Tyrosine (abbreviated as Tyr or Y) or 4-hydroxyphenylalanine, is one of the 20 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Greek ''tyri'', meaning ''cheese'', as it was first discovered in 1846 by German chemist Justus von Liebig in the protein casein from cheese. It is called tyrosyl when referred to as a functional group or side chain.
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 undergoes deprotonation of its phenolic OH-group. This radical is subsequently reduced in the photosystem II by the four core manganese clusters.
In plants and most microorganisms, tyr is produced via prephenate, an intermediate on the shikimate pathway. Prephenate is oxidatively decarboxylated with retention of the hydroxyl group to give ''p''-hydroxyphenylpyruvate, which is transaminated using glutamate as the nitrogen source to give tyrosine and α-ketoglutarate.
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 enzyme phenylalanine hydroxylase, a monooxygenase. This enzyme catalyzes the reaction causing the addition of an hydroxyl group to the end of the 6-carbon aromatic ring of phenylalanine, such that it becomes tyrosine.
The thyroid hormones triiodothyronine (T3) and thyroxine (T4) in the colloid of the thyroid also are derived from tyrosine.
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).
The next oxidation step catalyzes by ''p''-hydroxylphenylpyruvate-dioxygenase and splitting off CO2 homogentisate (2,5-dihydroxyphenyl-1-acetate). In order to split the aromatic ring of homogentisate, a further dioxygenase, homogentistate-oxygenase is required. Thereby, through the incorporation of a further O2 molecule, maleylacetoacetate is created.Fumarylacetate is created maleylacetoacetate-''cis''-''trans''-isomerase through rotation of the carboxyl group created from the hydroxyl group via oxidation. This ''cis-trans''-isomerase contains glutathione as a coenzyme. Fumarylacetoacetate is finally split via fumarylacetoacetate-hydrolase through the addition of a water molecule.
Thereby fumarate (also a metabolite of the citric acid cycle) and acetoacetate (3-ketobutyroate) are liberated. Acetoacetate is a ketone body, which is activated with succinyl-CoA, and thereafter it can be converted into acetyl-CoA which in turn can be oxidized by the citric acid cycle or be used for fatty acid synthesis.
Three isomers of tyrosine are known. In addition to 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 (''m''-tyr or 3-hydroxyphenylalanine or L-m-tyrosine) and ''ortho''-tyrosine (''o''-tyr or 2-hydroxyphenylalanine) which 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.
m-Tyrosine and analogues (rare in nature and therefore available synthetically) have shown application in Parkinson's Disease, Alzheimer's disease and arthritis.
A number of studies have found tyrosine to be useful during conditions of stress, cold, fatigue, loss of a loved one such as in death or divorce, prolonged work and sleep deprivation, with reductions in stress hormone levels, reductions in stress-induced weight loss seen in animal trials, improvements in cognitive and physical performance seen in human trials; however, because tyrosine hydroxylase is the rate-limiting enzyme, effects are less significant than those of l-dopa.
Tyrosine does not seem to have any significant effect on mood, cognitive or physical performance in normal circumstances. A daily dosage for a clinical test supported in the literature is about 100 mg/kg for an adult which amounts to about 6.8 grams at 150 lbs. The usual dosage amounts to 500–1500 mg per day (dose suggested by most manufacturers; usually an equivalent to 1–3 capsules of pure tyrosine). It is not recommended to exceed 12000 mg (12 g) per day. In fact, too high doses result in reduced levels of dopamine. Tyrosine may decrease the absorption of other amino acids in high or chronic doses. It decreases absorption of l-dopa.
Category:Proteinogenic amino acids Category:Glucogenic amino acids Category:Ketogenic amino acids Category:Aromatic amino acids Category:Phenols
ar:تيروزين zh-min-nan:Tyrosin ca:Tirosina cs:Tyrosin da:Tyrosin de:Tyrosin et:Türosiin es:Tirosina eo:Tirozino eu:Tirosina fa:تیروزین fr:Tyrosine gl:Tirosina ko:티로신 hr:Tirozin io:Tirozino id:Tirosina it:Tirosina he:טירוזין lv:Tirozīns lb:Tyrosin lt:Tirozinas mk:Тирозин nl:Tyrosine ja:チロシン oc:Tirosina pl:Tyrozyna pt:Tirosina ru:Тирозин sr:Тирозин su:Tirosin fi:Tyrosiini sv:Tyrosin ta:டைரோசின் tr:Tirozin uk:Тирозин zh:酪氨酸This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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