Aromatic amino acids, like other proteinogenic amino acids, are the building blocks of proteins and include phenylalanine, tryptophan, and tyrosine. All plants and micro-organisms synthesize their own aromatic amino acids to make proteins. However, animals have lost these costly metabolic pathways for aromatic amino acids synthesis and must instead obtain the amino acids through their diet. Herbicides and antibiotics take advantage of this by inhibiting enzymes involved in aromatic amino acid synthesis, thereby making them toxic to micro-organisms and plants but not to animals.

In animals and humans, aromatic amino acids serve as precursors for the synthesis of many biologically/neurologically active compounds that are essential for maintaining normal biological functions. Tyrosine is the initial precursor for the biosynthesis of dopa, dopamine, octopamine, norepinephrine and epinephrine, etc., that are fundamental by functioning as neurotransmitters or hormones for animals and humans. In addition, tyrosine is the precursor for melanin synthesis in most organisms including humans and animals, and is particularly important in insects for protection. Tryptophan is the initial precursor for the biosynthesis of tryptamine, serotonin, auxin, kynurenines, and melatonin. Kynurenic acid, a kynurenine, produced along the tryptophan-kynurenine pathway, is an antagonist at excitatory amino acid receptors and plays a role in protecting neurons from overstimulation by excitatory neurotransmitters. Many enzymes involved in aromatic amino acids metabolism have been drug targets for diseases including neurodegenerative diseases, schizophrenia, and cancers.

This Research Topic in Frontiers in Molecular Biosciences will provide recent advances in aromatic amino acid metabolism, including the following:

1) Structure and function relationship of enzymes involved in aromatic amino acid metabolism;

2) Aromatic amino acid metabolism regulation and abnormality in disease conditions;

3) Structural analysis, biochemical characterization, and inhibition of the aromatic amino acid metabolic enzymes as potential drug targets, i.e. indoleamine 2,3-dioxygenase, Kynurenine monooxygenase, kynurnenine aminotransferases, dopa decarboxylase, tyrosine hydroxylase, etc.

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