Aromatic amino acid
An aromatic amino acid is an amino acid that includes an aromatic ring. 

Among the 20 standard amino acids, the following are classically considered aromatic: phenylalanine, tryptophan and tyrosine.

Properties and function

Optical properties
Aromatic amino acids absorb ultraviolet light at a above 250 nm and fluoresce. This characteristic is used in quantitative analysis, notably in determining the concentrations of these amino acids in solution. Most proteins absorb at 280 nm due to the presence of aromatic amino acid residues. Of the aromatic amino acids, tryptophan has the highest extinction coefficient; its absorption maximum occurs at 280 nm. The absorption maximum of tyrosine occurs at 274 nm.

Role in protein structure and function 
Aromatic amino acids stabilize folded structures of many proteins.  Aromatic residues are found predominantly sequestered within the cores of globular proteins, although often comprise key portions of protein-protein or protein-ligand interaction interfaces on the protein surface.

Aromatic amino acids as precursors 
Aromatic amino acids often serve as the precursors to other important biochemicals.

Phenylalanine is the precursor to tyrosine.
Tyrosine is the precursor to  L-DOPA, dopamine, norepinephrine, and Epinephrine.  It is also precursor to octopamine and melanin in numerous organisms. Tyrosine is the precursor to the thyroid hormone thyroxine.
Tryptophan is the precursor to 5-hydroxytryptophan and then serotonin, tryptamine, auxin, kynurenines, and melatonin.

Biosynthesis

Shikimate pathway 
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. Glyphosate, a type of herbicide, is used to control the accumulation of excess greens. In addition to destroying greens, Glyphosate can easily affect the maintenance of the gut microbiota in host organisms by specifically inhibiting the 5-enolpyruvylshikinate-3-phosphate synthase which prevents the biosynthesis of essential aromatic amino acids. Inhibition of this enzyme results in disorders such as gastrointestinal diseases and metabolic diseases.

Nutritional requirements 
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. 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; therefore, it can be synthesized by the animal, but only from phenylalanine. Phenylketonuria, a genetic disorder that occurs as a result of the inability to breakdown phenylalanine, is due to a lack of the enzyme phenylalanine hydroxylase. A dietary lack of tryptophan can cause stunted skeletal development. Excessive intake of aromatic amino acids far beyond levels obtained through normal protein consumption might lead to hypertension, something which could go un-noticed for a long time in healthy individuals. It could be caused by other factors as well such as the use of various herbs and foods like chocolate which inhibit monoamine oxidase enzymes to varying degrees, and also some medications. Aromatic trace amines like tyramine can displace norepinephrine from peripheral monoamine vesicles and in people taking MAOIs this occurs to the extent of being life threatening. for Blue diaper syndrome is an autosomal recessive disease that is caused by poor tryptophan absorption in the body.

See also 
 Aromatic L-amino acid decarboxylase
 Expanded genetic code
Phenylketonuria
Tyrosine hydroxylase
Neurotransmitter

References

Further reading

External links 

Amino acids
 

