Candidates should be able to use the lock and key model to explain the properties of enzymes. They should also recognise its limitations and be able to explain why the induced fit model provides a better explanation of specific enzyme properties

The lock and key theory helps us to understand that an enzyme has an active site that bonds with one kind of substrate.

However it suggests that the structure is rigid and that bonds can only be made to the active site.

Induced fit theory shows us that the structure is flexible and can be changed by the substrate or by bonds at an allosteric site (not the active site.) This helps us see how reactions are sped up- because the substrate does not have to fit into the enzyme in exactly the right way as the enzyme moulds around it.