Tyrosine: An amino acid, one of the 20 building blocks of protein. A nonessential amino acid, tyrosine is produced from another amino acid, phenylalanine. Deficiency of the enzyme phenylalanine hydroxylase that catalyzes this reaction causes the genetic disease called phenylketonuria (PKU). Tyrosine is a precursor of thyroid hormones, catecholamines, and melanin. Symbol: Tyr.