Models & Functions of Enzyme Action (AQA A Level Biology)

Models of enzyme action

• 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 suggested that the rigid shape of the active site of the enzyme was 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
  • Prior to binding, the substrate and active site and not completely complementary in shape
  • When the substrate binds the active site alters shape and moulds around the substrate

• 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

The lock-and-key hypothesis

The induced-fit hypothesis