Reference
Tyrosine
**Tyrosine** is a conditionally essential amino acid that the body synthesizes from the essential amino acid phenylalanine, and which serves as the precursor for several signaling molecules including dopamine, noradrenaline, adrenaline, and the thyroid hormones.
Why tyrosine is “conditionally essential”
An essential amino acid cannot be made by the body and must come from food. Tyrosine sits one step removed from that category: a healthy adult can produce it internally by hydroxylating phenylalanine, so under ordinary conditions it is not strictly required in the diet. It becomes essential, however, when that conversion is impaired or when demand outpaces supply.
The clearest example is phenylketonuria (PKU), an inherited disorder in which the enzyme that converts phenylalanine to tyrosine is deficient. People with PKU restrict dietary phenylalanine and must obtain tyrosine directly, because their bodies cannot make it. This dependency is the reason tyrosine is classified as conditionally, rather than non-, essential.
What tyrosine is used for
Tyrosine is a building block for proteins like any amino acid, but its better-known role is as a metabolic starting material. It is the precursor in the pathway that produces the catecholamine neurotransmitters dopamine, noradrenaline, and adrenaline. It is also incorporated into thyroglobulin to form the thyroid hormones thyroxine (T4) and triiodothyronine (T3), and it contributes to the synthesis of melanin, the pigment in skin and hair.
Because tyrosine and phenylalanine are chemically linked, protein-quality scoring methods such as PDCAAS and DIAAS evaluate them together as the aromatic amino acid pair rather than scoring tyrosine in isolation. A diet adequate in phenylalanine generally supplies the raw material for tyrosine as well.
Tyrosine in potato protein
Potato protein isolate is a complete protein, meaning it supplies all nine essential amino acids, including the phenylalanine from which tyrosine is derived. Its overall protein quality is high: the Digestible Indispensable Amino Acid Score (DIAAS) for potato protein isolates has been reported as high as 100 (Food Science & Nutrition, Herreman et al., 2020, PMID:33133540), placing it among the better-scoring plant proteins. For a fuller account of the ingredient and its amino acid composition, see our complete guide to potato protein.
Note that tyrosine itself is a separate compound from the catecholamines and hormones it helps create; supplying dietary phenylalanine and tyrosine provides the substrate for those pathways but does not, on its own, increase their output.
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