Tyrosine (Tyr, Y)
Aromatic side chain, conditionally essential in the human diet
What is the charge of tyrosine at pH 7.4?
At pH 7.4, the pH of blood plasma, tyrosine carries a net charge of −0.02, and 98% of its molecules are the zwitterion, charge 0. Its phenol side chain, pKR 10.07, has lost its proton in 0.2% of molecules. Its isoelectric point is 5.66, from Lehninger’s pKa values of 2.20, 9.11 and 10.07.
Properties of tyrosine
| Three-letter code | Tyr |
|---|---|
| One-letter code | Y |
| Side-chain class (Lehninger) | Aromatic |
| Side-chain group | phenol |
| Formula (uncharged form) | C₉H₁₁NO₃ |
| Molar mass | 181.19 g/mol |
| pK₁ (α-carboxyl) | 2.20 |
| pK₂ (α-amino) | 9.11 |
| pKR (side chain) | 10.07 |
| Isoelectric point (pI) | 5.66 |
| Configuration at Cα (L form) | S |
| Hydropathy (Kyte and Doolittle) | −1.3 |
| mRNA codons | UAU, UAC |
| Human diet | Conditionally essential |
pKa values and pI from Nelson and Cox, Lehninger Principles of Biochemistry, 7th edition, Table 3-1. Formula and molar mass from the wwPDB Chemical Component Dictionary and the IUPAC standard atomic weights; codons from NCBI translation table 1; hydropathy from Kyte and Doolittle (1982); diet from the Institute of Medicine, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005), chapter 10.
The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.
Charge of tyrosine at common pH values
| pH | Net charge | Most common form | Share of molecules |
|---|---|---|---|
| 1 | +0.94 | cation (+1) | 94% |
| 3 | +0.14 | zwitterion (0) | 86% |
| 5 | 0.00 | zwitterion (0) | 99.8% |
| 7 | −0.01 | zwitterion (0) | 99% |
| 7.4 (blood plasma) | −0.02 | zwitterion (0) | 98% |
| 9 | −0.52 | zwitterion (0) | 52% |
| 11 | −1.88 | dianion (−2) | 88% |
| 13 | −2.00 | dianion (−2) | 99.9% |
The net charge is an average over all the molecules, so it can be a fraction; each molecule carries a whole number of charges, and the most common form is the one protonation state with the largest share.
Which form dominates between the pKas
- Below pH 2.20: the cation, net charge +1.
- pH 2.20 to 9.11: the zwitterion, net charge 0.
- pH 9.11 to 10.07: the anion, net charge −1.
- Above pH 10.07: the dianion, net charge −2.
Open tyrosine in the Amino Acid 3D Explorer and drag the pH to watch each group gain and lose its proton, with every pKa and its pI marked on the pH slider.
How the charge at pH 7.4 is calculated
Each group is treated on its own with the Henderson-Hasselbalch equation. An acidic group adds
−1/(1 + 10^(pKa − pH)), the share of molecules in which it has lost its proton, and a basic group
adds +1/(1 + 10^(pH − pKa)), the share in which it holds one.
Net charge of tyrosine at pH 7.4
- α-carboxyl (pK₁ = 2.20): −1/(1 + 10^(2.20 − 7.4)) = −1.0000
- α-amino (pK₂ = 9.11): +1/(1 + 10^(7.4 − 9.11)) = +0.9809
- phenol (pKR = 10.07): −1/(1 + 10^(10.07 − 7.4)) = −0.0021
- q = −1.0000 + 0.9809 − 0.0021 = −0.0212
- q ≈ −0.02
How to calculate the pI of tyrosine
Tyrosine has three ionizable groups. Taken from the lowest pKa up, its neutral form exists between pK₁ (2.20, the α-carboxyl) and pK₂ (9.11, the α-amino), so the pI is the mean of those two.
Isoelectric point of tyrosine
- pI = (pK₁ + pK₂)/2
- = (2.20 + 9.11)/2
- = 5.655 ≈ 5.66
The two printed pKas average to 5.655, exactly halfway between 5.65 and 5.66; Lehninger’s table gives 5.66, the value used on this page.
Averaging all three pKas, a common mistake, gives 7.13, 1.47 pH units above the pI.
The explorer’s net charge curve, which treats each group on its own, crosses zero at 5.63 rather than 5.66, and the table is right. The α-amino pKa, 9.11, and the phenol pKa, 10.07, are only 0.96 apart, so a titration measures them together, and treating them separately counts too many negative molecules near the pI.
Stereochemistry
L-tyrosine is S at its α-carbon, as 18 of the 19 chiral amino acids are: its amino group outranks its carboxyl, which outranks its side chain. The R and S Configuration Explorer ranks the groups on a stereocenter step by step.
How tyrosine compares with the other 19
- Tyrosine is one of 3 amino acids in the class Lehninger calls aromatic; the others are phenylalanine and tryptophan.
- Its molar mass, 181.19 g/mol, makes it the 2nd heaviest of the 20.
- Its pI, 5.66, is the 7th lowest of the 20.
- On Kyte and Doolittle’s hydropathy scale it scores −1.3, the 9th most hydrophilic of the 20.
- It has 2 codons, as do 8 of the other 19.
Is tyrosine an essential amino acid?
Only conditionally. Tyrosine is one of 6 conditionally essential amino acids: the body makes it, but needs it from food when its own synthesis cannot keep up with demand. It is made from phenylalanine, one of the essential nine. The other 5 are arginine, cysteine, glutamine, glycine and proline.
Classification from the Institute of Medicine, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005), chapter 10, which calls the three lists indispensable, conditionally indispensable and dispensable.
Codons for tyrosine
Tyrosine is encoded by 2 of the 61 sense codons in the standard genetic code: UAU and UAC in mRNA, written TAT and TAC on the DNA coding strand.
The DNA to Protein Translator reads codons in a sequence of your own, and the Protein Synthesis Simulator builds the chain on a ribosome, one amino acid per codon.
Work with tyrosine
- Amino Acid 3D Explorer: tyrosine in 3D, with its charge at any pH
- Molar mass of C₉H₁₁NO₃, worked out atom by atom in the Molar Mass Calculator
- Henderson-Hasselbalch Calculator: the acid to base ratio of one group at any pH
- Titration Curve Calculator: the curve for a weak acid titrated with a strong base
- All 20 amino acids: pKa values, pI, charge and codons in one table
- Tyrosine on Wikipedia