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Tertiary & Quaternary Structure

Biology: Cambridge International A Level, topic 2.3.3

Tertiary Structure of Proteins

The secondary polypeptide folds further to form a tertiary, three-dimensional polypeptide chain.

R groups

R groups

  • R groups are the side chains on amino acids.
  • Interactions between R groups creates the complex 3D tertiary structure of a protein.
  • The 3D structure is usually coiled or folded.
Holding the 3D structure

Holding the 3D structure

  • There are many weak and strong interactions that determine the final 3D shape of the protein (e.g. ionic bonds, hydrogen bonds and disulphide bridges).
  • When the protein loses its 3D shape, it may no longer be functional.
Ionic bonds

Ionic bonds

  • Charged amino acids have a positively or negatively charged ion in their side chain.
  • Charged amino acids can form relatively strong ionic bonds with other charged amino acids.
  • Ionic bonds between amino acids are quite rare.
Disulfide bridges

Disulfide bridges

  • Disulfide bridges are also known as S-S bonds or disulfide bonds.
  • These are covalent bonds set up within proteins containing cysteine (CYS) amino acids.
Hydrogen bonds

Hydrogen bonds

  • Hydrogen bonds also form between amino acids.
  • Hydrogen bonds are relatively weak but when there are many, the overall stability of the tertiary structure increases.

Quaternary Structure of Proteins

Multiple 3D polypeptides can come together to form a complex, quaternary structure. Examples of proteins with a quaternary structure are:

Insulin

Insulin

  • Weak interactions between the subunits in the insulin polypeptide help to stabilise the overall quaternary structure.
  • Insulin has a combination of hydrogen bonds and disulfide bridges that cause it to form a clumped, globular (ball) shape.
Haemoglobin

Haemoglobin

  • Haemoglobin contains four polypeptides (subunits) surrounding a central haem group.
Collagen

Collagen

  • Collagen is a fibrous protein found in skin.

Test yourself on Tertiary & Quaternary Structure

  • Which of these statements about the tertiary structure of proteins are true?

  • Which of these is NOT a bond that gives rise to the tertiary structure?

  • Which structure involves multiple 3D polypeptides in one complex structure?

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