Threonine is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group, a carboxyl group, and a side chain containing a hydroxyl group, making it a polar, uncharged amino acid. It is essential in humans, meaning the body cannot synthesize it de novo and must obtain it from dietary protein; the biologically active form is L-threonine with molecular formula C4H9NO3 and molecular weight 119.12 g mol, codons ACU ACC ACA ACG encode threonine in mRNA, the side chain hydroxyl is a site for post translational modification including phosphorylation and O linked glycosylation, and the amino and carboxyl groups give pKa values near 2.1 for the carboxyl and 9.1 for the amino group with an isoelectric point around 5.6.
Typical technical and pharmaceutical specifications for L threonine indicate assay (L threonine) 98.5 to 100.0 percent, water content (Karl Fischer) up to 0.5 percent, loss on drying up to 0.5 percent, chloride typically less than or equal to 0.05 percent, sulfate less than or equal to 0.03 percent, heavy metals not more than 10 parts per million, residue on ignition not more than 0.1 percent; related substances by HPLC typically report D threonine not more than 0.5 percent, serine and glycine impurities individually below 0.5 percent, other amino acid cross contaminants such as valine or leucine each below 0.2 percent, and unspecified organic impurities reported as total related substances below 1.0 percent, with limits adjusted to meet reagent or compendial grade requirements.
Dietary sources include animal proteins such as meat, poultry, fish, eggs, and dairy, and plant sources such as soy, legumes, nuts, seeds, and whole grains; it is also available as a purified supplement for nutritional and feed applications.
In humans deficiency is uncommon but can contribute to impaired protein synthesis presenting as poor growth in children, reduced muscle maintenance, and weakened immune responses; signs are typically nonspecific and occur in the context of overall protein malnutrition.
Threonine serves as a protein building block, contributes to structural motifs and active sites in proteins, is a substrate for post translational modifications such as phosphorylation and O linked glycosylation, and participates in intermediary metabolism as a precursor to compounds including glycine and acetyl CoA via catabolic pathways.
Elevated threonine levels in blood or tissues may reflect recent dietary intake or supplementation and in rare cases indicate metabolic disorders such as hyperthreoninemia; interpretation requires comparison to laboratory reference ranges and clinical context.
