Threonine
| Threonine | |
|---|---|
| Systematic name | (2S,3R)-2-Amino- 3-hydroxybutanoic acid |
| Abbreviations | Thr T |
| Chemical formula | C4H9NO3 |
| Molecular mass | 119.12 g mol-1 |
| Melting point | 256 °C |
| Density | ? g cm-3 |
| Isoelectric point | 5.60 |
| pKa | 2.20 8.96 |
| PubChem | 6288 |
| CAS number | [72-19-5] |
| EINECS number | 200-774-1 |
| SMILES | C[C@@H](O)[C@H](N)C(O)=O |
 
| |
| Disclaimer and references | |
Threonine is an α-amino acid that is common in many proteins and together with serine and tyrosine is one of three proteinogenic amino acids bearing an alcohol group. Like serine, threonine is sometimes in substantial concentrations in the outer regions of soluble proteins due to its hydrophilic nature. With an easily removed hydrogen on the hydroxyl side chain, threonine is often a hydrogen donor in enzymes.
The L-isomer of threonine, which is the only form that is involved in protein synthesis, is one of the 20 standard amino acids common in animal proteins and required for normal functioning in humans. Threonine is also classified as an "essential amino acid" since it cannot be synthesized by the human body from other compounds through chemical reactions and thus has to be taken in with the diet.
Threonine's three letter code is Thr, its one letter code is T, its codons are ACU and ACA, and its systematic name is 2-Amino-3-hydroxybutanoic acid (IUPAC-IUB 1983).
Structure
In biochemistry, the term amino acid is frequently used to refer specifically to alpha amino acids: those amino acids in which the amino and carboxylate groups are attached to the same carbon, the so-called α–carbon (alpha carbon). The general structure of these alpha amino acids is:
R
|
H2N-C-COOH
|
H
where R represents a side chain specific to each amino acid.
Most amino acids occur in two possible optical isomers, called D and L. The L amino acids represent the vast majority of amino acids found in proteins. They are called proteinogenic amino acids. As the name "proteinogenic" (literally, protein building) suggests, these amino acid are encoded by the standard genetic code and participate in the process of protein synthesis. In serine, only the L-stereoisomer is involved in synthesis of mammalian proteins.
Serine has the chemical formula HO-CH2-CH(NH2)-COOH (alternatively, HO2C-CH(NH2)-CH2-OH), or more generally, C3H7NO3.
with the chemical formula HO2CCH(NH2)CH(OH)CH3.
CH3-CH(OH)-CH(NH2)-COOH
Serine, like threonine, has a short group ended with a hydroxyl group. The hydroxyl group attached makes it a polar amino acid. Its hydrogen is easy to remove, so serine and threonine often act as hydrogen donors in enzymes. Both are very hydrophilic, therefore the outer regions of soluble proteins tend to be rich with them.
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.
Behaves similarly to serine.
The threonine residue is susceptible to numerous posttranslational modifications. The hydroxy side chain can undergo O-linked glycosylation. Additionally, 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 or, more commonly, 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.[1] 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
- It is converted to alpha-ketobutyrate, and thereby enter the pathway leading to succinyl CoA.
Synthesis
Racemic threonine can be prepared from crotonic acid by alpha-functionalization using mercury(II) acetate.[2]
Sources
Foods high in threonine include cottage cheese, poultry, fish, meat, lentils, and sesame seeds.[citation needed]
ReferencesISBN links support NWE through referral fees
- ↑ Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.
- ↑ Carter, H. E.; West, H. D. (1955). "dl-Threonine". Org. Synth.; Coll. Vol. 3: 813.
- Doolittle, R. F. 1989. Redundancies in protein sequences. In G. D. Fasman, ed., Prediction of Protein Structures and the Principles of Protein Conformation. New York: Plenum Press. ISBN 0306431319.
- International Union of Pure and Applied Chemistry and International Union of Biochemistry and Molecular Biology (IUPAC-IUB) Joint Commission on Biochemical Nomenclature. 1983. Nomenclature and symbolism for amino acids and peptides: Recommendations on organic & biochemical nomenclature, symbols & terminology. IUPAC-IUB. Retrieved June 14, 2007.
- Lehninger, A. L., D. L. Nelson, and M. M. Cox. 2000. Lehninger Principles of Biochemistry, 3rd ed. New York: Worth Publishing. ISBN 1572591536.
External links
Template:ChemicalSources
| Major families of biochemicals | ||
| Peptides | Amino acids | Nucleic acids | Carbohydrates | Nucleotide sugars | Lipids | Terpenes | Carotenoids | Tetrapyrroles | Enzyme cofactors | Steroids | Flavonoids | Alkaloids | Polyketides | Glycosides | ||
| Analogues of nucleic acids: | The 20 Common Amino Acids | Analogues of nucleic acids: |
| Alanine (dp) | Arginine (dp) | Asparagine (dp) | Aspartic acid (dp) | Cysteine (dp) | Glutamic acid (dp) | Glutamine (dp) | Glycine (dp) | Histidine (dp) | Isoleucine (dp) | Leucine (dp) | Lysine (dp) | Methionine (dp) | Phenylalanine (dp) | Proline (dp) | Serine (dp) | Threonine (dp) | Tryptophan (dp) | Tyrosine (dp) | Valine (dp) | ||
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