Methionine

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Methionin - Methionine.svg L-methionine-B-3D-balls.png

Chemical structure of Methionine

Methionine

Systematic (IUPAC) name
(S)-2-amino-4-(methylsulfanyl)-butanoic acid
Identifiers
CAS number 63-68-3
PubChem         876
Chemical data
Formula C5H11NO2S 
Molar mass 149.21 g/mol
SMILES CSCC[C@H](N)C(O)=O
Complete data

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Methionine (abbreviated as Met or M)[1] is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH2SCH3. This essential amino acid is classified as nonpolar. Together with cysteine, methionine is one of two sulfur-containing proteinogenic amino acids. Its derivative S-adenosyl methionine (SAM) serves as a methyl donor. Methionine is an intermediate in the biosynthesis of cysteine, carnitine, taurine, lecithin, phosphatidylcholine, and other phospholipids. Improper conversion of methionine can lead to atherosclerosis.

Methionine is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code (tryptophan, encoded by UGG, is the other). The codon AUG is also significant, in that it carries the "Start" message for a ribosome that signals the initiation of protein translation from mRNA. As a consequence, methionine is incorporated into the N-terminal position of all proteins in eukaryotes and archaea during translation, although it is usually removed by post-translational modification.

Biosynthesis

As an essential amino acid, methionine is not synthesized in humans, hence we must ingest methionine or methionine-containing proteins. In plants and microorganisms, methionine is synthesized via a pathway that uses both aspartic acid and cysteine. First, aspartic acid is converted via β-aspartyl-semialdehyde into homoserine, introducing the pair of contiguous methylene groups. Homoserine converts to O-succinyl homoserine, which then reacts with cysteine to produce cystathionine, which is cleaved to yield homocysteine. Subsequent methylation of the thiol group by folates affords methionine. Both cystathionine-γ-synthase and cystathionine-β-lyase require Pyridoxyl-5'-phosphate as a cofactor, whereas homocysteine methyltransferase requires Vitamin B12 as a cofactor.[2]

Enzymes involved in methionine biosynthesis:

  1. aspartokinase
  2. β-aspartate semialdehyde dehydrogenase
  3. homoserine dehydrogenase
  4. homoserine acyltransferase
  5. cystathionine-γ-synthase
  6. cystathionine-β-lyase
  7. methionine synthase (in mammals, this step is performed by homocysteine methyltransferase)
Met biosynthesis.gif

Other biochemical pathways

Although mammals cannot synthesize methionine, they can still utilize it in a variety of biochemical pathways:

Methionine is converted to S-adenosylmethionine (SAM) by (1) methionine adenosyltransferase. SAM serves as a methyl-donor in many (2) methyltransferase reactions and is converted to S-adenosylhomocysteine (SAH). (3) adenosylhomocysteinase converts SAH to homocysteine.

There are two fates of homocysteine:

Fates of methionine

Synthesis

Racemic methionine can be synthesized from diethyl sodium phthalimidomalonate by alkylation with chloroethylmethylsulfide (ClCH2CH2SCH3) followed by hydrolysis and decarboxylation.[3]

Dietary aspects

High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meats, and some other plant seeds. Most fruit and vegetables contain very little; however, some have significant amounts, such as spinach, potatoes, and boiled corn.st DL-methionine is sometimes added as an ingredient to pet foods.[4] Methionine, cysteine, and soy protein heated in a small amount of water creates a meat-like aroma.

See also

References

  1. IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. "Nomenclature and Symbolism for Amino Acids and Peptides". Recommendations on Organic & Biochemical Nomenclature, Symbols & Terminology etc. Retrieved 2007-05-17. 
  2. Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.
  3. Barger, G.; Weichselbaum, T. E. (1943). "dl-Methionine". Org. Synth.; Coll. Vol. 2: 384. 
  4. What's in your dog's food?

External links


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