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Methionine
Identifiers
Abbreviations Met, M
CAS number 59-51-8 Yes check.svgY
63-68-3 (L-isomer)
348-67-4 (D-isomer)
PubChem 876
ChemSpider 853
EC-number 200-432-1
ATC code V03AB26,QA05BA90, QG04BA90
SMILES
InChI
Properties[1]
Molecular formula C5H11NO2S
Molar mass 149.21 g mol−1
Appearance White crystalline powder
Density 1.340 g/cm3
Melting point

281 °C decomp.

Solubility in water Soluble
Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Methionine (pronounced /mɛˈθaɪ.ɵniːn, mɛˈθaɪ.ɵnɪn/; abbreviated as Met or M)[2] is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH2SCH3. This essential amino acid is classified as nonpolar.

Contents

Function

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.[citation needed]

This amino acid is also used by plants for synthesis of ethylene. The process is known as the Yang Cycle or the methionine cycle.

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 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.

Betaines

(S)-Methionine (left) and (R)-methionine (right) in zwitterionic form at neutral pH

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.[3]

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)
Methionine biosynthesis

Other biochemical pathways

Fates of methionine

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

Generation of homocysteine

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: it can be used to regenerate methionine, or to form cysteine.

Regeneration of methionine

Methionine can be regenerated from homocysteine via (4) methionine synthase.

It can also be remethylated using glycine betaine (NNN-trimethyl glycine) to methionine via the enzyme Betaine-homocysteine methyltransferase (E.C.2.1.1.5, BHMT). BHMT makes up to 1.5% of all the soluble protein of the liver, and recent evidence suggests that it may have a greater influence on methionine and homocysteine homeostasis than methionine synthase.

Conversion to cysteine

Homocysteine can be converted to cysteine.

Synthesis

Racemic methionine can be synthesized from diethyl sodium phthalimidomalonate by alkylation with chloroethylmethylsulfide (ClCH2CH2SCH3) followed by hydrolysis and decarboxylation.[4]

Dietary aspects

Food sources of Methionine[5]
Food g/100g
Sesame seeds flour (low fat) 1.656
Brazilnuts 1.008
Soy protein concentrate 0.814
Wheat germ 0.456
Oat 0.312
Peanuts 0.309
Chickpea 0.253
Corn, yellow 0.197
Almonds 0.151
Beans, pinto, cooked 0.117
Lentils, cooked 0.077
Rice, brown, medium-grain, cooked 0.052
Rice and beans provides a complete protein, the methionine in the rice complementing the proteins in the beans.

High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meats and some other plant seeds; methionine is also found in cereal grains. Most fruits and vegetables contain very little of it. Most legumes are also low in methionine. The complement of cereal (methionine) and legumes (lysine), providing a complete protein,[6] is a classic combination, found throughout the world, such as in rice and beans, and similar combinations discussed there.

Sesame seeds, such as in hummus, provide methionine in Arab cuisine.

The use of sesame seeds in cuisine, such as Indian cuisine and, especially in the form of tahini, in Arab cuisine, helps provide essential protein in vegetarian and vegan diets. For example, in hummus, sesame seeds are combined with chickpeas.

Racemic methionine is sometimes added as an ingredient to pet foods.[7]

DL-methionine is the active ingredient in dog supplements to prevent yellow nitrogen burns to grass from their urine. The action is by reducing the pH of the dog's urine. [8] One example is "Grass Saver" by NaturVet. [9] There are claims the supplements can cause bladder stone (animal). [10]

Methionine restriction

There is a growing body of evidence that shows restricting methionine consumption can increase lifespans in some animals.[11]

A 2005 study showed methionine restriction without energy restriction extends mouse lifespan.[12]

A study published in Nature showed adding just the essential amino acid methionine to fruit flies on a calorie restricted diet restored egg-laying without reducing lifespan.[13][14]

See also

References

  1. ^ Weast, Robert C., ed. (1981), CRC Handbook of Chemistry and Physics (62nd ed.), Boca Raton, FL: CRC Press, p. C-374, ISBN 0-8493-0462-8 .
  2. ^ "Nomenclature and symbolism for amino acids and peptides (IUPAC-IUB Recommendations 1983)", Pure Appl. Chem. 56 (5): 595–624, 1984, doi:10.1351/pac198456050595 .
  3. ^ Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2000), Principles of Biochemistry (3rd ed.), New York: W. H. Freeman, ISBN 1-57259-153-6 .
  4. ^ Barger, G.; Weichselbaum, T. E. (1934), "dl-Methionine", Org. Synth. 14: 58, http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV2P0384 ; Coll. Vol. 2: 384 .
  5. ^ National Nutrient Database for Standard Reference, U.S. Department of Agriculture, http://www.nal.usda.gov/fnic/foodcomp/search/, retrieved 2009-09-07 .
  6. ^ Nutritional Value – Idaho Bean Commission
  7. ^ What's in your dog's food?, Ojibwa Yorkies, http://www.yorkshire-terrier.com/dogfood.htm, retrieved 2009-09-07 .
  8. ^ Burn Baby Burn! Grass Burns from Dog Urine, About.Com, http://dogs.about.com/od/dogcarebasics/qt/grass_burns.htm, retrieved 2010-02-15 .
  9. ^ Grass Saver" by NaturVet, NaturVet, http://www.naturvet.com/index.php?option=com_dogcat&catid=1&subcat=3&Itemid=33 .
  10. ^ Bladder Stones, Ask.com, http://dogs.about.com/cs/disableddogs/p/bladder_stones.htm .
  11. ^ http://www.telegraph.co.uk/health/healthnews/6710896/Vegetarian-low-protein-diet-could-be-key-to-long-life.html
  12. ^ Miller, Richard A.; Buehner, Gretchen; Chang, Yayi; Harper, James M.; Sigler, Robert; Smith-Wheelock, Michael (2005), "Methionine-deficient diet extends mouse lifespan, slows immune and lens aging, alters glucose, T4, IGF-I and insulin levels, and increases hepatocyte MIF levels and stress resistance", Aging cell 4 (3): 119–125, doi:10.1111/j.1474-9726.2005.00152.x, PMID 15924568 .
  13. ^ Grandison, R. C.; Piper, M. D. W.; Partridge, L. (2009). "Amino-acid imbalance explains extension of lifespan by dietary restriction in Drosophila". Nature. doi:10.1038/nature08619. Lay summary.  edit
  14. ^ http://www.sciencenews.org/view/generic/id/50275/title/Amino_acid_recipe_could_be_right_for_long_life
  • Rudra, M. N.; Chowdhury, L. M. (30 September 1950), "Methionine Content of Cereals and Legumes", Nature 166 (568), doi:10.1038/166568a0 

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