Refer figure 9-3 (d):
The three-dimensional structure of a multi-subunit protein results in quaternary structure, which is stabilized by disulfide bonds and non-covalent interactions. Haemoglobin, which is a heterotetramer, is the best example for the quaternary structure made of different multiple subunits like two α-chains and two β-chains.
The α-subunit is encoded by four genes whereas, the β-subunit is encoded by two genes and the subunits attached to each other. Sometimes mutation may occur in the subunits resulting abnormal haemoglobin proteins and lose some of its functions.
The structure of the protein is important in determining the function of a protein. But the formation of the quaternary structure may get stopped due to a mutation in the β-subunit that inhibits the connection of α and β-subunits and blocks the function of the protein. A corresponding mutation in the α-subunit create the ability to bond with the β-subunit mutant and efficiently control the mutation in β-subunit thus, resulting in the formation of a normal protein structure.