The vast majority of the proteins found in living organisms are composed of only 20 different kinds of amino acids, repeated many times and strung together in a particular order. Interactions among the amino acids cause the protein chain to assume a characteristic secondary structure and, under some circumstances, a tertiary structure. The secondary structure is a function of the angles formed by the peptide bonds that link together the amino acids. These bond angles are held in position by the development of hydrogen bonds between the nitrogen-bound hydrogen atom of one amino acid unit and the oxygen atom of another. The tertiary structure refers to the looping and folding of the protein chain back upon itself.
Tertiary structure is determined largely by the side chains of the amino acids.
Some side chains are so large that they disrupt the regular helical secondary structure of the chain, causing it to have kinks and bends. Hydrophilic side chains, which readily form hydrogen bonds with water molecules, are left on the outside of the protein structure. The spatial conformation of these chains is known as the quaternary structure. Quaternary structure is maintained by the same kinds of forces that determine tertiary structure.