Now L-Tyrosine It is an essential amino acid formed from Phenylalanine through an enzyme called phenylalanine hydroxylase.  This amino acid acts as a precursor of catecholamines (Epinephrine, Norepinephrine and Dopamine), thus justifying its importance in the regulation of the Nervous System.  Catecholamines are neurotransmitters involved in alertness and focus, relaxation, and mood.