por Tyrosine (Tyr, Y) is one of the 22 amino acids that make up the proteins of all cells present in living beings. Unlike other amino acids such as valine, threonine, tryptophan, leucine, lysine, and others, tyrosine is a «conditionally» essential amino acid.
The name «tyrosine» derives from the Greek word «tiros», which means cheese, as this amino acid was first discovered in this food. The term was coined in 1846 by Liebig, who mixed cheese with potassium hydroxide and obtained an unknown compound, hardly soluble in water.
After the initial description, other researchers such as Warren de la Rue and Hinterberger obtained it from coccoid insects and horn proteins, respectively. Its separation from the hydrolysis of other proteins with hydrochloric acid was described in 1901 by Mörner.
Generally, this amino acid is obtained in mammals thanks to the hydroxylation of phenylalanine, although it is also absorbed in the intestine from proteins consumed with food.
Tyrosine has multiple functions in the human body and among these the most relevant are, perhaps, that of a substrate for the production of neurotransmitters and hormones such as adrenaline and thyroid hormone.
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Characteristics
Tyrosine weighs approximately 180 g/mol and its R group or side chain has a dissociation constant pKa of 10.07. Its relative abundance in cellular proteins does not exceed 4%, but it has multiple essential functions for human physiology.
This amino acid belongs to the group of aromatic amino acids, which also includes phenylalanine and tryptophan. Members of this group have aromatic rings in their R groups or side chains and are generally hydrophobic or nonpolar amino acids.
Like tryptophan, tyrosine absorbs ultraviolet light and is one of the amino acid residues responsible for the absorbance of light at 280 nm of many proteins, making it useful for characterization.
It is considered a “conditionally” essential amino acid since its biosynthesis in humans depends on phenylalanine, an essential amino acid. If the body meets its daily requirements for phenylalanine, tyrosine can be synthesized without problem and is not a limiting amino acid.
However, if the diet is deficient in phenylalanine, the body will not only have an imbalance of this amino acid, but also of tyrosine. It is also important to point out that the tyrosine synthesis reaction from phenylalanine is not reversible, so tyrosine cannot supply the cellular needs for phenylalanine.
Tyrosine also belongs to the group of amino acids with dual functions in the production of glucogenic and ketogenic metabolic intermediates, which are involved in glucose synthesis for the brain and in the formation of ketone bodies in the liver.
Structure
Like the rest of the amino acids, tyrosine, or β-parahydroxyphenyl-α-amino propionic acid, is an α-amino acid that has a central carbon atom, called the α carbon and that is chiral, since it is linked to four different substituent atoms or molecules.
This chiral carbon is attached to two characteristic groups of amino acids: an amino group (NH2) and a carboxyl group (COOH). It also shares one of its bonds with a hydrogen atom and the remaining bond is occupied by the R group or side chain proper to each amino acid.
In the case of tyrosine, this group consists of an aromatic ring associated with a hydroxyl (OH) group, which gives it the ability to form hydrogen bonds with other molecules and gives it essential functional characteristics for certain enzymes.
functions
Tyrosine is a fundamental component of many proteins with a great diversity of activities and biological functions.
In humans and other mammals, this amino acid is used in the nervous and renal tissues for the synthesis of dopamine, adrenaline, and norepinephrine, three related catecholaminergic neurotransmitters that are extremely important for bodily function.
It is also essential for the synthesis of ultraviolet (UV) radiation protectors such as melanin; some analgesics such as endorphins and antioxidant molecules such as vitamin E.
In the same way, this amino acid is used for the synthesis of tyramine, octopamine and thyroid hormones through the organization of iodine in the tyrosine residue of thyroglobulin.
Tyramine is a vasoactive molecule found in the human body, and octopamine is an amine related to norepinephrine.
All these functions of tyrosine are possible thanks to its obtaining from dietary proteins or by hydroxylation of phenylalanine with the liver as the main organ for the systemic supply of said amino acid.
functions in plants
Tyrosine and some of the intermediates generated during its biosynthesis feed the biosynthetic pathways of metabolites specialized in defense, pollinator attraction, electronic transport, and structural support.
Biosynthesis
In humans, tyrosine is obtained from the diet or synthesized in a single step by liver cells from phenylalanine, an essential amino acid, through a reaction catalyzed by the phenylalanine hydroxylase enzyme complex.
This complex has oxygenase activity and is only present in the liver of humans or other mammals. The tyrosine synthesis reaction then involves the transfer of an oxygen atom to the para position of the aromatic ring of phenylalanine.
Said reaction occurs at the same time that a water molecule is formed by the reduction of another molecular oxygen atom and the reducing power is provided directly by a NADPH conjugated with a tetrahydropterin molecule, which is similar to folic acid.
biosynthesis in plants
In plants, tyrosine is synthesized de novo downstream of the «shikimate» pathway, which feeds other biosynthetic pathways for other aromatic amino acids such as phenylalanine and tryptophan.
In these organisms, the synthesis starts from a compound known as «chorismate», which is the final product of the shikimate pathway and, furthermore, the common precursor for all aromatic amino acids, certain vitamins and plant hormones.
Chorismate is converted into prephenate by the catalytic action of the enzyme chorismate mutase and this is the first «compromised» step in the synthesis of tyrosine and phenylalanine in plants.
The prephenate is converted to tyrosine by oxidative decarboxylation and transamination, which can occur in any order.
In one of the biosynthetic pathways, these steps can be catalyzed by specific enzymes known as prephenate-specific tyrosine dehydrogenase (PDH) (which converts prephenate to 4-hydroxyphenylpyruvate (HPP)) and tyrosine aminotransferase (which produces tyrosine from HPP). ), respectively.
Another route of tyrosine synthesis from prephenate involves transamination of prephenate to a non-proteinogenic amino acid called L-arogenate, catalyzed by the enzyme prephenate aminotransferase.
L-arogenate is subsequently subjected to oxidative decarboxylation to form thyroxine, a reaction driven by an arogenate-specific tyrosine dehydrogenase enzyme, also known as ADH.
Plants preferentially employ the arogenate pathway, whereas most microbes synthesize tyrosine from the prephenate-derived HPP.
Regulation
As is true for most amino acid biosynthetic pathways, plants have a strict system for regulating the synthesis of aromatic amino acids, including tyrosine.
In these organisms, regulation occurs at many levels, since the mechanisms that control the shikimate pathway also control the production of tyrosine, a pathway for which there are also own regulation mechanisms.
However, the tyrosine requirements and, therefore, the rigidity in the regulation of its biosynthesis, are specific for each plant species.
Degradation
Degradation or catabolism of tyrosine results in the formation of fumarate and acetoacetate. The first step in this pathway involves the conversion of the amino acid to 4-hydroxyphenylpyruvate by a cytosolic enzyme known as tyrosine aminotransferase.
This amino acid can also be transaminated in the mitochondria of hepatocytes by an enzyme aspartate aminotransferase, although this enzyme is not very important under normal physiological conditions.
Through the degradation of tyrosine, succinyl-acetoacetate can be produced, which can be decarboxylated to succinyl-acetate. Succinyl acetate is the most potent inhibitor of the enzyme responsible for the synthesis of the heme group, the enzyme 5-aminolevulinic acid dehydratase.
Synthesis of adrenaline and norepinephrine
As mentioned, tyrosine is one of the main substrates for the synthesis of two very important neurotransmitters for the human body: adrenaline and norepinephrine.
This is initially used by an enzyme known as tyrosine hydroxylase, capable of adding an additional hydroxyl group to the aromatic ring of the R group of tyrosine, thereby forming the compound known as dopa.
Dopa gives rise to dopamine once it is enzymatically processed by a dopa decarboxylase enzyme, which removes the carboxyl group from the starting amino acid and requires a molecule of pyridoxal phosphate (FDP).
Dopamine is subsequently converted into norepinephrine by the action of the enzyme dopamine β-oxidase, which catalyzes the addition of a hydroxyl group to -CH that was part of the R group of tyrosine and that functioned as a «bridge» between the aromatic ring. and the α carbon.
Epinephrine is derived from norepinephrine by the action of phenylethanolamine N-methyltransferase, which is responsible for the S-adenosyl-methionine-dependent transfer of a methyl group (-CH3) to the free amino group of norepinephrine.
Foods rich in tyrosine
As discussed above, tyrosine is a «conditionally» essential amino acid, as it is synthesized in the human body by hydroxylation of phenylalanine, an essential amino acid.
Therefore, if phenylalanine intake meets the body’s demands, tyrosine is not a limiting factor for normal cell function. Tyrosine, however, is also acquired from the proteins that are consumed with daily food.
Some studies report that the minimum daily intake of both tyrosine and phenylalanine should be between 25 and 30 mg per kilogram of weight, so an average person should consume more or less 875 mg of tyrosine per day.
The foods with the highest tyrosine content are cheese and soybeans. Among these are also beef, lamb, pork, chicken and fish.
Some seeds and nuts such as walnuts…