Threonine

Threonine is a polar amino acid with a hydroxyl group and a branched side chain. It is encoded by the codons ACU, ACC, ACA and ACG and is an essential amino acid, meaning humans cannot make it and must obtain it from food.

Threonine has two chiral centers, so it exists in four stereoisomers, though proteins use only the L-threo form. It was the last of the twenty standard amino acids to be discovered, isolated in 1935 by William Cumming Rose. Good dietary sources include meat, dairy, eggs and legumes. Bacteria and plants make it from aspartate, in a pathway regulated by allosteric feedback.

Like serine, threonine can be phosphorylated by kinases, an on-off switch that controls many signaling pathways, as described under phosphorylation. Its hydroxyl is also a common attachment site for sugars in O-linked glycosylation, as in the mucins that coat the lining of the gut.

Threonine helps to build collagen and elastin and is a part of tooth enamel proteins. The body breaks it down to produce glycine and the energy intermediate acetyl-CoA or propionyl-CoA. Because it is essential, a shortage can limit protein synthesis, which matters in livestock feed formulation. It is also one of the amino acids that proteins are built from, as specified by the genetic code.

• Serine — a similar amino acid
• Phosphorylation — a key modification
• Amino Acid — the class it belongs to