Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a non-polar aromatic amino acid. It is essential in humans, meaning the body cannot synthesize it: it must be obtained from the diet. Tryptophan is also a precursor to the neurotransmitter [[serotonin]] and the hormone [[melatonin]].
It is encoded by the codon UGG.
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A methylated [[tryptophan]] with [[immune checkpoint inhibitor]]y activity. [[Indoximod]] inhibits the enzyme indoleamine 2,3-dioxygenase ([[IDO]]), which degrades the essential [[amino acid]] [[tryptophan]], and may increase or maintain tryptophan levels important to [[T cell]] function. Tryptophan depletion is associated with [[immunosuppression]] involving T cell arrest and [[anergy]]. 
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