Quaternary structure refers to the fact that some proteins are made up of more than one polypeptide subunit joined together, or a polypeptide and an inorganic component. Such proteins only function if all the subunits are present. Quaternary structure may involve two identical polypeptides coming together to form the final working protein, or it may involve a number of different polypeptide subunits coming together. Examples of proteins with quaternary structure include haemoglobin and insulin.
Haemoglobin – a transport protein:
Haemoglobins quaternary structure consists of four polypeptide subunits. 2 are called α- chains. The other 2 are β- chains. The 4 subunits together form 1 haemoglobin molecule, which is a water-soluble globular protein.
As with all proteins, the tertiary structure of each subunit is held in place by a number of bonds and interactions. These interactions give the subunits (and hence the complete molecule) a very specific shape. This shape is vital for the molecule to carry outs its function.
Haemoglobin’s function is to carry oxygen from the lungs to the tissues. It binds oxygen in the lungs and releases it in the tissues. A specialised part of each polypeptide, called a haem group, contains an iron (Fe2+) ion. The haem group is responsible for the colour of haemoglobin.
Haemoglobin + Oxygen àOxyhaemoglobin
(Purple-Red) (Bright Red)
An oxygen molecule can bind to the iron in the haem group. This means that 1 complete haemoglobin molecule…