threonine

Threonine is one of the nine essential amino acids for humans, classified as glucogenic (its carbon skeleton can be converted to succinyl-CoA or pyruvate for gluconeogenesis). Structurally, it contains a hydroxyl group, making it a substrate for O-linked glycosylation of proteins (along with serine), important in mucin and other glycoprotein synthesis. Threonine residues are also common phosphorylation sites for protein kinases (along with serine and tyrosine), playing a role in signal transduction pathways. Metabolically, threonine can be degraded via two pathways: one leading to glycine and acetyl-CoA (via threonine aldolase), and another via threonine dehydratase to alpha-ketobutyrate, which feeds into propionyl-CoA and eventually succinyl-CoA in the TCA cycle. This connects threonine metabolism to vitamin B12 and biotin-dependent pathways (relevant in methylmalonic acidemia and propionic acidemia if metabolism is disrupted). Clinically, essential amino acid deficiencies including threonine can occur in severe malnutrition (e.g., kwashiorkor) or in inborn errors of amino acid metabolism, though isolated threonine deficiency syndromes are rare. On the USMLE, threonine is often tested in the context of classifying amino acids as essential vs. nonessential, glucogenic vs. ketogenic, and its role in post-translational modifications (glycosylation and phosphorylation sites) rather than a dedicated disease topic.