Tyrosine Calculators

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Tyrosine (Tyr, Y) is a conditionally essential aromatic amino acid that serves as the biosynthetic precursor to dopamine, norepinephrine, epinephrine, thyroid hormones (T3 and T4), and melanin. Under normal conditions, it is synthesized in the liver from the essential amino acid phenylalanine by phenylalanine hydroxylase (PAH). In phenylketonuria (PKU), PAH is deficient, making tyrosine conditionally essential. Tyrosine also plays a key role in cell signaling through tyrosine phosphorylation — the mechanism exploited by receptor tyrosine kinases and targeted by many cancer drugs.

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Tyrosine Structure and Properties

Tyrosine is 4-hydroxyphenylalanine — phenylalanine with a para-hydroxyl (−OH) on the benzene ring. MW = 181.19 g/mol. The phenolic ring absorbs UV at 274–280 nm (ε₂₈₀ ≈ 1480 L/mol/cm), contributing to protein absorbance at A280 alongside tryptophan. The hydroxyl group can be phosphorylated, making Tyr a key regulatory site in signal transduction.

Biosynthesis from Phenylalanine

Phenylalanine + O₂ + tetrahydrobiopterin (BH₄) → Tyrosine, catalyzed by phenylalanine hydroxylase (PAH). In PKU, PAH activity is absent or severely reduced — phenylalanine accumulates to neurotoxic levels while tyrosine becomes dietary-essential. Treatment requires phenylalanine-restricted diet with tyrosine supplementation.

Tyrosine as a Biosynthetic Precursor

  • Catecholamines: Tyr → L-DOPA (tyrosine hydroxylase) → Dopamine → Norepinephrine → Epinephrine
  • Thyroid hormones: Tyr residues in thyroglobulin are iodinated to form T3 and T4
  • Melanin: Tyr → DOPA → dopaquinone → melanin (skin, hair, eye pigment)

Tyrosine Kinase Signaling and Cancer

Protein tyrosine kinases (PTKs) — including EGFR, HER2, PDGFR, BCR-ABL, and Src — phosphorylate tyrosine residues on target proteins to activate growth and survival pathways. Oncogenic PTK mutations drive many cancers. Targeted tyrosine kinase inhibitors (imatinib/Gleevec for CML, erlotinib for lung cancer, trastuzumab for HER2+ breast cancer) are among the most successful cancer therapeutics.

Glossary

Phenylalanine Hydroxylase (PAH)
The liver enzyme converting phenylalanine to tyrosine using tetrahydrobiopterin; deficient in phenylketonuria (PKU), causing phenylalanine accumulation and making tyrosine conditionally essential.
Catecholamine
A class of neurotransmitters and hormones (dopamine, norepinephrine, epinephrine) derived from tyrosine; regulate stress response, mood, cardiovascular function, and attention.
Protein Tyrosine Kinase (PTK)
An enzyme that phosphorylates tyrosine residues on target proteins using ATP; key regulators of cell growth; frequently mutated in cancer and targeted by approved oncology drugs.

Frequently Asked Questions

Tyrosine is conditionally essential. Normally the body synthesizes sufficient tyrosine from phenylalanine via phenylalanine hydroxylase (PAH). In phenylketonuria (PKU), PAH is deficient, blocking this pathway — tyrosine becomes truly essential and must be supplied in the diet. In premature infants and severe metabolic illness, endogenous synthesis may also be insufficient. For healthy adults with adequate phenylalanine intake, tyrosine is nonessential.

Tyrosine is the precursor to the catecholamine pathway: tyrosine → L-DOPA (by tyrosine hydroxylase) → dopamine → norepinephrine → epinephrine. It is also the structural basis of thyroid hormones — iodination of tyrosine residues in thyroglobulin produces monoiodotyrosine (MIT) and diiodotyrosine (DIT), which couple to form T3 and T4. Additionally, tyrosinase converts tyrosine to melanin in melanocytes, determining skin, hair, and eye color.

Tyrosine phosphorylation occurs when protein tyrosine kinases (PTKs) transfer a phosphate group from ATP to the hydroxyl group of a tyrosine residue on a target protein, activating downstream signaling. This is a central regulatory mechanism in growth factor signaling, immune activation, and cell division. In cancer, PTKs are frequently mutated to constitutively active forms, driving uncontrolled proliferation — making them prime drug targets. PTK inhibitors represent one of the major success stories of targeted cancer therapy.

Tyrosine's para-hydroxyphenyl ring undergoes π→π* electronic transitions that absorb UV light at ~274–280 nm. This absorption contributes to the A280 signal used to estimate protein concentration, alongside tryptophan (which absorbs more strongly, ε₂₈₀ ≈ 5500 vs. tyrosine's ~1480 L/mol/cm). Protein molar extinction coefficient at 280 nm can be calculated from the number of Trp and Tyr residues using the Pace formula or online tools like ExPASy ProtParam.