In tertiary structure, the polypeptide chain having secondary structure is twisted and folded to produce a stable, three-dimensional structure, which may be maintained by three types of bonds e.g. ionic bond, hydrogen bond and disulphide bond. In a large protein like haemoglobin, or in the case of an enzyme, the molecule undergoes further folding and coiling to attain functional conformation and is termed as the tertiary structure. The coils and folds of the protein molecule are so arranged as to hide non-polar amino acid side chains inside and expose the polar side chains and is absolutely necessary for many biological activities. Quaternary structure is the most complex structure.