Methionine Identifiers Abbreviations Met, M CAS number , (L-isomer) , (D-isomer) PubChem ChemSpider , (L-isomer) UNII EC-number KEGG ChEBI ChEMBL ATC code V03,QA05 , QG04 Jmol-3D images Image 1
Properties 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 Acidity (pKa) 2.28 (carboxyl), 9.21 (amino) Supplementary data page Structure and
n, εr, etc. Thermodynamic
Solid, liquid, gas
Spectral data UV, IR, NMR, MS (what is: /?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Methionine ( // or //; abbreviated as Met or M) is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein begins.
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 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. In bacteria, the derivative N-formylmethionine is used as the initial amino acid.
As an essential amino acid, methionine is not synthesized de novo 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.
Enzymes involved in methionine biosynthesis:
- β-aspartate semialdehyde dehydrogenase
- homoserine dehydrogenase
- homoserine O-transsuccinylase
- methionine synthase (in mammals, this step is performed by homocysteine methyltransferase)
Other biochemical pathways
Although mammals cannot synthesize methionine, they can still use it in a variety of biochemical pathways:
Generation of homocysteine
Methionine is converted to S-adenosylmethionine (SAM) by (1) methionine adenosyltransferase.
There are two fates of homocysteine: it can be used to regenerate methionine, or to form cysteine.
Regeneration of methionine
Homocysteine can also be remethylated using glycine betaine (NNN-trimethyl glycine, TMG) to methionine via the enzyme betaine-homocysteine methyltransferase (E.C.22.214.171.124, 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.
- (5) Cystathionine-β-synthase (a PLP-dependent enzyme) combines homocysteine and serine to produce cystathionine. Instead of degrading cystathionine via cystathionine-β-lyase, as in the biosynthetic pathway, cystathionine is broken down to cysteine and α-ketobutyrate via (6) cystathionine-γ-lyase.
- (7) The enzyme α-ketoacid dehydrogenase converts α-ketobutyrate to propionyl-CoA, which is metabolized to succinyl-CoA in a three-step process (see propionyl-CoA for pathway).
Racemic methionine can be synthesized from diethyl sodium phthalimidomalonate by alkylation with chloroethylmethylsulfide (ClCH2CH2SCH3) followed by hydrolysis and decarboxylation.
Food sources of Methionine Food g/100g Egg, white, dried, powder, glucose reduced 3.204 Sesame seeds flour (low fat) 1.656 Brazil nuts 1.008 Soy protein concentrate 0.814 Chicken, broilers or fryers, roasted 0.801 Fish, tuna, light, canned in water, drained solids 0.755 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
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, is a classic combination, found throughout the world, such as in rice and beans or tortilla and beans.
There is a growing body of evidence that shows restricting methionine consumption can increase lifespans in some animals.
A 2005 study showed methionine restriction without energy restriction extends mouse lifespan.
DL-methionine is sometimes given as a supplement to dogs; it helps keep dogs from damaging grass by reducing the pH of the urine.
Methionine is allowed as a supplement to organic poultry feed under the US certified organic program.
- Paracetamol poisoning - A Methionine-Paracetamol preparation that might prevent hepatotoxicity.
- Photo-reactive methionine
- ^ Dawson, R.M.C., et al., Data for Biochemical Research, Oxford, Clarendon Press, 1959.
- ^ 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. .
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- ^ Oz HS, Chen TS, Neuman M (2008), "Methionine deficiency and hepatic injury in a dietary steatohepatitis model", Digestive Diseases and Sciences 53 (3): 767–776, doi:10.1007/s10620-007-9900-7, PMC 2271115, PMID 17710550, http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2271115.
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- ^ 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 .
- ^ National Nutrient Database for Standard Reference, U.S. Department of Agriculture, http://www.nal.usda.gov/fnic/foodcomp/search/, retrieved 2009-09-07 .
- ^ Nutritional Value – Idaho Bean Commission
- ^ What's in your dog's food?, Ojibwa Yorkies, ISBN 087605467X, http://www.yorkshire-terrier.com/dogfood.htm, retrieved 2009-09-07 .
- ^ Alleyne, Richard (2009-12-03). "Vegetarian low protein diet could be key to long life". The Daily Telegraph (London). http://www.telegraph.co.uk/health/healthnews/6710896/Vegetarian-low-protein-diet-could-be-key-to-long-life.html. Retrieved 2010-05-12.
- ^ 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 .
- ^ Grandison, R. C.; Piper, M. D. W.; Partridge, L. (2009). "Amino acid imbalance explains extension of lifespan by dietary restriction in Drosophila". Nature 462 (7276): 1061–1064. Bibcode 2009Natur.462.1061G. doi:10.1038/nature08619. PMC 2798000. PMID 19956092. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2798000. Lay summary.
- ^ http://www.sciencenews.org/view/generic/id/50275/title/Amino_acid_recipe_could_be_right_for_long_life
- ^ Burn Baby Burn! Grass Burns from Dog Urine, About.Com, http://dogs.about.com/od/dogcarebasics/qt/grass_burns.htm, retrieved 2010-02-15 .
- ^ Federal Register, US, http://edocket.access.gpo.gov/2011/2011-5716.htm, retrieved 2011-03-12 .
The 20 common amino acids By propertiesAliphaticAromaticPolar, unchargedPositive charge (pKa)Negative charge (pKa)General Other classifications Antidotes (V03AB) Nervous systemBarbiturate overdoseBemegride • EthamivanBenzodiazepine overdoseGHB overdoseReversal of neuromuscular blockade Cardiovascular OtherParacetamol toxicity (Acetaminophen)Other Emetic K→acetyl-CoA GG→pyruvate→citrateG→glutamate→
α-ketoglutarateothermethionine→G→fumarateG→oxaloacetatesee urea cycle
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