Threonine (Thr, T) - Structure, pKa, pI, and Codons
What is threonine and what is its side chain?
Threonine (Thr, T) has the side chain -CH(OH)CH₃ - 1-hydroxyethyl; a secondary alcohol, beta-branched, carrying a second stereocenter. It is classified as polar, uncharged, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.87.
| Three-letter code | Thr |
| One-letter code | T |
| Molecular formula | C₄H₉NO₃ |
| Molecular weight | 119.12 g/mol |
| Residue mass | 101.10 Da |
| Side chain | -CH(OH)CH₃ - 1-hydroxyethyl; a secondary alcohol, beta-branched, carrying a second stereocenter. |
| Classification | Polar, uncharged |
| pKa (alpha-COOH) | 2.11 |
| pKa (alpha-NH3+) | 9.62 |
| Isoelectric point (pI) | 5.87 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Essential |
| Hydropathy (Kyte-Doolittle) | -0.7 |
| Configuration | L-form is (2S,3R) |
| Codons | ACU, ACC, ACA, ACG (4) |
Side chain
The R group of threonine is -CH(OH)CH₃. 1-hydroxyethyl; a secondary alcohol, beta-branched, carrying a second stereocenter. 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
Threonine has 2 ionizable groups: alpha-COOH (pKa 2.11), alpha-NH3+ (pKa 9.62). 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 = (2.11 + 9.62) / 2 = 5.87
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-threonine is (2S,3R). One of two amino acids with a second stereocenter in the side chain (the other is isoleucine).
Genetic code
4 codons encode threonine: ACU, ACC, ACA, ACG.
Features
- Hydroxyl-containing
- Phosphorylatable
- O-linked glycosylation site
- Beta-branched
- Two stereocenters
MCAT notes
- Beta-branched with two stereocenters (2S,3R).
- A phosphorylation site and an O-linked glycosylation site.
- Essential. Sources disagree slightly on its exact pKa values, so treat its pI as approximate.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View threonine in 3D