How Enzymes Work (Cambridge International AS Biology)

How enzymes work

The lock-and-key hypothesis

• Enzymes are globular proteins

• This means their shape (as well as the shape of their active site) is determined by the complex tertiary structure of the protein that makes up the enzyme

  • This means their shape is highly specific

• In the 1890’s the first model of enzyme activity was described by Emil Fischer:

  • He suggested that both enzymes and substrates were rigid structures that locked into each other very precisely, much like a key going into a lock
  • This is known as the ‘lock-and-key hypothesis’

• This was later modified and adapted to our current understanding of enzyme activity, permitted by advances in techniques in the molecular sciences

The lock-and-key hypothesis

The induced-fit hypothesis

• The modified model of enzyme activity is known as the ‘induced-fit hypothesis’

• Although it is very similar to the lock and key hypothesis, in this model the enzyme and substrate interact with each other:

  • 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 an ideal binding arrangement between the enzyme and substrate is achieved
  • This maximises the ability of the enzyme to catalyse the reaction

The induced-fit hypothesis

Exam Hint Don't forget – our current understanding of enzyme-substrate interactions is based on the induced-fit hypothesis.