CIE A Level Biology (9700) 2019-2021

Revision Notes

3.1.3 How Enzymes Work

How Enzymes Work

The lock-and-key hypothesis

  • Enzymes are globular proteins
  • This means their shape (as well as the shape of the active site of an enzyme) is determined by the complex tertiary structure of the protein that makes up the enzyme and is therefore 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 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

Exam Tip

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


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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