Tyrosine: an aromatic amino acid important in proteins and metabolism

Biological role and synthesis

In humans tyrosine is typically classified as non-essential because the body can produce it from the essential amino acid phenylalanine through the action of phenylalanine hydroxylase. In metabolic disorders such as phenylketonuria (PKU), conversion from phenylalanine is impaired and tyrosine becomes conditionally essential and must be obtained from the diet or supplements. Tyrosine is the precursor for several important biomolecules, including the catecholamine neurotransmitters dopamine, norepinephrine and epinephrine, the thyroid hormones, and the pigment melanin.

Dietary sources

• High-protein foods in general provide tyrosine.
• Chicken and fish are common animal sources.
• Milk, yogurt and other dairy products such as cottage cheese and cheese contribute useful amounts.
• Peanuts, almonds and other nuts supply tyrosine in plant-based diets.
• Seeds like pumpkin seeds and sesame seeds, plus soy products and legumes, are also significant sources.
• Some fruits and vegetables such as avocado and bananas, along with eggs, provide smaller amounts useful in varied diets.

Chemical properties and functions

The side chain of tyrosine contains a phenolic hydroxyl that makes it moderately polar and capable of forming hydrogen bonds. That hydroxyl group is the site of reversible phosphorylation in many signaling proteins; when present as a substituent it is called a tyrosyl group or tyrosyl residue. Phosphorylation of tyrosine residues plays a central role in signal transduction pathways and regulation of enzymatic activity. In plants and microbes, tyrosine is also a branching point for secondary metabolites such as alkaloids and pigments.

Medical, biochemical and practical notes

L‑tyrosine appears in research and as an ingredient in some dietary supplements marketed for stress, alertness or cognitive support, though clinical evidence varies and safety depends on individual health conditions and medications. Inherited defects of tyrosine metabolism can lead to elevated tyrosine or related intermediates with clinical consequences. In biochemistry and molecular biology the term tyrosyl is commonly used to describe the side chain when it participates in enzyme mechanisms or protein modification.

For further reading about dietary sources and functions see general nutrition and biochemical resources: basic amino acid info, genetic code, and product- or food-specific guides such as pages on poultry, seafood, dairy, fermented dairy, cottage cheese, cheese varieties, peanuts, tree nuts, pumpkin seeds, sesame, avocado and other plant foods.