Some of the tyrosine residues can be tagged (at the hydroxyl group) with a phosphate group (phosphorylated) by 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 residues may also be modified by the addition of a sulfate group, a process known as tyrosine sulfation.
In dopaminergic cells in the brain, tyrosine is converted to L-DOPA by the enzyme tyrosine hydroxylase (TH). TH is the rate-limiting enzyme involved in the synthesis of the neurotransmitter dopamine. Dopamine can then be converted into other catecholamines, such as norepinephrine (noradrenaline) and epinephrine (adrenaline).
The thyroid hormones triiodothyronine (T3) and thyroxine (T4) in the colloid of the thyroid are also derived from tyrosine.
The latex of Papaver somniferum, the opium poppy, has been shown to convert tyrosine into the alkaloid morphine and the bio-synthetic pathway has been established from tyrosine to morphine by using Carbon-14 radio-labelled tyrosine to trace the in-vivo synthetic route.[citation needed]
Tyrosine ammonia lyase (TAL) is an enzyme in the natural phenols biosynthesis pathway. It transforms L-tyrosine into p-coumaric acid.
Tyrosine is also the precursor to the pigment melanin.
Tyrosine (or its precursor phenylalanine) is needed to synthesize the benzoquinone structure which forms part of coenzyme Q10.
