Threonine

Threonine (abbreviated as Thr or T) is an α-amino acid with the chemical formula HO2CCH(NH2)CH(OH)CH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar. Together with serine and tyrosine, threonine is one of three proteinogenic amino acids bearing an alcohol group.

The threonine residue is susceptible to numerous posttranslational modifications. The hydroxy side chain can undergo O-linked glycosylation. In addition, threonine residues undergo phosphorylation through the action of a threonine kinase. In its phosphorylated form, it can be referred to as phosphothreonine.

 
Allo-threonine

With two chiral centers, threonine can exist in four possible stereoisomers, or two possible diastereomers of L-threonine. However, the name L-threonine is used for one single enantiomer, (2S,3R)-2-amino-3-hydroxybutanoic acid. The second diastereomer (2S,3S), which is rarely present in nature, is called L-allo-threonine.

Biosynthesis

As an essential amino acid, threonine is not synthesized in humans, hence we must ingest threonine in the form of threonine-containing proteins. In plants and microorganisms, threonine is synthesized from aspartic acid via α-aspartyl-semialdehyde and homoserine. Homoserine undergoes O-phosphorylation; this phosphate ester undergoes hydrolysis concomitant with relocation of the OH group.[2] Enzymes involved in a typical biosynthesis of threonine include:

aspartokinase
α-aspartate semialdehyde dehydrogenase
homoserine dehydrogenase
homoserine kinase
threonine synthase.

Metabolism
Threonine is metabolized in two ways:

It is converted to pyruvate via threonine dehydrogenase. An intermediate in this pathway can undergo thiolysis with CoA to produce Acetyl-CoA and glycine.

In humans, it is converted to alpha-ketobutyrate in a less common pathway via the enzyme serine dehydratase, and thereby enters the pathway leading to succinyl-CoA.

Sources

Foods high in threonine include cottage cheese, poultry, fish, meat, lentils, and sesame seeds.[citation needed]

Racemic threonine can be prepared from crotonic acid by alpha-functionalization using mercury(II) acetate.

Threonine. (2009, February 13). In Wikipedia, The Free Encyclopedia. Retrieved 08:47, February 17, 2009, from http://en.wikipedia.org/w/index.php?title=Threonine&oldid=270547548

Leucine

Leucine (abbreviated as Leu or L) is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH(CH3)2. It is an essential amino acid, which means that humans cannot synthesise it. Its codons are UUA, UUG, CUU, CUC, CUA, and CUG. With a hydrocarbon side chain, leucine is classified as a hydrophobic amino acid. It has an isobutyl R group. Leucine is a major component of the sub units in ferritin, astacin and other 'buffer' proteins.

  Subscribe in a reader Biosynthesis

As an essential amino acid, leucine is not synthesized in animals, hence it must be ingested, usually as a component of proteins. It is synthesized in plants and microorganisms via several steps starting from pyruvic acid. The initial part of the pathway also leads to valine. The intermediate α-ketovalerate is converted to α-isopropylmalate and then β-isopropylmalate, which is dehydrogenated to α-ketoisocaproate, which in the final step undergoes reductive amination. Enzymes involved in a typical leucine biosynthesis include.

• Acetolactate synthase,
• Acetohydroxy acid isomeroreductase,
• Dihydroxyacid dehydratase,
• α-Isopropylmalate synthase,
• α-Isopropylmalate isomerase,
• Leucine aminotransferase.

As a dietary supplement, leucine has been found to slow the degradation of muscle tissue by increasing the synthesis of muscle proteins. Leucine is utilized in the liver, adipose tissue, and muscle tissue. In adipose and muscle tissue, leucine is used in the formation of sterols, and the combined usage of leucine in these two tissues is seven times greater than its use in the liver.