The complex chemistry of threonine: a protein building block with a twist

Science · Published 19 October 2026 Swipe more like this →

In short: Threonine is more than just a structural component of proteins. As one of the few amino acids with two stereogenic centers, its specific spatial configuration determines its biological utility. From its distinct melting point to its role in vitamin synthesis, threonine is a master of molecular geometry.

L-threonine is a polar, uncharged amino acid with the molecular formula C4H9NO3. Its chemical identity is defined by the IUPAC name (2S,3R)-2-amino-3-hydroxybutanoic acid. This specific configuration is what distinguishes the naturally occurring L-threonine from other possible stereoisomers, such as L-allothreonine, which is rarely found in nature. Physically, it presents as a colorless crystalline powder with a slight savory aroma and a melting point of 256 °C (decomposing). The compound is also known by several synonyms, including 2-amino-3-hydroxybutyric acid. Its molecular weight is 119.12 g/mol. It is notably insoluble in common neutral solvents.

The molecule's unique structure allows it to participate in specific hydrogen-bonded motifs, such as ST turns and staples, which are common in alpha helices. Beyond structure, threonine is chemically versatile. It can undergo O-linked glycosylation or phosphorylation by threonine kinases. In the latter case, the resulting phosphothreonine provides three potential coordination sites—carboxyl, amine, and phosphate groups—which are critical for interacting with metal ions in various biological processes. The hydroxyl side-chain is particularly susceptible to these posttranslational modifications. These interactions are often based on motifs involving serine. These specific structural motifs are frequently found at the beginning of alpha helices.

Metabolically, threonine serves as a vital precursor in diverse pathways. While many animals convert it to pyruvate, humans possess an inactive pseudogene for threonine dehydrogenase, leading instead to its conversion to α-ketobutyrate. In bacteria, threonine is a key intermediate in the biosynthesis of cobalamin (Vitamin B12). Because humans cannot synthesize it, threonine is classified as an essential amino acid, requiring an intake of approximately 20 mg/kg of body weight per day to maintain health. In the brain and liver of rats, threonine helps synthesize glycine during L-carnitine production. Bacteria synthesize this amino acid from aspartate. Plants and microorganisms follow a similar biosynthetic route starting from aspartic acid.

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