CIE A Level Biology (9700) 2019-2021

Revision Notes

3.1.2 Enzyme Action

Mode of Enzyme Action

  • Enzymes have an active site where specific substrates bind forming an enzyme-substrate complex
  • The active site of an enzyme has a specific shape to fit a specific substrate
  • Extremes of heat or pH can change the shape of the active site, preventing substrate binding – this is called denaturation
  • Substrates collide with the enzymes active site and this must happen at the correct orientation and speed in order for a reaction to occur

Active site, downloadable AS & A Level Biology revision notes

The active site of an enzyme has a specific shape to fit a specific substrate (when the substrate binds an enzyme-substrate complex is formed)

  • The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
  • The shape of the active site (and therefore the specificity of the enzyme) is determined by the complex tertiary structure of the protein that makes up the enzyme:
    • Proteins are formed from chains of amino acids held together by peptide bonds
    • The order of amino acids determines the shape of an enzyme
    • If the order is altered, the resulting three-dimensional shape changes

Enzyme Specificity Examples, downloadable IGCSE & GCSE Biology revision notes

 

An example of enzyme specificity – the enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not

  • An enzyme-substrate complex forms when an enzyme and its substrate join together
  • The enzyme-substrate complex is only formed temporarily, before the enzyme catalyses the reaction and the product(s) are released

Enzyme-substrate complex, downloadable AS & A Level Biology revision notes

The temporary formation of an enzyme-substrate complex

  • Enzyme reactions can either be catabolic or anabolic
  • Catabolic reactions involve the breakdown of complex molecules into simpler products, which happens when a single substrate is drawn into the active site and broken apart into two or more distinct molecules
  • Examples of catabolic reactions include cellular respiration and hydrolysis reactions

Catabolic reaction, downloadable AS & A Level Biology revision notes

A catabolic reaction

  • Anabolic reactions involve the building of more complex molecules from simpler ones by drawing two or more substrates into the active site, forming bonds between them and releasing a single product
  • Examples of anabolic reactions include protein synthesis and photosynthesis

Anabolic reaction, downloadable AS & A Level Biology revision notes

An anabolic reaction

Enzymes work by lowering the activation energy of a reaction

  • All chemical reactions are associated with energy changes
  • For a reaction to proceed there must be enough activation energy
  • Activation energy is the amount of energy needed by the substrate to become just unstable enough for a reaction to occur and for products to be formed
    • Enzymes speed up chemical reactions because they influence the stability of bonds in the reactants
    • The destabilisation of bonds in the substrate makes it more reactive
  • Enzymes work by lowering the activation energy of a reaction and in doing so they provide an alternative energy pathway

Activation energy with and without catalyst, downloadable AS & A Level Biology revision notes

The activation energy of a chemical reaction is lowered by the presence of a catalyst (ie. an enzyme

Exam Tip

Don’t forget that both enzymes and their substrates are highly specific to each other – this is known as enzyme-substrate specificity.

Author:

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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