When taken up into the human body from the diet, the 22 standard amino acids either are used to synthesize proteins and other biomolecules or are oxidized to urea and carbon dioxide as a source of energy. The amino acid 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 amino acid transamidation is a keto acid that enters the citric acid cycle. Glucogenic amino acids can also be converted into glucose, through gluconeogenesis.
Pyrrolysine trait is restricted to several microbes, and only one organism has both Pyl and Sec. Of the 22 standard amino acids, 8 are called essential amino acids because the human body cannot synthesize them from other compounds at the level needed for normal growth, so Amino Acids must be obtained from food. In addition, cysteine, taurine, tyrosine, histidine, and arginine are semiessential amino-acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed. The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids.
In humans, non-protein amino acids also have important roles as metabolic intermediates, such as in the biosynthesis of the neurotransmitter gamma-aminobutyric acid. Many amino acids are used to synthesize other molecules, for example:
Tryptophan is a precursor of the neurotransmitter amino acid serotonin. Tyrosine Amino Acids is a precursor of the neurotransmitter dopamine.
Glycine is a precursor of porphyrins such as heme.
Arginine is a precursor of nitric oxide.
Ornithine and S-adenosylmethionine are precursors of polyamines.
Aspartate, glycine, and glutamine are precursors of nucleotides.
Phenylalanine is a precursor of various phenylpropanoids, which are important in amino acid plant metabolism.
However, not all of the functions of other abundant non-standard amino acids are known. For example, taurine is a major amino acid in muscle and brain tissues, but, although many functions have been proposed, its precise amino acid role in the body has not been determined.
Some non-standard amino acids are used as defenses against herbivores in plants.For example canavanine is an analogue of arginine that is found in many legumes,and in particularly large amounts in Canavalia gladiata (sword bean). This amino acid protects the plants from predators such as insects and can cause illness in people if some types of legumes are eaten without processing this amino acid. The non-protein amino acid mimosine is found in other species of legume, particularly Leucaena leucocephala. This Amino Acids compound is an analogue of tyrosine and can poison animals that graze on these plants.