Aside from being a proteinogenic amino acid, tyrosine has a special role by virtue of the phenol functionality. It occurs in proteins that are part of signal transduction processes and functions as a receiver of phosphate groups that are transferred by way of protein kinases. Phosphorylation of the hydroxyl group can change the activity of the target protein, or may form part of a signaling cascade via SH2 domain binding.
A tyrosine residue also plays an important role in photosynthesis. In chloroplasts (photosystem II), it acts as an electron donor in the reduction of oxidized chlorophyll. In this process, it loses the hydrogen atom of its phenolic OH-group. This radical is subsequently reduced in the photosystem II by the four core manganese clusters.
Dietary requirements and sources
The Dietary Reference Intake (recommended dietary allowance, RDA) for phenylalanine and tyrosine is 42 mg per kilogram of body weight. For a 70 kg person, this is 2.31 g (phenylalanine + tyrosine).
Mammals synthesize tyrosine from the essential amino acid phenylalanine (phe), which is derived from food. The conversion of phe to tyr is catalyzed by the enzymephenylalanine 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.
Conversion of phenylalanine and tyrosine to its biologically important derivatives.
Phosphorylation and sulfation
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. Phosphotyrosine can be detected through specific antibodies. Tyrosine residues may also be modified by the addition of a sulfate group, a process known as tyrosine sulfation.Tyrosine sulfation is catalyzed by tyrosylprotein sulfotransferase (TPST). Like the phosphotyrosine antibodies mentioned above, antibodies have recently been described that specifically detect sulfotyrosine.
Tyrosine is a precursor to trace amine compounds and the catecholamines.
Precursor to alkaloids
The latex of Papaver somniferum, the opium poppy, has been shown to convert tyrosine into the alkaloidmorphine 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.
The decomposition of L-tyrosine (syn. para-hydroxyphenylalanine) begins with an α-ketoglutarate dependent transamination through the tyrosine transaminase to para-hydroxyphenylpyruvate. The positional description para, abbreviated p, mean that the hydroxyl group and side chain on the phenyl ring are across from each other (see the illustration below).
Three structural isomers of L-tyrosine are known. In addition to the common amino acid L-tyrosine, which is the para isomer (para-tyr, p-tyr or 4-hydroxyphenylalanine), there are two additional regioisomers, namely meta-tyrosine (also known as 3-hydroxyphenylalanine, L-m-tyrosine, and m-tyr) and ortho-tyrosine (o-tyr or 2-hydroxyphenylalanine), that occur in nature. The m-tyr and o-tyr isomers, which are rare, arise through non-enzymatic free-radical hydroxylation of phenylalanine under conditions of oxidative stress.
Tyrosine is a precursor to neurotransmitters and increases plasma neurotransmitter levels (particularly dopamine and norepinephrine), but has little if any effect on mood in normal subjects. However, a number of studies have found tyrosine to be useful during conditions of stress, cold, fatigue (in mice), prolonged work and sleep deprivation, with reductions in stress hormone levels, reductions in stress-induced weight loss seen in animal trials, and improvements in cognitive and physical performance seen in human trials.
Tyrosine does not seem to have any significant effect on cognitive or physical performance in normal circumstances, but does help sustain working memory better during multitasking.
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Tyrosine MS Spectrum
Phenylalanine and tyrosine biosynthesis
Phenylalanine, Tyrosine, and tryptophan biosynthesis