Information AboutMethionine |
| CATEGORIES ABOUT METHIONINE | |
| proteinogenic amino acids | |
| glucogenic amino acids | |
| sulfur amino acids | |
| thioethers | |
| essential amino acids | |
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Methionine is an Essential Nonpolar Amino Acid , and a Lipotropic . Methionine and Cysteine are the only Sulfur -containing proteinogenic amino acids. The methionine derivative S-adenosyl Methionine (SAM) serves as a Methyl donor. Methionine plays a role in cysteine, Carnitine and Taurine synthesis by the Transsulfuration Pathway , Lecithin production, the synthesis of Phosphatidylcholine and other Phospholipid s. Improper conversion of methionine can lead to Atherosclerosis . Methionine is a Chelating Agent . 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. Methionine can also occur at other positions in the protein. Food s containing methionine include Fruit s, Meat , Vegetable s, Nut s and Legume s. High levels of methionine can be found in Spinach , green Pea s, Garlic , some Cheese s, Corn , Brazil Nut s, Pistachio s, Cashew Nut s, Kidney Bean s, Black Turtle Bean s, Tofu , and Tempeh . Most meat is also a rich source of Methionine including Chicken , Beef and Fish BIOSYNTHESIS Since methionine is an essential amino acid, it cannot be synthesized in humans. However, in plants and microorganisms, methionine is synthesized from Aspartic Acid and Cysteine . First, aspartic acid is converted to β-aspartyl-semialdehyde , an important intermediate in the biosynthesis of methionine, Lysine , and, Threonine . Of homoserine by Homoserine Acyltransferase , puts a good Leaving Group on homoserine allowing it to react with cysteine to produce Cystathionine . Enzymatic cleavage of cystathionine yilds homocysteine, which can then be methylated by Folates to give methionine. Both Cystathionine-γ-synthase and Cystathionine-β-lyase require Pyridoxyl-5'-phosphate as a Cofactor , while Homocysteine Methyltransferase requires Vitamin B12 as a cofactor. Enzymes invovled in methionine biosynthesis: # Aspartokinase # β-aspartate Semialdehyde Dehydrogenase # Homoserine Dehydrogenase # Homoserine Acyltransferase # Cystathionine-γ-synthase # Cystathionine-β-lyase # Methionine Synthase (in mammals, this step is performed by Homocysteine Methyltransferase ) 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 conveted 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 . 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). SEE ALSO EXTERNAL LINKS |
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