Tryptophan (Trp, W) - Structure, pKa, pI, and Codons
What is tryptophan and what is its side chain?
Tryptophan (Trp, W) has the side chain -CH₂-indole - Indol-3-ylmethyl; a bicyclic aromatic, benzene fused to pyrrole. It is classified as aromatic, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.89.
| Three-letter code | Trp |
| One-letter code | W |
| Molecular formula | C₁₁H₁₂N₂O₂ |
| Molecular weight | 204.23 g/mol |
| Residue mass | 186.21 Da |
| Side chain | -CH₂-indole - Indol-3-ylmethyl; a bicyclic aromatic, benzene fused to pyrrole. |
| Classification | Aromatic |
| pKa (alpha-COOH) | 2.38 |
| pKa (alpha-NH3+) | 9.39 |
| Isoelectric point (pI) | 5.89 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Essential |
| Hydropathy (Kyte-Doolittle) | -0.9 |
| Configuration | L-form is (S) |
| Codons | UGG (1) |
Side chain
The R group of tryptophan is -CH₂-indole. Indol-3-ylmethyl; a bicyclic aromatic, benzene fused to pyrrole. 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
Tryptophan has 2 ionizable groups: alpha-COOH (pKa 2.38), alpha-NH3+ (pKa 9.39). 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 = (2.38 + 9.39) / 2 = 5.89
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-tryptophan is (S).
Genetic code
1 codon encode tryptophan: UGG. Having only one codon makes it unusually sensitive to point mutations, since any single-base change produces a different amino acid.
Features
- UV-absorbing at 280 nm
MCAT notes
- The largest amino acid, with a bicyclic indole side chain.
- The dominant contributor to absorbance at 280 nm (about 3.7 times tyrosine), which is why protein concentration is measured there.
- Precursor to serotonin and then melatonin, and to niacin and NAD+ - which is why severe deficiency mimics pellagra.
- One of only two amino acids with a single codon (UGG), making it highly sensitive to point mutations.
W for tryptophan - the double-ring indole looks like a W.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View tryptophan in 3D