Tryptophan is one of the 22 standard amino acids and an essential amino acid in the human diet. It is encoded in the standard genetic code as the codonUGG.
Only the L-stereoisomer of tryptophan is used in structural or enzyme proteins, but the R-stereoisomer is occasionally found in naturally produced peptides (for example, the marine venom peptide contryphan).The distinguishing structural characteristic of tryptophan is that it contains an indole functional group.
In bacteria that synthesize tryptophan, high cellular levels of this amino acid activate a repressor protein, which binds to the trp operon.Binding of this repressor to the tryptophan operon prevents transcription of downstream DNA that codes for the enzymes involved in the biosynthesis of tryptophan. So high levels of tryptophan prevent tryptophan synthesis through a negative feedback loop and, when the cell's tryptophan levels are reduced, transcription from the trp operon resumes.
The genetic organisation of the trp operon thus permits tightly regulated and rapid responses to changes in the cell's internal and external tryptophan levels.