Aromatic amino acid

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An aromatic amino acid (AAA)[1] is an amino acid that includes an aromatic ring.

Examples include:

PhenylalanineTyrosineL-DOPA → (Dopamine) → (Norepinephrine) → (Epinephrine)

Tryptophan5-hydroxytryptophan → (Serotonin)


Phenylalanine, tryptophan, and histidine 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. A lack of the enzyme phenylalanine hydroxylase, used in tyrosine synthesis, causes phenylketonuria and concurrently renders tyrosine an essential amino acid.

Amino acids do not absorb light in the visible region of the electromagnetic spectrum, however, all absorb light from the infrared region due to their conjugated double bonds and the aromatic amino acids also absorb ultraviolet radiation. The aromatic amino acids phenylalanine, tyrosine, and tryptophan have the ability to significantly absorb ultraviolet radiation above 250 nm.This characteristic of aromatic amino acids is used to quantify the concentration of proteins in an unknown sample. These amino acids are able to absorb light which excites its electron to the excited state. When the electron falls back to its ground state, it will either emit light or release energy. If the molecule is able to emit light it is known as a fluorescent molecule. Tryptophan is widely used as a fluorescent molecule.[2]

Animals obtain aromatic amino acids from their diet, but all plants and micro-organisms must synthesize their aromatic amino acids through the metabolically costly shikimate pathway in order to make them.

In plants, the shikimate pathway first leads to the formation of chorismate, which is the precursor of phenylalanine, tyrosine, and tryptophan. These aromatic amino acids are the derivatives of many secondary metabolites, all essential to a plant's biological functions, such as the hormones salicylate and auxin. This pathway contains enzymes that can be regulated by inhibitors, which can cease the production of chorismate, and ultimately the organism's biological functions. Herbicides and antibiotics work by inhibiting these enzymes involved in the biosynthesis of aromatic amino acids, thereby rendering them toxic to plants.[3]

See also[edit]


  1. ^ Logan, Carolynn M.; Rice, M. Katherine (1987). Logan's Medical and Scientific Abbreviations. Philadelphia: J. B. Lippincott Company. p. 3. ISBN 978-0-397-54589-6.
  2. ^ Möller, Matías; Denicola, Ana (2002-05-01). "Protein tryptophan accessibility studied by fluorescence quenching". Biochemistry and Molecular Biology Education. 30 (3): 175–178. doi:10.1002/bmb.2002.494030030035. ISSN 1539-3429.
  3. ^ Tzin, Vered; Galili, Gad (2010-05-17). "The Biosynthetic Pathways for Shikimate and Aromatic Amino Acids in Arabidopsis thaliana". The Arabidopsis Book / American Society of Plant Biologists. 8: e0132. doi:10.1199/tab.0132. ISSN 1543-8120. PMC 3244902. PMID 22303258.

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