Asparagine (Asn, N) - Structure, pKa, pI, and Codons
What is asparagine and what is its side chain?
Asparagine (Asn, N) has the side chain -CH₂CONH₂ - Carboxamide; an amide side chain. It is classified as polar, uncharged, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.41.
| Three-letter code | Asn |
| One-letter code | N |
| Molecular formula | C₄H₈N₂O₃ |
| Molecular weight | 132.12 g/mol |
| Residue mass | 114.10 Da |
| Side chain | -CH₂CONH₂ - Carboxamide; an amide side chain. |
| Classification | Polar, uncharged |
| pKa (alpha-COOH) | 2.02 |
| pKa (alpha-NH3+) | 8.80 |
| Isoelectric point (pI) | 5.41 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Nonessential |
| Hydropathy (Kyte-Doolittle) | -3.5 |
| Configuration | L-form is (S) |
| Codons | AAU, AAC (2) |
Side chain
The R group of asparagine is -CH₂CONH₂. Carboxamide; an amide side chain. 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
Asparagine has 2 ionizable groups: alpha-COOH (pKa 2.02), alpha-NH3+ (pKa 8.80). 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 = (2.02 + 8.8) / 2 = 5.41
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-asparagine is (S).
Genetic code
2 codons encode asparagine: AAU, AAC.
Features
- Amide side chain
- N-linked glycosylation site
MCAT notes
- An amide side chain, which does not ionize anywhere between pH 2 and 12. Amides are neither amines nor acids.
- The site of N-linked glycosylation, via the Asn-X-Ser/Thr sequon where X is not proline. Added co-translationally in the ER.
- The amide of aspartate: D and N differ only by -OH versus -NH2.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View asparagine in 3DRelated Topics
Serine (Ser)
Another polar amino acid
Threonine (Thr)
Another polar amino acid
Cysteine (Cys)
Another polar amino acid
Aspartic acid (Asp)
Its acid partner - they differ only by -OH versus -NH2
Asparagine stereochemistry
Assign R/S using CIP priority rules on the alpha carbon
Amino Acid Explorer
All 20 amino acids in 3D with flashcards and titration curves