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First teaching 2023

First exams 2025

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Protein Shape (CIE AS Biology)

Revision Note

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Cara Head

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Cara Head

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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 proteins with a tertiary structure the following bonds occur:
    • Strong covalent disulfide
    • Weak hydrophobic interactions
    • Weak hydrogen
    • Ionic

Bonds in Proteins Diagram

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

Disulfide

  • Disulfide bonds are strong covalent bonds that form between two cysteine R groups (as this is the only amino acid with an available sulfur atom in its R group)
  • These bonds are the strongest within a protein, but occur less frequently, and help stabilise the proteins
  • These are also known as disulfide bridges
  • Can be broken by reduction
  • Disulfide bonds are common in proteins secreted from cells e.g. insulin

Ionic

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

Hydrogen

  • 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

Examiner Tip

Note that an interaction is not the same as a bond. An interaction refers to a low energy attraction between two groups, whereas a bond is a high energy and typically involves the electrons of the molecules concerned.

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

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding