Glycine (Gly, G) - Structure, pKa, pI, and Codons
What is glycine and what is its side chain?
Glycine (Gly, G) has the side chain -H - A single hydrogen atom. No side chain proper. It is classified as nonpolar, aliphatic, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.97.
| Three-letter code | Gly |
| One-letter code | G |
| Molecular formula | C₂H₅NO₂ |
| Molecular weight | 75.07 g/mol |
| Residue mass | 57.05 Da |
| Side chain | -H - A single hydrogen atom. No side chain proper. |
| Classification | Nonpolar, aliphatic |
| pKa (alpha-COOH) | 2.34 |
| pKa (alpha-NH3+) | 9.60 |
| Isoelectric point (pI) | 5.97 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Conditionally essential |
| Hydropathy (Kyte-Doolittle) | -0.4 |
| Configuration | Achiral |
| Codons | GGU, GGC, GGA, GGG (4) |
Side chain
The R group of glycine is -H. A single hydrogen atom. No side chain proper. 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
Glycine has 2 ionizable groups: alpha-COOH (pKa 2.34), alpha-NH3+ (pKa 9.60). 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 = (2.34 + 9.6) / 2 = 5.97
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
Glycine is the only achiral amino acid: its alpha carbon carries two hydrogens, so it has no stereocenter and is neither D nor L. The only achiral amino acid. Its alpha carbon carries two hydrogens, so it has no stereocenter and is neither D nor L.
Genetic code
4 codons encode glycine: GGU, GGC, GGA, GGG.
Features
- Achiral
- Helix breaker
MCAT notes
- The smallest amino acid and the only achiral one - its alpha carbon bears two hydrogens.
- Maximum conformational freedom (widest allowed Ramachandran region), so it breaks helices by flexibility and is essential at tight turns.
- Every third residue in collagen (Gly-X-Y, X often Pro, Y often hydroxyproline) because only glycine fits inside the triple helix core.
- An inhibitory neurotransmitter in the spinal cord, and a co-agonist at NMDA receptors.
G for Glycine, the one with nothing on it.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View glycine in 3D