Tyrosine is an amino acid, one of the building blocks of protein. The body produces it from another amino acid, phenylalanine, so it is considered a “non-essential” amino acid. However, under specific circumstances like illness or stress, the body may not be able to synthesize enough, making it “conditionally essential.”
How Tyrosine is Formed
The formation of tyrosine begins with an essential amino acid called phenylalanine, which must be obtained from food. Aside from being used to build proteins, the primary function of phenylalanine is its irreversible conversion into tyrosine.
The reaction occurs almost exclusively in the liver and is catalyzed by the enzyme phenylalanine hydroxylase (PAH). This enzyme changes the structure of phenylalanine into tyrosine. For the PAH enzyme to function correctly, it requires assistance from a cofactor.
This helper molecule is tetrahydrobiopterin (BH4). During the conversion, BH4 is oxidized and must be regenerated by another enzyme to participate in further reactions. This regulated process ensures the body can produce the tyrosine it needs from available phenylalanine.
Key Functions of Tyrosine
Tyrosine serves as a precursor for several important substances. A primary role is forming a group of neurotransmitters called catecholamines, including dopamine, norepinephrine, and epinephrine. These chemicals transmit signals between nerve cells and are involved in regulating mood, managing stress responses, and maintaining attention.
Tyrosine is also integral to the function of the thyroid gland. It is a component for the synthesis of thyroid hormones, thyroxine (T4) and triiodothyronine (T3). These hormones regulate the body’s overall metabolism, influencing factors like heart rate and body temperature.
This amino acid is also required for the production of melanin, the pigment that determines skin, hair, and eye color. Melanin also serves a protective function by absorbing ultraviolet (UV) radiation from the sun, which helps shield the skin from damage.
Dietary Intake and Tyrosine
The body’s supply of tyrosine is linked to the dietary intake of its precursor, phenylalanine. Phenylalanine is found in a wide variety of protein-rich foods, including animal products like meat, fish, eggs, and dairy, as well as plant-based options such as soy products, nuts, and seeds.
Tyrosine is also present in these same high-protein foods. For most healthy individuals, a balanced diet with adequate protein supplies enough phenylalanine to produce all the tyrosine the body needs, which is why it is considered non-essential.
Tyrosine can become conditionally essential when a medical condition impairs the body’s ability to convert phenylalanine. In these cases, it becomes necessary to obtain tyrosine directly from dietary sources or supplementation to meet the body’s requirements.
Conditions Related to Tyrosine Metabolism
The most well-known condition involving the conversion of phenylalanine to tyrosine is Phenylketonuria (PKU). PKU is a rare inherited disorder caused by a defective gene for the phenylalanine hydroxylase (PAH) enzyme. This defect means the enzyme is either missing or severely reduced in function, impairing the conversion process.
As a result of the enzyme deficiency, phenylalanine from the diet accumulates to toxic levels in the blood and tissues. This buildup can lead to serious health issues, including severe intellectual disability and other neurological problems if left untreated. Simultaneously, because the conversion is blocked, tyrosine levels become deficient.
Due to the serious consequences of untreated PKU, newborns in many countries are screened for the condition shortly after birth. Management of PKU involves a lifelong, strict diet that severely limits phenylalanine intake. This diet is often supplemented with special medical foods and tyrosine to prevent deficiency and support normal development.