Leucine (Leu, L) - Structure, pKa, pI, and Codons
What is leucine and what is its side chain?
Leucine (Leu, L) has the side chain -CH₂CH(CH₃)₂ - Isobutyl; branched at the gamma carbon, not the beta carbon. It is classified as nonpolar, aliphatic, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.98.
| Three-letter code | Leu |
| One-letter code | L |
| Molecular formula | C₆H₁₃NO₂ |
| Molecular weight | 131.17 g/mol |
| Residue mass | 113.16 Da |
| Side chain | -CH₂CH(CH₃)₂ - Isobutyl; branched at the gamma carbon, not the beta carbon. |
| Classification | Nonpolar, aliphatic |
| pKa (alpha-COOH) | 2.36 |
| pKa (alpha-NH3+) | 9.60 |
| Isoelectric point (pI) | 5.98 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Essential |
| Hydropathy (Kyte-Doolittle) | +3.8 |
| Configuration | L-form is (S) |
| Codons | UUA, UUG, CUU, CUC, CUA, CUG (6) |
Side chain
The R group of leucine is -CH₂CH(CH₃)₂. Isobutyl; branched at the gamma carbon, not 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
Leucine has 2 ionizable groups: alpha-COOH (pKa 2.36), 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 = (2.36 + 9.6) / 2 = 5.98
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-leucine is (S).
Genetic code
6 codons encode leucine: UUA, UUG, CUU, CUC, CUA, CUG. This is the maximum degeneracy in the genetic code.
Features
- Branched-chain (BCAA)
MCAT notes
- The most abundant amino acid in proteins, and a branched-chain amino acid.
- Branched at the gamma carbon rather than the beta carbon, so unlike valine and isoleucine it has high helix propensity - the basis of the leucine zipper DNA-binding motif.
- Purely ketogenic (along with lysine).
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View leucine in 3D