Isoleucine
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. Essential amino acids are necessary in our diet. In plants isoleucine can be synthesized from threonine and methionine. In plants and bacteria, isoleucine is synthesized from pyruvate employing leucine biosynthesis enzymes. It is encoded by the codons AUU, AUC, and AUA. In plants and microorganisms, isoleucine is synthesized via several steps starting from pyruvate and alpha-ketobutyrate. As an essential nutrient, this pathway is not present in humans. 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 is oxidised and split into propionyl-CoA and acetyl-CoA. Propionyl-CoA is converted into succinyl-CoA, a TCA cycle intermediate which can be converted into oxaloacetate for gluconeogenesis (hence glucogenic). In mammals acetyl-CoA cannot be converted to carbohydrate but can be either fed into the TCA cycle by condensing with oxaloacetate to form citrate or used in the synthesis of ketone bodies (hence ketogenic) or fatty acids. The degradation of isoleucine is impaired in the following metabolic diseases: Combined malonic and methylmalonic aciduria (CMAMMA) Maple syrup urine disease (MSUD) Methylmalonic acidemia Propionic acidemia 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.
