Isoleucine (Ile, I) - Structure, pKa, pI, and Codons
What is isoleucine and what is its side chain?
Isoleucine (Ile, I) has the side chain -CH(CH₃)CH₂CH₃ - sec-butyl; beta-branched, carrying a second stereocenter. It is classified as nonpolar, aliphatic, carries a neutral (0) charge at pH 7, and has an isoelectric point of 6.02.
| Three-letter code | Ile |
| One-letter code | I |
| Molecular formula | C₆H₁₃NO₂ |
| Molecular weight | 131.17 g/mol |
| Residue mass | 113.16 Da |
| Side chain | -CH(CH₃)CH₂CH₃ - sec-butyl; beta-branched, carrying a second stereocenter. |
| Classification | Nonpolar, aliphatic |
| pKa (alpha-COOH) | 2.36 |
| pKa (alpha-NH3+) | 9.68 |
| Isoelectric point (pI) | 6.02 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Essential |
| Hydropathy (Kyte-Doolittle) | +4.5 |
| Configuration | L-form is (2S,3S) |
| Codons | AUU, AUC, AUA (3) |
Side chain
The R group of isoleucine is -CH(CH₃)CH₂CH₃. sec-butyl; beta-branched, carrying a second stereocenter. 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
Isoleucine has 2 ionizable groups: alpha-COOH (pKa 2.36), alpha-NH3+ (pKa 9.68). 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 + 9.68) / 2 = 6.02
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-isoleucine is (2S,3S). One of two amino acids with a second stereocenter in the side chain (the other is threonine).
Genetic code
3 codons encode isoleucine: AUU, AUC, AUA.
Features
- Branched-chain (BCAA)
- Beta-branched
- Two stereocenters
MCAT notes
- The most hydrophobic residue on the Kyte-Doolittle scale (+4.5).
- A branched-chain amino acid, beta-branched, with two stereocenters (2S,3S).
- Both ketogenic and glucogenic.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View isoleucine in 3D