What is Quaternary protein structure and why is it important?

Proteins serve many functions important for life and their function depends on their structure. We can say that proteins are organised into 4 layers of 'structure'. At the first level we have many molecular units called 'Peptides' linked in a chain or 'poly-peptide' as 'poly' means 'many'. The order of the peptides is the Primary structure. Depending on this order of the different types of peptide, certain electrostatic attractions called hydrogen bonds form and pulls the chain into either a spiral, or layered sheets (like a pastry) called an ‘alpha helix’ and ‘beta sheets’ respectively. This is Secondary structure. The overall shape of the polypeptide caused by both alpha helices and beta sheets we call tertiary structure. When we find more than one polypeptide (each with their own primary, secondary and tertiary structure) functioning together, we call this overall structure quaternary structure. This is seen in haemoglobin, the oxygen carrying protein, found in red blood cells which has 4 sub-units/ polypeptides each of which bind to a molecule of oxygen. Due to its quaternary structure, the strength of binding or ‘affinity’ each polypeptide has to oxygen (altered via its shape or ‘conformation’) depends on whether its ‘neighbour’ polypeptide unit is bound to oxygen (co-affinity). This becomes very important in the dissociation and distribution of oxygen appropriately to the different tissues of the body….