AQA AS Biology

Revision Notes

1.4.2 Enzyme Specificity

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: Amelia

While studying Biochemistry at Oxford University, Amelia started her own tutoring service, helping to connect Science tutors with students in her local area. Amelia has experience teaching the sciences and Maths at all levels to UK and international students and, as well as being our Biology Lead, designs revision resources for Chemistry.
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