Protein Shape (Cambridge (CIE) AS Biology): Revision Note

Proteins: interactions & shape

• A polypeptide chain will fold differently due to the interactions (and 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

  • This means that 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

  • Ionic

  • Weak hydrogen

  • Weak hydrophobic interactions

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