threonine

Threonine is an essential amino acid, one of the 20 standard amino acids used in protein synthesis. In cells, the L-enantiomer, L-threonine, is incorporated into proteins by ribosomes; the D-enantiomer is not used in human proteins and is comparatively rare.

Chemically, threonine is an α-amino acid with the formula C4H9NO3. It has a stereocenter at the alpha carbon and a hydroxy-bearing side chain (–CH(OH)–CH3). The side chain makes threonine a polar, hydrophilic residue.

Genetic codons for threonine are ACN (ACA, ACC, ACG, ACU). Threonine is synthesized by plants and many microbes, but in humans it is essential and must be obtained from the diet. Common dietary sources include dairy products, eggs, meat, fish, and legumes.

Function and metabolism: Threonine is required for normal protein synthesis and tissue maintenance. It can be degraded to intermediates such as glycine and serine in some organisms, contributing to one-carbon metabolism, and is involved in mucin production in the gut.

Deficiency is rare under normal dietary conditions but can occur in malnutrition, potentially leading to fatigue, poor growth, or immune dysfunction. Threonine is included in many parenteral nutrition formulations to ensure adequate amino acid supply.