How Enzymes Work (AQA AS Biology): Revision Note

The induced-fit model

• Also known as the ‘induced-fit hypothesis’

• In this model, the enzyme and substrate interact with each other in the following way:

  • The enzyme and its active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme

  • These changes in shape are known as conformational changes

  • This ensures that an ideal binding arrangement between the enzyme and substrate is achieved

  • This maximises the ability of the enzyme to catalyse the reaction

Development of the induced-fit enzyme model

• Scientists often use models to explain their observations from experiments

• As technology and research advances within a field, new models can be developed and old ones disproven

• The lock and key model covered at GCSE was originally thought to be an accurate model of enzyme action

  • It was suggested that the rigid shape of the active site of the enzyme is a precise fit for the specific shape of the substrate

• New techniques have allowed scientists to discover that proteins are not rigid structures

  • Experiments showed that multiple regions of an enzyme molecule moved in response to the environment

  • Many of these movements were minimal, but some of them were more significant

  • The larger movements occurred when the substrate bound to the enzyme

• These findings led to the now widely accepted induced fit model

• There is evidence to support the induced fit model:

  • X-ray diffraction techniques allow for 3D pictures of molecules to be formed

  • This technique was used to produce pictures of the enzyme hexokinase before and after it bound to its substrate glucose

  • The images confirmed that the active site of the enzyme changed shape after the substrate bound