Enzymes: how they work

Active site and specificity

Enzymes are globular proteins. The active site is a region with a specific shape, determined by the enzyme's tertiary structure. Only a substrate with a complementary shape can bind, forming an enzyme-substrate complex. This is why each enzyme is specific to one substrate (or a small group).

In the induced fit model, the active site changes shape slightly as the substrate binds, fitting more closely around it.

Lowering activation energy

Activation energy is the energy needed to start a reaction. Binding in the active site strains bonds in the substrate or brings reacting molecules close together in the right orientation, so the reaction happens with less energy. The enzyme is not used up and can be reused.

On an energy diagram, the hump is lower for the catalysed reaction; the energy of reactants and products is unchanged.

Inside and outside cells

• Intracellular enzymes work inside cells, for example catalase, which breaks down hydrogen peroxide, and enzymes of respiration and DNA replication.
• Extracellular enzymes are secreted by exocytosis and work outside the cell that made them, for example amylase and trypsin in the gut, or enzymes released by fungi to digest food.

Worked example

Explain why amylase breaks down starch but not proteins.

1. The active site of amylase has a specific shape.
2. Only starch is complementary to it, so only starch forms an enzyme-substrate complex.

Answer: Proteins do not fit the active site, so no enzyme-substrate complex forms.

Check yourself

1. Why are enzymes specific?
   • They are made of lipids
   • They are used up in the reaction
   • The active site has a shape complementary to only one substrate
   • They raise the activation energy
   Show answer

   The active site has a shape complementary to only one substrate. Shape complementarity determines which substrate binds.

2. How do enzymes speed up reactions?
   • They lower the activation energy
   • They raise the temperature
   • They increase the energy of the products
   • They supply ATP
   Show answer

   They lower the activation energy. Less energy is needed to start the reaction.

3. What does the induced fit model propose?
   • The active site is rigid and never changes
   • The substrate changes into the enzyme
   • The active site changes shape slightly as the substrate binds
   • Enzymes bind to any substrate
   Show answer

   The active site changes shape slightly as the substrate binds. Binding causes a small change in the active site to fit closely.

4. Which is an extracellular enzyme?
   • Catalase inside liver cells
   • Trypsin in the small intestine
   • DNA polymerase in the nucleus
   • An enzyme of respiration in mitochondria
   Show answer

   Trypsin in the small intestine. Trypsin is secreted and works in the gut lumen.

5. What happens to an enzyme at the end of a reaction?
   • It is broken down
   • It becomes part of the product
   • It is converted into substrate
   • It is unchanged and can catalyse another reaction
   Show answer

   It is unchanged and can catalyse another reaction. Catalysts are not used up.

6. Which level of protein structure determines the shape of the active site?
   • Primary structure alone
   • Tertiary structure
   • Secondary structure alone
   • Quaternary structure only
   Show answer

   Tertiary structure. Folding into the 3D tertiary shape forms the active site (though it depends on the primary sequence).

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.