Tyrosine (Tyr, Y) - Structure, pKa, pI, and Codons
What is tyrosine and what is its side chain?
Tyrosine (Tyr, Y) has the side chain -CH₂C₆H₄OH - 4-hydroxybenzyl; a phenol - aromatic ring with a para hydroxyl. It is classified as aromatic / polar, uncharged, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.66.
| Three-letter code | Tyr |
| One-letter code | Y |
| Molecular formula | C₉H₁₁NO₃ |
| Molecular weight | 181.19 g/mol |
| Residue mass | 163.18 Da |
| Side chain | -CH₂C₆H₄OH - 4-hydroxybenzyl; a phenol - aromatic ring with a para hydroxyl. |
| Classification | Aromatic / Polar, uncharged |
| pKa (alpha-COOH) | 2.20 |
| pKa (alpha-NH3+) | 9.11 |
| pKa (side chain phenol) | 10.07 |
| Isoelectric point (pI) | 5.66 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Conditionally essential |
| Hydropathy (Kyte-Doolittle) | -1.3 |
| Configuration | L-form is (S) |
| Codons | UAU, UAC (2) |
Side chain
The R group of tyrosine is -CH₂C₆H₄OH. 4-hydroxybenzyl; a phenol - aromatic ring with a para hydroxyl. This is what distinguishes it from the other 19 amino acids, since all of them share the same backbone: an alpha carbon bonded to an amino group, a carboxyl group, and a hydrogen.
Ionization and isoelectric point
Tyrosine has 3 ionizable groups: alpha-COOH (pKa 2.20), alpha-NH3+ (pKa 9.11), side chain phenol (pKa 10.07). The isoelectric point is the pH at which the molecule carries no net charge, found by averaging the two pKa values that bracket the neutral species.
pI = (2.2 + 9.11) / 2 = 5.66
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-tyrosine is (S).
Genetic code
2 codons encode tyrosine: UAU, UAC.
Features
- Hydroxyl-containing
- Phosphorylatable
- UV-absorbing at 280 nm
MCAT notes
- Aromatic and polar - both classifications are defensible, and textbooks split on it.
- A phosphorylation site: the substrate of receptor tyrosine kinases such as the insulin receptor and EGFR, and the target of drugs like imatinib.
- Precursor to dopamine, norepinephrine, epinephrine, melanin, and the thyroid hormones.
- Made from phenylalanine, so it is conditionally essential and becomes essential in PKU.
- Its phenol deprotonates after the alpha-amino group (10.07 versus 9.11), so its pI uses the neutral rule despite having an ionizable side chain.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View tyrosine in 3D