CIE A Level Biology (9700) 2019-2021

Revision Notes

3.2.7 Enzyme Inhibitors

Enzyme Inhibitors

  • An enzyme’s activity can be reduced or stopped, temporarily, by a reversible inhibitor
  • There are two types of reversible inhibitors:
    • Competitive inhibitors have a similar shape to that of the substrate molecules and therefore compete with the substrate for the active site
    • Non-competitive inhibitors bind to the enzyme at an alternative site, which alters the shape of the active site and therefore prevents the substrate from binding to it

Competitive and non-competitive inhibition, downloadable AS & A Level Biology revision notesCompetitive and non-competitive inhibition

  • Reversible inhibitors can act as regulators in metabolic pathways
  • Metabolic reactions must be very tightly controlled and balanced, so that no single enzyme can ‘run wild’ and continuously and uncontrollably generate more and more of a particular product
  • Metabolic reactions can be controlled by using the end-product of a particular sequence of metabolic reactions as a non-competitive, reversible inhibitor:
    • As the enzyme converts substrate to product, the process is itself slowed down as the end-product of the reaction chain binds to an alternative site on the original enzyme, changing the shape of the active site and preventing the formation of further enzyme-substrate complexes
    • The end-product can then detach from the enzyme and be used elsewhere, allowing the active site to reform and the enzyme to return to an active state
    • This means that as product levels fall, the enzyme begins catalysing the reaction once again, in a continuous feedback loop
    • This process is known as end-product inhibition

End-product inhibition, downloadable AS & A Level Biology revision notesEnd-product inhibition


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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