Tyrosine (from the Greek tyros = cheese) is a non-essential amino acid that humans can synthesize completely from phenylalanine. An enzyme defect in phenylalanine hydroxylase leads to phenylketonuria and thus to a lack of L-tyrosine.tyrosine is significantly involved in the formation of melanin, and tyrosine is also a precursor of neurotransmitters such as dopamine and noradrenaline, as well as thyroxine, triiodothyronine and melanin.