Aromatic Amino Acids

Aromatic Amino Acids • Aromatic amino acids (AAAs) are amino acids that include an aromatic ring. Tryptophan (Trp), Phenylalanine (Phe), and Tyrosine (Tyr) are aromatic amino acids (AAAs) required for protein biosynthesis in all living cells. Phenylalanine and tryptophan are essential amino acids for animals. Since they are not synthesized in the human body, they must be derived from the diet. Tyrosine is semi-essential; it can be synthesized, but only from phenylalanine. Animals obtain aromatic amino acids from their diet, but all plants and microorganisms must synthesize their aromatic amino acids through the metabolically costly shikimate pathway in order to make them.

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• In plants, these AAAs also serve as precursors of a wide variety of plant natural products that play crucial roles in plant growth, development, reproduction, defense, and environmental responses. Trp is a precursor of alkaloids, phytoalexins, and indole glucosinolates as well as the plant hormone auxin, whereas Tyr is a precursor of isoquinoline alkaloids, pigment betalains, and quinones (tocochromanols and plastoquinone). Phe is a common precursor of numerous phenolic compounds, which include flavonoids, condensed tannins, lignans, lignin, and phenylpropanoid/benzenoid volatiles. Of the three AAAs, the highest carbon flux is often directed to Phe, as Phe-derived compounds can constitute up to 30% of organic matter in some plant species. All three AAAs are produced from the final product of the shikimate pathway, chorismate, which is also a precursor for vitamins K1 and B9 and the plant defense hormone salicylic acid. The AAA pathways consist of the shikimate pathway and individual postchorismate pathways leading to Trp, Phe, and Tyr. These pathways are found in bacteria, fungi, plants, and some protists but are absent in animals. Therefore, AAAs and some of their derivatives (vitamins) are essential nutrients in the human diet.