Tryptophan Hydroxylase: ProductsTryptophan hydroxylase (TPH) belongs to the pterin-dependent, aromatic amino-acid hydroxylase family. It is found in peripheral tissues and pineal gland, and catalyzes the rate-limiting step in serotonin generation from L-tryptophan. The molecule contains two domains, an N-terminal regulatory domain and a C-terminal catalytic domain that contains an iron atom. TPH is constitutively active but unstable. Phosphorylation by PKA at S58 stabilizes the enzyme and increases its activity by 25%. This action is complemented by 14-3-3 proteins that physically interact with phosphorylated TPH to increase both its stability and activity by another 15% to 45% of baseline. Binding of the acidic carboxy terminus of 14-3-3 with the regulatory region of TPH also blocks phosphatase activity directed towards TPH, prolonging its activity.