One of the three branched-chain amino acids — the group that muscles burn directly for fuel, bypassing the liver entirely.
Symbol
Ile · I
Discovered
1904
Mol. Weight
131.18 g/mol
Essential
Yes
I
Discovery: The Isomer Problem
L-Isoleucine
In 1904, German physician Felix Ehrlich was studying the fermentation products of proteins when he isolated a new amino acid from fibrin — the protein that forms blood clots. He recognized it as an isomer of leucine: same molecular formula (C₆H₁₃NO₂), same molecular weight, but a different structure. He named it isoleucine — quite literally, "leucine's isomer." Its composition was established three years later through a degradation to d-isoamylamine and a subsequent Strecker synthesis from d-isovaleraldehyde.
The structural difference is subtle but significant. Where leucine has a simple branched side chain with the branch at the second carbon, isoleucine branches at the first carbon of the side chain, creating a more compact and differently shaped molecule. That shape difference is enough that the ribosome — the cell's protein-building machine — can tell them apart precisely and insert each one only where the genetic code specifies.
🔬 Two Chiral Centers — Four Possible Forms
Most amino acids have one chiral center (the alpha carbon) and therefore two possible forms: L and D. Isoleucine is unusual in having two chiral centers — the alpha carbon and the beta carbon in its branched side chain. This means four possible stereoisomers exist in principle. Life uses only one of them: L-isoleucine, also known as (2S,3S)-isoleucine. The other three forms — including the naturally-named "D-alloisoleucine" — don't appear in proteins. They have identical chemical formulas but are as different to a ribosome as left and right hands.
Identifiers and Properties of Isoleucine
Identity
IUPAC Name(2S,3S)-2-Amino-3-methylpentanoic acid
FormulaC₆H₁₃NO₂
Mol. Weight131.18 g/mol
CAS Number73-32-5
MDL NumberMFCD00064222
Physical
Melting point286–288 °C
Solubility41.2 g/L (50 °C)
pKa₁ (COOH)2.32
pKa₂ (NH₃⁺)9.76
pI6.04
Chiral centers2
Rf (BuOH/AcOH/H₂O = 12:3:5)0.72
Identifiers
Canonical SMILESCCC(C)C(C(=O)O)N
Isomeric SMILESCC[C@H](C)[C@@H](C(=O)O)N
InChIKeyAGPKZVBTJJNPAG-WHFBIAKZSA-N
CategoryNonpolar
EssentialYes
The Branched-Chain Amino Acids
Isoleucine belongs to a group of three amino acids — along with leucine and valine — called the branched-chain amino acids (BCAAs). They share a structural feature: a branched aliphatic side chain. They also share a metabolic peculiarity that sets them apart from all other amino acids: they are metabolized primarily in muscle tissue rather than in the liver.
For most amino acids, the liver is the processing hub. BCAAs skip this step and go directly to peripheral tissues — especially skeletal muscle — where they are broken down for energy. During prolonged exercise, when glucose stores begin to deplete, muscles increasingly rely on BCAAs as fuel. Isoleucine's carbon skeleton enters the citric acid cycle directly, generating energy within the muscle cell itself.
Glucogenic and Ketogenic
Amino acids are classified by what happens to their carbon skeletons after the nitrogen is removed: glucogenic amino acids produce glucose precursors, while ketogenic amino acids produce ketone bodies. Most amino acids fall into one category. Isoleucine is one of only a handful that are both — it produces both succinyl-CoA (glucogenic) and acetyl-CoA (ketogenic) when degraded. This metabolic flexibility makes isoleucine a particularly versatile fuel source under varied dietary conditions, and also means it can help maintain normal blood glucose levels during periods of low carbohydrate intake or prolonged fasting.
Functions of L-Isoleucine in the Body
As an essential amino acid, isoleucine must be obtained through diet. Beyond its role as a BCAA fuel source, it participates in several other important physiological processes.
Hemoglobin synthesis
Isoleucine is one of the amino acids required for the synthesis of hemoglobin, the oxygen-carrying protein in red blood cells. Adequate dietary isoleucine, along with other essential amino acids, is necessary for normal hemoglobin production. Isoleucine also participates in the regulation of blood sugar levels through its glucogenic conversion pathway in the liver.
Tissue repair and muscle protein synthesis
As part of the BCAA family, isoleucine contributes to muscle protein synthesis, though leucine carries the primary signaling role for this process. Isoleucine supports the repair of muscle and other tissues after injury or surgery, contributing both as a structural amino acid for new protein construction and as an energy substrate during the recovery period.
Did You Know?
Isoleucine is one of only two standard amino acids with two chiral centers (the other is threonine). This gives it four possible mirror-image forms — but living things use only one of them: (2S,3S)-isoleucine.
Interesting Facts
🧬
Sickle cell anemia and a single substitution. Sickle cell disease is caused by replacing just one amino acid in hemoglobin — glutamic acid at position 6 of the beta chain is replaced by valine. Isoleucine substitutions at other positions in hemoglobin produce different hemoglobin variants with their own distinct properties. Single amino acid changes in a 287-residue protein can completely alter how blood carries oxygen.
🏋️
The BCAA supplement story. Branched-chain amino acids — isoleucine, leucine, and valine — became popular in sports nutrition because of their direct muscle metabolism. Leucine in particular is a signal for muscle protein synthesis. BCAA supplements deliver all three together, with leucine doing most of the signaling work and isoleucine and valine contributing to energy production during exercise.
🌿
Signals plant defenses. In plants, isoleucine plays a surprising role in immune signaling. The plant hormone jasmonate — which triggers defensive responses to insect attack and wounding — is active as its isoleucine conjugate. The jasmonate-isoleucine complex is the molecular key that unlocks the plant's defense response. Isoleucine is not just a structural building block but a signaling component in plant immunity.
🧪
Maple syrup urine disease. A rare genetic disorder called maple syrup urine disease (MSUD) involves a defect in the enzyme that breaks down all three BCAAs — isoleucine, leucine, and valine. Without this enzyme, the three amino acids accumulate to toxic levels. The disease takes its name from the distinctive sweet odor of urine in affected infants. Untreated, BCAA accumulation causes severe neurological damage within days of birth.
Where Isoleucine Is Found
As an essential amino acid, isoleucine must come from food. It is found in high concentrations in most complete proteins. Values below are approximate per standard serving:
