Quaternary structure of proteins
The quaternary structure appears when the protein is made up of several polypeptide chains, whether or not they have a tertiary structure. Each of these polypeptide chains is called a protomer, monomer, or subunit.
These protomers are linked by weak bonds, such as hydrogen bonds, Van der Waals forces, and even disulfide bridges, as is the case with immunoglobulins.
The collagen is a fibrous protein composed of three polypeptide chains intertwined to form helical larger triple helix conferring great resistance such fibers.
The hemoglobin (see bottom) is a globular protein comprised of four polypeptide chains.
