Alanine (Ala, A) - Structure, pKa, pI, and Codons
What is alanine and what is its side chain?
Alanine (Ala, A) has the side chain -CH₃ - Methyl group. It is classified as nonpolar, aliphatic, carries a neutral (0) charge at pH 7, and has an isoelectric point of 6.01.
| Three-letter code | Ala |
| One-letter code | A |
| Molecular formula | C₃H₇NO₂ |
| Molecular weight | 89.09 g/mol |
| Residue mass | 71.08 Da |
| Side chain | -CH₃ - Methyl group. |
| Classification | Nonpolar, aliphatic |
| pKa (alpha-COOH) | 2.34 |
| pKa (alpha-NH3+) | 9.69 |
| Isoelectric point (pI) | 6.01 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Nonessential |
| Hydropathy (Kyte-Doolittle) | +1.8 |
| Configuration | L-form is (S) |
| Codons | GCU, GCC, GCA, GCG (4) |
Side chain
The R group of alanine is -CH₃. Methyl group. 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
Alanine has 2 ionizable groups: alpha-COOH (pKa 2.34), alpha-NH3+ (pKa 9.69). 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 + 9.69) / 2 = 6.01
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-alanine is (S).
Genetic code
4 codons encode alanine: GCU, GCC, GCA, GCG.
MCAT notes
- The simplest chiral amino acid - a single methyl side chain.
- Central to the glucose-alanine (Cahill) cycle: muscle transaminates pyruvate to alanine to ship nitrogen to the liver.
- Has the highest alpha-helix propensity of the 20.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View alanine in 3D