In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. In biochemistry, this term refers to alpha-amino acids with the general formula H₂NCHRCOOH, where R is an organic substituent. Proline is an exception to this general formula. It lacks the NH₂ group because of the cyclization of the side chain. In the alpha amino acids, the amino and carboxylate groups are attached to the same carbon, which is called the α–carbon. The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon. They can vary in size from just a hydrogen atom in glycine through a methyl group in alanine to a large heterocyclic group in tryptophan.

Beyond the amino acids that are found in all forms of life, many non-natural amino acids have vital roles in technology and industry. For example, the chelating agents EDTA and nitrilotriacetic acid are alpha amino acids that are important in the chemical industry.

Overview

Twenty standard amino acids are used by cells in protein biosynthesis, and these are specified by the general genetic code. These 20 amino acids are biosynthesized from other molecules, but organisms differ in which ones they can synthesize and which ones must be provided in their diet. The ones that cannot be synthesized by an organism are called essential amino acids.

Functions in proteins

Non-protein functions

The 20 standard amino acids are either used to synthesize proteins and other biomolecules or oxidized to urea and carbon dioxide as a source of energy. The oxidation pathway starts with the removal of the amino group by a transaminase, the amino group is then fed into the urea cycle. The other product of transamidation is a keto acid that enters the citric acid cycle. Glucogenic amino acids can also be converted into glucose, through gluconeogenesis.

Hundreds of types of non-protein amino acids have been found in nature and they have multiple functions in living organisms. Microorganisms and plants can produce uncommon amino acids. In microbes, examples include 2-aminoisobutyric acid and lanthionine, which is a sulfide-bridged alanine dimer. Both these amino acids are found in peptidic lantibiotics such as alamethicin. While in plants, 1-aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is a key intermediate in the production of the plant hormone ethylene.

In humans, non-protein amino acids also have important roles, such as the neurotransmitter gamma-aminobutyric acid. Many amino acids are used to synthesize other molecules, for example: