Valine (Val, V) - Structure, pKa, pI, and Codons
What is valine and what is its side chain?
Valine (Val, V) has the side chain -CH(CH₃)₂ - Isopropyl; branched at the beta carbon. 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 | Val |
| One-letter code | V |
| Molecular formula | C₅H₁₁NO₂ |
| Molecular weight | 117.15 g/mol |
| Residue mass | 99.13 Da |
| Side chain | -CH(CH₃)₂ - Isopropyl; branched at the beta carbon. |
| Classification | Nonpolar, aliphatic |
| pKa (alpha-COOH) | 2.32 |
| pKa (alpha-NH3+) | 9.62 |
| Isoelectric point (pI) | 5.97 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Essential |
| Hydropathy (Kyte-Doolittle) | +4.2 |
| Configuration | L-form is (S) |
| Codons | GUU, GUC, GUA, GUG (4) |
Side chain
The R group of valine is -CH(CH₃)₂. Isopropyl; branched at the beta carbon. 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
Valine has 2 ionizable groups: alpha-COOH (pKa 2.32), alpha-NH3+ (pKa 9.62). 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 = (2.32 + 9.62) / 2 = 5.97
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-valine is (S).
Genetic code
4 codons encode valine: GUU, GUC, GUA, GUG.
Features
- Branched-chain (BCAA)
- Beta-branched
MCAT notes
- A branched-chain amino acid (with leucine and isoleucine), essential, and strongly hydrophobic.
- Glu to Val at position 6 of beta-globin causes sickle cell disease - a charged surface residue replaced by a hydrophobic one creates a sticky patch that drives HbS polymerization.
- Beta-branching sterically disfavors the alpha helix.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View valine in 3D