Glutamic acid (Glu, E) - Structure, pKa, pI, and Codons
What is glutamic acid and what is its side chain?
Glutamic acid (Glu, E) has the side chain -CH₂CH₂COOH - 2-carboxyethyl; a carboxylic acid, one carbon longer than aspartate. It is classified as acidic (negative at ph 7), carries a negative (-1) charge at pH 7, and has an isoelectric point of 3.22.
| Three-letter code | Glu |
| One-letter code | E |
| Molecular formula | C₅H₉NO₄ |
| Molecular weight | 147.13 g/mol |
| Residue mass | 129.12 Da |
| Side chain | -CH₂CH₂COOH - 2-carboxyethyl; a carboxylic acid, one carbon longer than aspartate. |
| Classification | Acidic (negative at pH 7) |
| pKa (alpha-COOH) | 2.19 |
| pKa (side chain COOH) | 4.25 |
| pKa (alpha-NH3+) | 9.67 |
| Isoelectric point (pI) | 3.22 |
| Charge at pH 7 | negative (-1) |
| Essentiality | Nonessential |
| Hydropathy (Kyte-Doolittle) | -3.5 |
| Configuration | L-form is (S) |
| Codons | GAA, GAG (2) |
Side chain
The R group of glutamic acid is -CH₂CH₂COOH. 2-carboxyethyl; a carboxylic acid, one carbon longer than aspartate. 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
Glutamic acid has 3 ionizable groups: alpha-COOH (pKa 2.19), side chain COOH (pKa 4.25), alpha-NH3+ (pKa 9.67). 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 = (2.19 + 4.25) / 2 = 3.22
Note that this uses the side chain pKa rather than both alpha group values, because the side chain ionizes within the range that brackets the neutral form.
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-glutamic acid is (S).
Genetic code
2 codons encode glutamic acid: GAA, GAG.
MCAT notes
- Negatively charged at pH 7 (side chain pKa 4.25).
- The principal excitatory neurotransmitter of the central nervous system.
- The hub of nitrogen metabolism, via glutamate dehydrogenase - the only enzyme that both fixes and releases free ammonia - and transamination to alpha-ketoglutarate.
- Carboxylated to gamma-carboxyglutamate in clotting factors in a vitamin K dependent step, which is the target of warfarin.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View glutamic acid in 3D