Proteins are said to have quaternary structure if they consist of 2 or more polypeptide chains united by forces other than covalent bonds (i.e., not peptide or disulphide bonds). The forces that stabilise these aggregates are hydrogen bonds and electrostatic bonds formed between amino acids on the polypeptide chains. Haemoglobin, the oxygen-transporting protein of blood, is an example of quaternary structure. This protein consists of four polypeptide chains of two types- and chains that occur in pairs.