A comprehensive classification of amino acids is based on their structure and chemical nature. Each amino acid is assigned a three-letter or one-letter symbol. The twenty amino acids found in protein are divided into seven distinct groups. Example: Glycine.

Glycine has the smallest functional group (hydrogen) of any of the α amino acids

1. On the basis of composition of side chain.
2. On the basis of number of amino and carboxylic groups.
3. On the basis of polarity of the side chain.
4. On the basis of metabolic fate.
5. On the basis of nutritional requirement.