Amino Acids

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 both 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:

Tryptophan is a precursor of the neurotransmitter serotonin.
Glycine is a precursor of porphyrins such as heme.
Arginine is a precursor of nitric oxide.
Carnitine is used in lipid transport within the cell.
Ornithine and S-adenosylmethionine are precursors of polyamines.
Homocysteine is an intermediate in S-adenosylmethionine recycling.

Hydroxyproline, hydroxylysine, and sarcosine are also non-protein amino acids. The thyroid hormones are also alpha-amino acids. Some amino acids have even been detected in meteorites, especially in a type known as carbonaceous chondrites. This observation has prompted the suggestion that life may have arrived on earth from an extraterrestrial source.

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Non-protein Functions

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