Glutamine (Gln, Q) - Structure, pKa, pI, and Codons
What is glutamine and what is its side chain?
Glutamine (Gln, Q) has the side chain -CH₂CH₂CONH₂ - 2-carbamoylethyl; an amide side chain, one carbon longer than asparagine. It is classified as polar, uncharged, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.65.
| Three-letter code | Gln |
| One-letter code | Q |
| Molecular formula | C₅H₁₀N₂O₃ |
| Molecular weight | 146.14 g/mol |
| Residue mass | 128.13 Da |
| Side chain | -CH₂CH₂CONH₂ - 2-carbamoylethyl; an amide side chain, one carbon longer than asparagine. |
| Classification | Polar, uncharged |
| pKa (alpha-COOH) | 2.17 |
| pKa (alpha-NH3+) | 9.13 |
| Isoelectric point (pI) | 5.65 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Conditionally essential |
| Hydropathy (Kyte-Doolittle) | -3.5 |
| Configuration | L-form is (S) |
| Codons | CAA, CAG (2) |
Side chain
The R group of glutamine is -CH₂CH₂CONH₂. 2-carbamoylethyl; an amide side chain, one carbon longer than asparagine. 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
Glutamine has 2 ionizable groups: alpha-COOH (pKa 2.17), alpha-NH3+ (pKa 9.13). 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 = (2.17 + 9.13) / 2 = 5.65
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-glutamine is (S).
Genetic code
2 codons encode glutamine: CAA, CAG.
Features
- Amide side chain
MCAT notes
- An amide side chain that does not ionize in the physiological range.
- The most abundant free amino acid in blood, and the principal non-toxic carrier of ammonia from tissues to the liver and kidney.
- Conditionally essential - a major fuel for enterocytes and rapidly dividing immune cells.
- The amide of glutamate: E and Q 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 glutamine in 3DRelated Topics
Serine (Ser)
Another polar amino acid
Threonine (Thr)
Another polar amino acid
Cysteine (Cys)
Another polar amino acid
Glutamic acid (Glu)
Its acid partner - they differ only by -OH versus -NH2
Glutamine 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