Tyrosiini

Tyrosiini, known in English as tyrosine, is an aromatic amino acid that participates in protein synthesis and serves as a key metabolic precursor. It is categorized as non-essential because most humans can synthesize it from phenylalanine, though it becomes conditionally essential when phenylalanine intake is limited or certain metabolic disorders are present.

Biosynthesis and metabolism

Tyrosine is formed in the liver from phenylalanine through the action of phenylalanine hydroxylase, using the cofactor tetrahydrobiopterin (BH4). Once formed, tyrosine can be incorporated into proteins or metabolized through the tyrosine degradation pathway. The catabolic sequence begins with tyrosine transamination to p-hydroxyphenylpyruvate, followed by steps that convert intermediates to fumarate and acetoacetate, which enter energy production and other metabolic routes. Disruptions in this pathway cause inherited tyrosinemias, a group of rare metabolic disorders.

Biological roles

Tyrosine is a precursor to several important biological molecules. It gives rise to catecholamines (dopamine, norepinephrine, epinephrine), which are involved in mood, stress response, and cardiovascular function. It is also a precursor to thyroid hormones (thyroxine T4 and triiodothyronine T3) through incorporation into thyroglobulin. Additionally, tyrosine is a substrate for melanin synthesis in pigment-forming cells and contributes to the structural proteins in various tissues.

Diet and nutrition

Dietary sources rich in tyrosine include dairy products, meat, fish, eggs, soy products, legumes, nuts, and whole grains. In most individuals, adequate protein intake provides sufficient tyrosine. In phenylketonuria or other conditions limiting phenylalanine conversion, tyrosine becomes essential to support protein synthesis and downstream functions.

Clinical notes

Tyrosinemias are metabolic disorders involving defects in tyrosine degradation, with varying severity. Tyrosine supplementation may be considered in specific dietary management plans for certain conditions, but is generally addressed within the broader context of total protein and amino acid balance.