Valine

Valine
Systematic name	(S)-2-amino-3-methyl-butanoic acid
Abbreviations	Val V
Chemical formula	C5H11NO2
Molecular mass	117.15 g mol-1
Melting point	315 °C
Density	1.230 g cm-3
Isoelectric point	5.96
pKa	2.27 9.52
PubChem	1182
CAS number	[72-18-4]
EINECS number	200-773-6
SMILES	CC(C)[C@H](N)C(O)=O
Disclaimer and references

Valine is an α-amino acid that is found in most proteins and is essential in the human diet. It is similar to leucine and isoleucine in being a branched-chain amio acid and whose buildup in the blood and urine, due a particular enzyme deficiency, causes the serious metabolic disorder maple syrup urine disease.

In humans, the L-isomer of valine, 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. Valine 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.

Valine is named after the plant valerian. In sickle-cell disease, valine substitutes for the hydrophilic (binds with and dissolves in water) amino acid glutamic acid in hemoglobin. Because valine is hydrophobic (repelled by water), the hemoglobin does not fold correctly.

Valine's three letter code is Val, its one letter code is V, its codons are GUU, GUC, GUA, and GUG, and its systematic name is 2-Amino-3-methylbutanoic 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 asparagine, only the L-stereoisomer is involved in synthesis of mammalian proteins.

Asparagine's chemical formula is H₂N-CO-CH₂-CH(NH₂)-COOH, or more generally, C₄H₈N₂O₃. Arginine's chemical formula is H₂N-C(=NH)-NH-[CH₂]₃-CH(NH₂)-COOH or in general form C₆H₁₄N₄O₂(IUPAC-IUB 1983).

Valine: with the chemical formula HO₂CCH(NH₂)CH(CH₃)2. (CH3)2CH-CH(NH2)-COOH

Biosynthesis

As an essential amino acid, valine is not synthesized in animals, hence it must be ingested, usually as a component of proteins. It is synthesized in plants via several steps starting from pyruvic acid. The initial part of the pathway also leads to leucine. The intermediate α-ketovalerate undergoes reductive amination with glutamate. Enzymes involved in this biosynthesis include:

1. acetolactate synthase
2. acetohydroxy acid isomeroreductase
3. dihydroxyacid dehydratase
4. valine aminotransferase

Other aspects

In a 1994 report released by five top cigarette companies, valine is one of the 599 additives to cigarettes. Like most cigarette additives, the reason for its inclusion is unknown.

Dietary aspects

Nutritional sources of valine include cottage cheese, fish, poultry, peanuts, sesame seeds, and lentils.

Maple syrup urine disease

Maple syrup urine disease (MSUD) is an autosomal recessive metabolic disorder of amino acid metabolism. It is also called branched chain ketoaciduria.

This amino acid acidopathy is due to a deficiency of the metabolic enzyme branched chain α-keto acid dehydrogenase (BCKDH) leading to a buildup of branched-chain amino acids (leucine, isoleucine, and valine) in the blood and urine.

Characterized by an infant with sweet-smelling urine with an odor similar to that of maple syrup, infants with this disease seem healthy at birth but if left untreated suffer severe brain damage and eventually die. Because of a genetic bottleneck effect, MSUD has a much higher prevalence in children of Amish and Mennonite descent.

From early infancy, the condition is characterized by poor feeding, vomiting, lack of energy (lethargy), seizures, and mental health issues. The urine of affected infants has a distinctive sweet odor, much like burned caramel, that gives the condition its name.

Treatment of the MSUD, like diabetes, requires careful monitoring of blood chemistry and involves both special diet and frequent testing. A diet with minimal levels of the amino acids leucine, isoleucine, and valine must be maintained in order to prevent neurological damage. Usually, patients, or parents of patients are assisted by a physician or dietician. This diet must be adhered to strictly and permanently. However, with proper treatment those afflicted are able to live healthy, normal lives and not suffer the severe neurological damage that characterizes the untreated disease.

References

ISBN links support NWE through referral fees

• 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

• Isoleucine and valine biosynthesis

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