Tyrosine (4-hydroxyphenylalanine, or 2-amino-3(4-hydroxyphenyl)-propanoic acid) is one of the 20 amino acids that are used by cells to synthesize proteins. Tyrosine cannot be completely synthesized by animals, although it can be made by hydroxylation of phenylalanine if the latter is in abundant supply. There are three structural isomers of Tyr, namely para-Tyr (p-Tyr), meta-Tyr (m-Tyr) and ortho-Tyr (o-Tyr). Enzymatically, only the first isomer (p-Tyr) is produced from L-Phe by the Phe-hydroxylase enzyme. The other two isoforms, m-Tyr and o-Tyr can be produced as a consequence of free radical attack on Phe in states with increased oxidative stress. Tyrosine is converted to DOPA by the enzyme, tyrosine hydroxylase. It plays a key role in signal transduction, since it can be tagged with a phosphate group (phosphorylated) by protein kinases to alter the functionality and activity of certain enzymes. (In its phosphorylated state, it is sometimes referred to as phosphotyrosine.) Tyrosine is also a precursor to the thyroid hormones thyroxine and tri-iodothyronine, the pigment melanin, and the biologically-active catecholamines dopamine, norepinephrine and epinephrine.