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In humans, non-protein amino acids also have important roles as metabolic intermediates, such as in the biosynthesis of the neurotransmitter gamma-amino-butyric acid (GABA). Many amino acids are used to synthesize other molecules, for example: Tryptophan is a precursor of the neurotransmitter serotonin.[57] Tyrosine (and its precursor phenylalanine) are precursors of the catecholamine neurotransmitters dopamine, epinephrine and norepinephrine. Glycine is a precursor of porphyrins such as heme.[58] Arginine is a precursor of nitric oxide.[59] Ornithine and S-adenosylmethionine are precursors of polyamines.[60] Aspartate, glycine, and glutamine are precursors of nucleotides.[61] Phenylalanine is a precursor of various phenylpropanoids, which are important in plant metabolism. However, not all of the functions of other abundant non-standard amino acids are known. Some non-standard amino acids are used as defenses against herbivores in plants.[62] For example, canavanine is an analogue of arginine that is found in many legumes,[63] and in particularly large amounts in Canavalia gladiata (sword bean).[64] 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.[65] The non-protein amino acid mimosine is found in other species of legume, in particular Leucaena leucocephala.[66] This compound is an analogue of tyrosine and can poison animals that graze on these plants. |
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