Methionine

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 phospholipids. 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 proteins in eukaryotes and archaea during translation, although it is usually removed by post-translational modification. Methionine can also occur at other positions in the protein, but during the creation of amino acid chains, methionine is always created first.

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

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 yields 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 involved in methionine biosynthesis:
 * 1) aspartokinase
 * 2) B-aspartate semialdehyde dehydrogenase
 * 3) Homserine dehydrogenase
 * 4) homoserine acyltransferase
 * 5) cystathionine-γ-synthase
 * 6) cystathionine-β-lyase
 * 7) 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 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).