1. Quarternary structure

2. Many proteins contain more than one polypeptide chain. The quaternary structure describes the number and type of these sub-units. Sub units are held together by the same interactions as tertiary structures.

3. Eg Haemoglobin has four sub units, 2 ά and 2 β

4. ά sub units have 141 amino acids. β sub units have 146. Both are c70% ά helices separated by bends. There are few interactions between like subunits. Foetal haemoglobin has γ sub units instead of β.

5. Prosthetic groups Some proteins also contain non-protein prosthetic groups. Eg; Each sub-unit of haemoglobin consists of globin, a polypeptide chain, and haem. Haem contains Fe 2+ at the centre of a porphyrin ring.

6. Myoglobin Myoglobin has a similar structure to one sub-unit of haemoglobin, complete with prosthetic group.