Difference between revisions of "Methionine" - New World Encyclopedia

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{| id="bioChemInfoBox" align="right" border="1" cellspacing="0" cellpadding="3" style="margin: 0 0 0 0.5em; background: #FFFFFF; border-collapse: collapse; border-color: #C0C090; width: 320px;"
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! {{chembox header}}| '''Methionine'''
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|-
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| style="width: 30%;" | [[IUPAC nomenclature|Systematic name]]
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| (''S'')-2-amino-4-(methylsulfanyl)-<br/>butanoic acid
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|-
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| Abbreviations
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| '''Met<br/>M'''
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|-
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| [[Chemical formula]]
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| C<sub>5</sub>H<sub>11</sub>NO<sub>2</sub>S
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|-
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| [[Molecular mass]]
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| 149.21 g mol<sup>-1</sup>
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|-
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| [[Melting point]]
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| 281 °C
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|-
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| [[Density]]
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| 1.340 g cm<sup>-3</sup>
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|-
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| [[Isoelectric point]]
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| 5.74
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|-
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| [[Acid dissociation constant|p''K''<sub>a</sub>]]
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| 2.16<br/>9.08
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|-
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| [[CAS registry number|CAS number]]
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| [63-68-3]
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|-
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| [[PubChem]]
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| [http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=876 876]
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|-
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| [[EINECS number]]
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| 200-562-9
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|-
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| {{chembox SMILES|value=CSCC[C@H](N)C(O)=O}}
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|-
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| align="center" colspan="2" | [[Image:L-methionine-skeletal.png|120px|Chemical structure of methionine]] [[Image:L-methionine-3D-sticks.png|120px|Chemical structure of methionine]] [[Image:Met-stick.png|Chemical structure of methionine]]
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| {{chembox header}} | <small>[[wikipedia:Chemical infobox|Disclaimer and references]]</small>
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'''Methionine''' is an α-[[amino acid]] with the [[chemical formula]] HO<sub>2</sub>CCH(NH<sub>2</sub>)CH<sub>2</sub>CH<sub>2</sub>SCH<sub>3</sub>.  This [[Essential amino acid|essential]] 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 plays a role in the biosynthesis of cysteine, [[carnitine]], and [[taurine]] (by the [[transsulfuration pathway]]), [[lecithin]] production, the synthesis of [[phosphatidylcholine]], and other [[phospholipid]]s. Improper conversion of methionine can lead to [[atherosclerosis]].
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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]] to begin protein translation from mRNA. As a consequence, methionine is incorporated into the N-terminal position of all [[protein]]s in [[eukaryote]]s and [[archaea]] during translation, although it is usually removed by [[post-translational modification]].
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== Biosynthesis ==
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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 [[folic acid|folates]] affords methionine.  Both [[cystathionine-γ-synthase]] and [[cystathionine-β-lyase]] require [[Pyridoxal-phosphate|Pyridoxyl-5'-phosphate]] as a [[cofactor]], whereas [[homocysteine methyltransferase]] requires [[Cyanocobalamin|Vitamin B12]] as a cofactor.<ref>Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.</ref>
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Enzymes involved in methionine biosynthesis:
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# [[aspartokinase]]
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# β-aspartate semialdehyde [[dehydrogenase]]
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# homoserine dehydrogenase
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# homoserine [[acyltransferase]]
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# cystathionine-γ-[[synthase]]
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# cystathionine-β-[[lyase]]
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# [[methionine synthase]] (in mammals, this step is performed by [[homocysteine methyltransferase]])
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<br style="clear: both;" />
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[[Image:Met biosynthesis.gif|center]]
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== Other biochemical pathways ==
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Although mammals cannot synthesize methionine, they can still utilize it in a variety of biochemical pathways:
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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]].
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There are two fates of [[homocysteine]]. 
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* First, 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 sythase. 
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* Alternatively, 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) [[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).
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[[Image:Met pathway.gif|center|frame|Fates of methionine]]
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==Synthesis==
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[[Racemic]] methionine can be synthesized from diethyl sodium phthalimidomalonate, (C<sub>6</sub>H<sub>4</sub>(CO)<sub>2</sub>NC(CO<sub>2</sub>Et)<sub>2</sub>), by alkylation with chloroethylmethylsulfide, ClCH<sub>2</sub>CH<sub>2</sub>SCH<sub>3</sub> followed by hydrolysis and decarboxylation.<ref>Barger, G.; Weichselbaum, T. E. “dl-Methionine” Organic Syntheses, Collected Volume 2, p.384 (1943). http://www.orgsyn.org/orgsyn/pdfs/CV2P0384.pdf.</ref>
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==Dietary aspects==
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High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meat, and some seeds. Most fruit and vegetables contain very little, although peppers and spinach are the best sources.
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== See also ==
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* [[Allantoin]]
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* [[Formylmethionine]]
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==References==
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<references/>
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==External links==
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* [http://www.hcusupport.com/diet.htm Foods containing methionine]
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* [http://www.compchemwiki.org/index.php?title=Methionine Computational Chemistry Wiki]
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{{ChemicalSources}}
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{{AminoAcids}}
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[[Category:Proteinogenic amino acids]]
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[[Category:Glucogenic amino acids]]
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[[Category:Sulfur amino acids]]
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[[Category:Thioethers]]
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[[Category:Essential amino acids]]
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{{credit|136972990}}
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[[Category:Life sciences]]

Revision as of 15:31, 15 June 2007

Methionine
Systematic name (S)-2-amino-4-(methylsulfanyl)-
butanoic acid
Abbreviations Met
M
Chemical formula C5H11NO2S
Molecular mass 149.21 g mol-1
Melting point 281 °C
Density 1.340 g cm-3
Isoelectric point 5.74
pKa 2.16
9.08
CAS number [63-68-3]
PubChem 876
EINECS number 200-562-9
SMILES CSCC[C@H](N)C(O)=O
Chemical structure of methionine Chemical structure of methionine Chemical structure of methionine
Disclaimer and references

Methionine is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH2SCH3. This essential 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 plays a role in the biosynthesis of cysteine, carnitine, and taurine (by the transsulfuration pathway), lecithin production, the synthesis of 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 to begin 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.[1]

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.

  • First, 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 sythase.
  • Alternatively, 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) 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).
Fates of methionine

Synthesis

Racemic methionine can be synthesized from diethyl sodium phthalimidomalonate, (C6H4(CO)2NC(CO2Et)2), by alkylation with chloroethylmethylsulfide, ClCH2CH2SCH3 followed by hydrolysis and decarboxylation.[2]

Dietary aspects

High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meat, and some seeds. Most fruit and vegetables contain very little, although peppers and spinach are the best sources.

See also

  • Allantoin
  • Formylmethionine

References
ISBN links support NWE through referral fees

  1. ↑ Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.
  2. ↑ Barger, G.; Weichselbaum, T. E. “dl-Methionine” Organic Syntheses, Collected Volume 2, p.384 (1943). http://www.orgsyn.org/orgsyn/pdfs/CV2P0384.pdf.

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 AcidsAnalogues 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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