Now l-tyrosine with L-tirosine 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 regulating the nervous system. Catecholamines are neurotransmitters involved in alert and focus, relaxation and mood of mind.