Question: why is haemoglobin a tetramer?
Answer: Allostery: Haemoglobin. Haemoglobin, like myoglobin is an oxygen-binding protein. Whereas myoglobin exists as a monomer, haemoglobin is a tetramer: each of the four subunits is similar in terms of fold to myoglobin. ... As haemoglobin binds successive oxygens, the oxygen affinity of the subunits increases.