The quaternary structure
The quaternary structure is formed by interactions between multiple polypeptide chains. This includes non-protein groups (known as cofactors) that may associate with the protein.
The molecular interactions involved (same as interactions at tertiary structures) are as follows:
- Hydrogen bond
- Disulphide bond
- Ionic bond
- Hydrophobic interaction
Fibrous proteins and globular proteins are the two main classes of protein within the quaternary structure.
Fibrous proteins are long and thin and tend to take on structural roles.
Collagen, the main component in connective tissue, is a fibrous protein. It is formed of three strands coiled into a triple helix.
Globular proteins are roughly spherical and usually water-soluble. Globular proteins tend to participate in metabolic reactions.
Haemoglobin (the oxygen carrier in red blood cells) is an example of a globular protein. It is made up of four polypeptide chains.