Tryptophan is among the less common amino acids found in proteins, but it plays important structural or functional roles whenever it occurs. For instance, tryptophan and tyrosine residues play special roles in "anchoring" membrane proteins within the cell membrane.
Tryptophan functions as a biochemical precursor for the neurotransmitter serotonin, the neurohormone melatonin, niacin (vitamin B3), and a class of phytohormones called auxins.
* Amino acids which are essential amino acids which cannot be made by the human body and, therefore, must be obtained in the diet.
Tillery, Enger and Ross