phenylalanine, tyrosine and tryptophan biosynthetic pathway KEGG:00400 PW:0000075 Those metabolic reactions involved in the synthesis of phenylalanine, tyrosine and tryptophan and which are intimately related. Although the body can not manufacture phenylalanine on its own, phenylalanine is the precursor of tyrosine. Phenylalanine together with tryptophan governs the release of an intestinal hormone - cholecystokinin. pathway amino acid metabolic pathway (KEGG)