CIE A Level Biology (9700) 2019-2021

Revision Notes

2.3.3 Protein Shape

Proteins: Interactions & Shape

  • A polypeptide chain will fold differently due to the interactions (and hence the bonds that form) between R groups. The three-dimensional configuration that forms is called the tertiary structure of a protein
  • Each of the twenty amino acids that make up proteins has a unique R group and therefore many different interactions can occur creating a vast range of protein configurations and therefore functions
  • Within tertiary structured proteins are the following bonds:
    • Strong covalent disulphide
    • Weak hydrophobic interactions
    • Weak hydrogen
    • Ionic

Interactions within a protein, downloadable AS & A Level Biology revision notes
The interactions that occur between the R groups of amino acids determines the shape and function of a protein. These interactions are found within tertiary structures of proteins


  • Disulphide bonds are strong covalent bonds that form between two cysteine R groups (as this is the only amino acid with a sulphur atom)
  • These bonds are the strongest within a protein, but occur less frequently, and help stabilise the proteins
  • These are also known as disulphide bridges
  • Can be broken by oxidation
  • Disulphide bonds are common in proteins secreted from cells eg. insulin


  • Ionic bonds form between positively (amine group -NH3+) and negatively charged (carboxylic acid -COO) R groups
  • Ionic bonds are stronger than hydrogen bonds but they are not common
  • The bonds are broken by pH changes


  • Hydrogen bonds form between strongly polar R groups. These are the weakest bonds that form but the most common as they form between a wide variety of R groups

Hydrophobic interactions

  • Hydrophobic interactions form between the non-polar (hydrophobic) R groups within the interior of proteins

Exam Tip

You need to be able to determine which bonds are found in tertiary structures and recognise them in diagrams.

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