The side chains of amino acids are called R-groups (or organic substituents). All twenty amino acids that are found in biological systems have the functional groups of an amine and a carboxylic acid. Some R-groups of the amino acids also have amine or carboxylic acid functional groups. All of the functional groups of amino acid side chains are instrumental in determining protein function and shape.
The twenty amino acids can be divided into four groups, with the classifications being determined by their R-groups: