2.3.3 Protein 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 disulfide
  • Weak hydrophobic interactions
  • Weak hydrogen
  • Ionic

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 eg. 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