock. This is called the lock and key model.

The substrate collides with the active site of the enzyme.
The substrate fits into the active site, forming an enzyme-substrate complex.
The reaction takes place at the active site.
The product or products leave the active site and the enzyme is free to be used again.

Enzymes are specific. Only a substrate with a shape that fits the active site can bind to it, so each enzyme catalyses only one type of reaction.

Four-step lock and key model: the substrate collides with the active site, fits to form an enzyme-substrate complex, the reaction takes place, then the products leave and the enzyme is unchanged. — The lock and key model. The substrate is complementary to the active site, and the enzyme is not used up.

Exam tip:

The mark scheme word is complementary. Say the shape of the substrate is complementary to the shape of the active site, not that they are 'the same'.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

Frequently asked questions

How do enzymes speed up reactions?

Enzymes speed up reactions by acting as biological catalysts, without being used up. The substrate collides with the enzyme's active site and fits into it because their shapes are complementary, forming an enzyme-substrate complex. The reaction happens at the active site, the products leave, and the enzyme can be used again.

Why do enzymes denature at high temperatures?

Enzymes denature at high temperatures because the bonds that hold the enzyme in shape are broken. The enzyme and its active site change shape, so the substrate no longer fits and the reaction cannot be catalysed. This change is permanent, so above the optimum temperature the rate falls quickly towards zero.

What happens to enzymes at low temperatures?

At low temperatures enzymes work slowly, but they are not denatured. The particles have less kinetic energy and move more slowly, so the enzyme and substrate collide less often and the rate of reaction is lower. As the temperature rises towards the optimum, collisions become more frequent and the rate increases.

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