Enzyme action and factors affecting it
How enzymes work, and why temperature and pH change their rate.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 5.1.1Describe enzymes as proteins that function as biological catalysts, using the lock-and-key model.
- 5.1.2Explain the effects of temperature and pH on enzyme activity, including denaturation.
6 minute read
Enzyme action and factors affecting it
Chemical reactions in cells would be far too slow at body temperature without help. Enzymes provide it.
What an enzyme is
An enzyme is a protein that acts as a biological catalyst: it speeds up a chemical reaction without being changed or used up, so one enzyme molecule works over and over.
The lock-and-key model
Each enzyme has an active site with a specific shape. Only a substrate with a complementary shape fits into it, forming an enzyme-substrate complex. The reaction happens, products leave, and the active site is free again. This is why enzymes are specific — amylase breaks down starch and nothing else, because only starch fits its active site.
Temperature
As temperature rises, molecules move faster, collide more often, and the rate increases — up to the optimum, around 37 °C in humans. Above the optimum the rate falls sharply, because the heat breaks the bonds holding the protein in shape. The active site changes shape, the substrate no longer fits, and the enzyme is denatured. This is permanent — cooling it down does not restore activity.
pH
Each enzyme has an optimum pH. Pepsin in the stomach works best at about pH 2; amylase in the mouth at about pH 7. Move too far either side of the optimum and the active site is again distorted and the enzyme denatures.
Think of it like this
The active site is a keyhole and the substrate is the key. Denaturing is not losing the key — it is melting the lock, so that no amount of trying will make the key fit again.
Worked examples
Method, step by step
A reaction catalysed by an enzyme is fastest at 40 °C. Explain why the rate is lower at 20 °C and also lower at 60 °C.
- 1At 20 °C molecules have less kinetic energy, so there are fewer collisions between enzyme and substrate per second — the enzyme is not damaged.
- 2At 60 °C the enzyme has been denatured: the bonds holding its shape break.
- 3The active site changes shape, the substrate can no longer bind, so few or no enzyme-substrate complexes form.
At 20 °C the rate is low because of fewer successful collisions (reversible). At 60 °C the enzyme is denatured — the active site has changed shape so substrate no longer fits (permanent).
Common misconceptions
- Saying an enzyme is "killed" by heat. Enzymes are molecules, not organisms — the correct word is **denatured**.
- Thinking denaturation is reversible. Once the active site's shape is lost, cooling does not bring the activity back.
- Claiming the rate rises steadily then stops. Beyond the optimum, the rate falls, and it falls quickly.
In the exam
- When explaining why rate falls above the optimum, you must mention that the **active site changes shape** so the substrate no longer fits — "the enzyme is denatured" alone is often only one of two marks.
- For low temperature, do not say denatured. The enzyme is simply working slowly because there is less kinetic energy and fewer successful collisions; it recovers on warming.