Isoleucine (symbol Ile or I) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+
3 form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and a hydrocarbon side chain with a branch (a central carbon atom bound to three other carbon atoms). It is classified as a non-polar, uncharged (at physiological pH), branched-chain, aliphatic amino acid. It is essential in humans, meaning the body cannot synthesize it, and must be ingested in our diet. Isoleucine is synthesized from pyruvate employing leucine biosynthesis enzymes in other organisms such as bacteria. It is encoded by the codons AUU, AUC, and AUA.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||131.175 g·mol−1|
|Supplementary data page|
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Dielectric constant (εr), etc.
|UV, IR, NMR, MS|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Inability to break down isoleucine, along with other amino acids, is associated with the disease called maple syrup urine disease, which results in discoloration and a sweet smell in the patient's urine, which is where the name comes from. However, in severe cases, MSUD can lead to damage to the brain cells and ultimately death.
As an essential nutrient, it is not synthesized in the body, hence it must be ingested, usually as a component of proteins. In plants and microorganisms, it is synthesized via several steps, starting from pyruvic acid and alpha-ketoglutarate. Enzymes involved in this biosynthesis include:
- Acetolactate synthase (also known as acetohydroxy acid synthase)
- Acetohydroxy acid isomeroreductase
- Dihydroxyacid dehydratase
- Valine aminotransferase
Isoleucine is both a glucogenic and a ketogenic amino acid. After transamination with alpha-ketoglutarate the carbon skeleton can be converted into either succinyl CoA, and fed into the TCA cycle for oxidation or converted into oxaloacetate for gluconeogenesis (hence glucogenic). It can also be converted into acetyl CoA and fed into the TCA cycle by condensing with oxaloacetate to form citrate. In mammals Acetyl CoA cannot be converted back to carbohydrate but can be used in the synthesis of ketone bodies or fatty acids, hence ketogenic.
Isoleucine, like other branched-chain amino acids, is associated with insulin resistance: higher levels of isoleucine are observed in the blood of diabetic mice, rats, and humans. Mice fed a isoleucine deprivation diet for one day have improved insulin sensitivity, and feeding of a isoleucine deprivation diet for one week significantly decreases blood glucose levels. In diet-induced obese and insulin resistant mice, a diet with decreased levels of isoleucine and the other branched-chain amino acids results in reduced adiposity and improved insulin sensitivity. In humans, a protein restricted diet lowers blood levels of isoleucine and decreases fasting blood glucose levels.
The Food and Nutrition Board (FNB) of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002. For isoleucine, for adults 19 years and older, 19 mg/kg body weight/day.
Even though this amino acid is not produced in animals, it is stored in high quantities. Foods that have high amounts of isoleucine include eggs, soy protein, seaweed, turkey, chicken, lamb, cheese, and fish.
|Forms of Isoleucine|
|PubChem:||CID 791 from PubChem||CID 94206 from PubChem||CID 6306 from PubChem||CID 76551 from PubChem|
|l-isoleucine (2S,3S) and d-isoleucine (2R,3R)|
|l-alloisoleucine (2S,3R) and d-alloisoleucine (2R,3S)|
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- "Foods highest in Isoleucine". Self Nutrition Data. Condé Nast.
List is in order of highest to lowest of per 200 Calorie serving of the food, not volume or weight.
- Marvel, C. S. (1941). "dl-Isoleucine (α-Amino-β-methylvaleric acid)". Organic Syntheses. 21: 60. doi:10.15227/orgsyn.021.0060.; Collective Volume, 3, p. 495
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