Tyrosine (Tyr) or L-tyrosine is an alpha-amino acid. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (alpha carbon).  Amino acids are organic compounds that contain amino (‚ÄìNH2) and carboxyl (‚ÄìCOOH) functional groups, along with a side chain (R group) specific to each amino acid. L-tyrosine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Tyrosine is found in all organisms ranging from bacteria to plants to animals. It is classified as a non-polar, uncharged (at physiological pH) aromatic amino acid.  Tyrosine is a non-essential amino acid, meaning the body can synthesize it ‚Äì usually from phenylalanine. The conversion of phenylalanine to tyrosine is catalyzed by the enzyme phenylalanine hydroxylase, a monooxygenase. This enzyme catalyzes the reaction causing the addition of a hydroxyl group to the end of the 6-carbon aromatic ring of phenylalanine, such that it becomes tyrosine. Tyrosine is found in many high-protein food products such as chicken, turkey, fish, milk, yogurt, cottage cheese, cheese, peanuts, almonds, pumpkin seeds, sesame seeds, soy products, lima beans, avocados and bananas. Tyrosine is one of the few amino acids that readily passes the blood-brain barrier. Once in the brain, it is a precursor for the neurotransmitters dopamine, norepinephrine and epinephrine, better known as adrenalin. These neurotransmitters are an important part of the body's sympathetic nervous system, and their concentrations in the body and brain are directly dependent upon dietary tyrosine. Tyrosine is not found in large concentrations throughout the body, probably because it is rapidly metabolized. Folic acid, copper and vitamin C are cofactor nutrients of these reactions. Tyrosine is also the precursor for hormones, including thyroid hormones (diiodotyrosine), catecholestrogens and the major human pigment, melanin. Tyrosine is an important amino acid in many proteins, peptides and even enkephalins, the body's natural pain reliever. Valine and other branched amino acids, and possibly tryptophan and phenylalanine may reduce tyrosine absorption. A number of genetic errors of tyrosine metabolism have been identified, such as hawkinsinuria and tyrosinemia I. The most common feature of these diseases is the increased amount of tyrosine in the blood, which is marked by decreased motor activity, lethargy and poor feeding. Infection and intellectual deficits may occur. Vitamin C supplements can help reverse these disease symptoms. Some adults also develop elevated tyrosine in their blood. This typically indicates a need for more vitamin C. More tyrosine is needed under stress, and tyrosine supplements prevent the stress-induced depletion of norepinephrine and can help aleviate biochemical depression. However, tyrosine may not be good for treating psychosis. Many antipsychotic medications apparently function by inhibiting tyrosine metabolism. L-Dopa, which is directly used in Parkinson's, is made from tyrosine. Tyrosine, the nutrient, can be used as an adjunct in the treatment of Parkinson's. Peripheral metabolism of tyrosine necessitates large doses of tyrosine, however, compared to L-Dopa (http://www.dcnutrition.com). In addition to its role as a precursor for neurotransmitters, tyrosine plays an important role for the function of many proteins.  Within many proteins or enzymes, certain tyrosine residues can be tagged (at the hydroxyl group) with a phosphate group (phosphorylated) by specialized protein kinases. In its phosphorylated form, tyrosine is called phosphotyrosine. Tyrosine phosphorylation is considered to be one of the key steps in signal transduction and regulation of enzymatic activity.  Tyrosine (or its precursor phenylalanine) is also needed to synthesize the benzoquinone structure which forms part of coenzyme Q10.