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Methionine
 
 
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Methionine (; abbreviated as Met or M) is an a-amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
 with the chemical formula
Chemical formula

A chemical formula is a way of expressing information about the atoms that constitute a particular chemical compound, and how the relationship between those atoms changes in chemical reactions....
 HO2CCH(NH2)CH2CH2SCH3. This essential amino acid
Essential amino acid

File:BakedFish.jpgAn essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo synthesis by the organism , and therefore must be supplied in the diet....
 is classified as nonpolar. Together with cysteine
Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
, methionine is one of two sulfur
Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
-containing proteinogenic amino acids. Its derivative S-adenosyl methionine
S-Adenosyl methionine

S-Adenosyl methionine is a coenzyme involved in methyl group transfers. SAM was first discovered in Italy by G. L. Cantoni in 1952. It is made from adenosine triphosphate and methionine by methionine adenosyltransferase ....
 (SAM) serves as a methyl donor. Methionine is an intermediate in the biosynthesis of cysteine
Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
, carnitine
Carnitine

Carnitine is a Quaternary ammonium cation biosynthesized from the amino acids lysine and methionine. In living cells, it is required for the transport of fatty acids from the cytosol into the mitochondria during the breakdown of lipids for the generation of metabolic energy....
, taurine
Taurine

Taurine, or 2-aminoethanesulfonic acid, is an organic acid. It is also a major constituent of bile and can be found in the lower intestine and in small amounts in the tissues of many animals and in humans as well....
, lecithin
Lecithin

Lecithin is any of a group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids ....
, phosphatidylcholine
Phosphatidylcholine

Phosphatidylcholines are a class of phospholipids which incorporate choline as a headgroup.They are a major component of biological membranes and can be isolated from either egg yolk or soy beans from which they are mechanically extracted or chemically extracted using hexane....
, and other phospholipid
Phospholipid

File:Phospholipid.svgFile:phospholipid_structure.pngFile:Phosphatidyl-Choline.svgPhospholipids are a class of lipids and are a major component of all cell membranes....
s.






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Methionine (; abbreviated as Met or M) is an a-amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
 with the chemical formula
Chemical formula

A chemical formula is a way of expressing information about the atoms that constitute a particular chemical compound, and how the relationship between those atoms changes in chemical reactions....
 HO2CCH(NH2)CH2CH2SCH3. This essential amino acid
Essential amino acid

File:BakedFish.jpgAn essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo synthesis by the organism , and therefore must be supplied in the diet....
 is classified as nonpolar. Together with cysteine
Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
, methionine is one of two sulfur
Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
-containing proteinogenic amino acids. Its derivative S-adenosyl methionine
S-Adenosyl methionine

S-Adenosyl methionine is a coenzyme involved in methyl group transfers. SAM was first discovered in Italy by G. L. Cantoni in 1952. It is made from adenosine triphosphate and methionine by methionine adenosyltransferase ....
 (SAM) serves as a methyl donor. Methionine is an intermediate in the biosynthesis of cysteine
Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
, carnitine
Carnitine

Carnitine is a Quaternary ammonium cation biosynthesized from the amino acids lysine and methionine. In living cells, it is required for the transport of fatty acids from the cytosol into the mitochondria during the breakdown of lipids for the generation of metabolic energy....
, taurine
Taurine

Taurine, or 2-aminoethanesulfonic acid, is an organic acid. It is also a major constituent of bile and can be found in the lower intestine and in small amounts in the tissues of many animals and in humans as well....
, lecithin
Lecithin

Lecithin is any of a group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids ....
, phosphatidylcholine
Phosphatidylcholine

Phosphatidylcholines are a class of phospholipids which incorporate choline as a headgroup.They are a major component of biological membranes and can be isolated from either egg yolk or soy beans from which they are mechanically extracted or chemically extracted using hexane....
, and other phospholipid
Phospholipid

File:Phospholipid.svgFile:phospholipid_structure.pngFile:Phosphatidyl-Choline.svgPhospholipids are a class of lipids and are a major component of all cell membranes....
s. Improper conversion of methionine can lead to atherosclerosis
Atherosclerosis

Atherosclerosis is a syndrome affecting artery blood vessels. It is a chronic inflammatory response in the walls of arteries, in large part due to the accumulation of macrophage white blood cells and promoted by low density lipoproteins without adequate removal of fats and cholesterol from the macrophages by functional high density lipoprot...
. Methionine is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code
Genetic code

The genetic code is the set of rules by which information encoded in genetic material is Translation into proteins by living cell s. The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids....
 (tryptophan
Tryptophan

Tryptophan is one of the 20 List of standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG....
, encoded by UGG, is the other). The codon AUG is also the "Start" message for a ribosome
Ribosome

Ribosomes are complexes of RNA and protein that are found in all cell s. Ribosomes from bacteria, archaea and eukaryotes, the three domains of life on Earth, have significantly different structure and RNA....
 that signals the initiation of protein translation from mRNA. As a consequence, methionine is incorporated into the N-terminal position of all protein
Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s in eukaryote
Eukaryote

Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
s and archaea
Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . Archaea, like bacteria, are prokaryotic....
 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
Aspartic acid

Aspartic acid is an a-amino acid with the chemical formula HO2CCHCH2CO2H. The carboxylate anion of aspartic acid is known as aspartate....
 and cysteine
Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
. First, aspartic acid is converted via ß-aspartyl-semialdehyde into homoserine, introducing the pair of contiguous methylene groups. Homoserine converts to O-succinyl homoserine
Homoserine

Homoserine is an a-amino acid with the chemical formula HO2CCHCH2CH2OH. L-Homoserine is not one of the common amino acids encoded by DNA....
, which then reacts with cysteine to produce cystathionine
Cystathionine

Cystathionine is an intermediate in the synthesis of cysteine.It is generated from homocysteine and serine by cystathionine beta synthase.It is cleaved into cysteine and a-ketobutyrate by cystathionine gamma-lyase....
, which is cleaved to yield homocysteine
Homocysteine

Homocysteine is an amino acid with the formula HSCH2CH2CHCO2H. It is a homologous series of the amino acid cysteine, differing by an additional methylene group....
. Subsequent methylation of the thiol
Thiol

In organic chemistry, a thiol is a compound that contains the functional group composed of a sulfur atom and a hydrogen atom . Being the sulfur analogue of an alcohol group , this functional group is referred to either as a thiol group or a sulfhydryl group....
 group by folates
Folic acid

Folic acid and Folate are forms of the water-soluble B vitamins. Vitamin B9 is essential to numerous bodily functions ranging from nucleotide synthesis to the remethylation of homocysteine....
 affords methionine. Both cystathionine-?-synthase
Cystathionine-?-synthase

Cystathionine-y-synthase is an enzyme which plays an important role in regulating the biosynthesis of methionine in bacteria and plants....
 and cystathionine-ß-lyase require Pyridoxyl-5'-phosphate
Pyridoxal-phosphate

Pyridoxal-phosphate is a prosthetic group of some enzymes. It is the active form of vitamin B6 which comprises three natural organic compounds, pyridoxal, pyridoxamine and pyridoxine....
 as a cofactor
Cofactor

Cofactor may refer to any of the following:* Cofactor The signed minor of a matrix* Minor as an alternative name for the determinant of a smaller Matrix than that which it describes...
, whereas homocysteine methyltransferase
Homocysteine methyltransferase

Homocysteine methyltransferase is an enzyme which converts homocysteine to methionine.External links...
 requires Vitamin B12
Cyanocobalamin

Cyanocobalamin is an especially common vitamer of the Vitamin B12. It is the most famous vitamer of the family, because it is chemically the most air-stable, and it is the easiest to crystallize and therefore easiest to purify after it is produced by bacterial fermentation....
 as a cofactor.

Enzymes involved in methionine biosynthesis:
  1. aspartokinase
    Aspartokinase

    Aspartokinase is an enzyme that catalysis the phosphorylation of the amino acid aspartate. This reaction is the first step in the biosynthesis of three essential amino acids: methionine, lysine, and Threonine, known as the "aspartate family"....
  2. ß-aspartate semialdehyde dehydrogenase
    Dehydrogenase

    A dehydrogenase is an enzyme that oxidizes a substrate by transferring one or more hydrides to an acceptor, usually Nicotinamide adenine dinucleotide/NADP or a flavin coenzyme such as FAD or FMN....
  3. homoserine dehydrogenase
  4. homoserine acyltransferase
    Acyltransferase

    Acyltransferase is a type of transferase enzyme which acts upon acyl groups.Examples include:* Glyceronephosphate O-acyltransferase* Lecithin-cholesterol acyltransferase...
  5. cystathionine-?-synthase
    Synthase

    In biochemistry, a synthase is an enzyme which catalyzes a synthesis process.Following the EC number classification, they belong to the group of ligases, with lyases catalysing the reverse reaction....
  6. cystathionine-ß-lyase
    Lyase

    In biochemistry, a lyase is an enzyme that catalyzes the breaking of various chemical bonds by means other than hydrolysis and oxidation, often forming a new double bond or a new ring structure....
  7. methionine synthase (in mammals, this step is performed by homocysteine methyltransferase
    Homocysteine methyltransferase

    Homocysteine methyltransferase is an enzyme which converts homocysteine to methionine.External links...
    )


Met Biosynthesis



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
Methionine adenosyltransferase

Methionine adenosyltransferase is an enzyme which catalyses the synthesis of S-adenosylmethionine from methionine and Adenosine triphosphate....
. SAM serves as a methyl-donor in many (2) methyltransferase
Methyltransferase

Methyltransferase is a type of transferase enzyme which transfers a methyl group from a donor to an acceptor.Often methylation occurs on nucleic bases in DNA or amino acids in protein structures....
 reactions and is converted to S-adenosylhomocysteine (SAH). (3) adenosylhomocysteinase
Adenosylhomocysteinase

Adenosylhomocysteinase is an enzyme which converts S-adenosylhomocysteine to homocysteine.External links...
 converts SAH to homocysteine
Homocysteine

Homocysteine is an amino acid with the formula HSCH2CH2CHCO2H. It is a homologous series of the amino acid cysteine, differing by an additional methylene group....
.

There are two fates of homocysteine
Homocysteine

Homocysteine is an amino acid with the formula HSCH2CH2CHCO2H. It is a homologous series of the amino acid cysteine, differing by an additional methylene group....
:
  • 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
    Betaine-homocysteine methyltransferase

    Betaine-homocysteine methyltransferase is a zinc metallo-enzyme that catalyzes the transfer of a methyl group from betaine to homocysteine to produce dimethylglycine and methionine respectively:...
     (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.
  • Homocysteine can be converted to cysteine. (5) Cystathionine-ß-synthase (a PLP-dependent enzyme) combines homocysteine and serine to produce cystathionine
    Cystathionine

    Cystathionine is an intermediate in the synthesis of cysteine.It is generated from homocysteine and serine by cystathionine beta synthase.It is cleaved into cysteine and a-ketobutyrate by cystathionine gamma-lyase....
    . Instead of degrading cystathionine
    Cystathionine

    Cystathionine is an intermediate in the synthesis of cysteine.It is generated from homocysteine and serine by cystathionine beta synthase.It is cleaved into cysteine and a-ketobutyrate by cystathionine gamma-lyase....
     via cystathionine-ß-lyase, as in the biosynthetic pathway, cystathionine is broken down to cysteine
    Cysteine

    Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
     and a-ketobutyrate via (6) cystathionine-?-lyase. (7) a-ketoacid dehydrogenase converts a-ketobutyrate to propionyl-CoA
    Propionyl-CoA

    Propionyl-CoA is a coenzyme A derivative of propionic acid....
    , which is metabolized to succinyl-CoA
    Succinyl-CoA

    Succinyl-Coenzyme A, generally abbreviated as Succinyl-CoA or SucCoA is a combination of succinic acid and coenzyme A....
     in a three-step process (see propionyl-CoA
    Propionyl-CoA

    Propionyl-CoA is a coenzyme A derivative of propionic acid....
     for pathway).


Met Pathway

Synthesis

Racemic
Racemic

In chemistry, a racemic mixture, or racemate, is one that has equal Amount of substance of left- and right-handed enantiomer of a Chirality molecule....
 methionine can be synthesized from diethyl sodium phthalimidomalonate by alkylation with chloroethylmethylsulfide (ClCH2CH2SCH3) followed by hydrolysis and decarboxylation.

Dietary aspects

High levels of methionine can be found in sesame seeds, Brazil nuts, fish, meats, and some other plant seeds. Most fruits and vegetables contain very little of it; however, some have significant amounts, such as spinach, potatoes, and boiled corn. Most legume
Legume

A legume is a plant in the family Fabaceae , or a fruit of these specific plants. A legume fruit is a Fruit#Simple fruit that develops from a simple carpel and usually Dehiscence on two sides....
s, though high in protein, are also low in methionine. DL-methionine is sometimes added as an ingredient to pet food
Pet food

Pet food is typically sold in pet stores and supermarkets. It is usually specific to the type of pet ....
s. Methionine, cysteine, and soy protein heated in a small amount of water creates a meat-like aroma.

See also

  • Allantoin
    Allantoin

    Allantoin is a chemical compound with formula C4H6N4O3. It is also called 5-ureidohydantoin or glyoxyldiureide....
  • Formylmethionine
  • Paradote
    Paracetamol

    Paracetamol or acetaminophen is a widely used over-the-counter drug analgesic and antipyretic . It is commonly used for the relief of fever, headaches, and other minor aches and pains, and is a major ingredient in numerous common cold and Influenza remedies....
     - A Methionine-Paracetamol preparation that might prevent hepatotoxicity.
  • Photo-reactive methionine
    Photo-reactive amino acid analog

    Photo-reactive amino acid analogs for in-vivo crosslinking of protein complexes were introduced in 2005 by researchers from the Max Planck Institute In this method, cells are grown with photoreactive diazirine analogs to leucine and methionine, which are incorporated into proteins....


External links