isoleucine

Isoleucine is one of the three branched-chain amino acids (BCAAs), along with leucine and valine. It is essential, meaning humans cannot synthesize it and must obtain it through dietary intake (found in meat, fish, eggs, dairy, and legumes). Structurally, it has a branched side chain and is classified as both glucogenic and ketogenic because its catabolism yields both acetyl-CoA and succinyl-CoA, which can enter the TCA cycle for either gluconeogenesis or ketone body production. Metabolism of isoleucine begins with transamination (removing the amino group) to form its corresponding alpha-ketoacid, followed by oxidative decarboxylation catalyzed by the branched-chain alpha-ketoacid dehydrogenase (BCKD) complex, a mitochondrial enzyme that requires thiamine (vitamin B1) as a cofactor. This step is clinically significant because a deficiency in BCKD activity causes Maple Syrup Urine Disease (MSUD), an autosomal recessive metabolic disorder characterized by accumulation of branched-chain amino acids and their ketoacids in the blood and urine, leading to a characteristic sweet, maple syrup-like odor. Untreated MSUD can cause severe neurological damage, encephalopathy, and death in infants due to toxic accumulation of these metabolites. Management involves dietary restriction of BCAAs and thiamine supplementation in some cases. BCAAs including isoleucine are also of interest in muscle protein synthesis and are metabolized primarily in skeletal muscle rather than the liver, distinguishing them from other amino acids. This is relevant in conditions like hepatic encephalopathy, where BCAA supplementation has been studied as a potential therapy since aromatic amino acids competitively cross the blood-brain barrier and contribute to altered neurotransmission when BCAAs are low.