S-adenosyl methionine SAM -dependent

Histone methylation is a common posttranslational modification fond in histones. Histone methylations have been identified on lysine and arginine residues. In case of lysines S-adenosyl-methionine (SAM) dependent methyl transferases catalyze the transfer of one, two or three methyl groups. Lysine methylation is reversible and lysine specific demethylases have been... 

One of the hydroxyl groups of (S)-norcoclaurine is methylated by a S-adenosyl methionine-(SAM)-dependent O-methyl transferase to yield (S)-coclaurine. This enzyme has been cloned, and the heterologously expressed enzyme exhibited the expected activity (15-17). The resulting intermediate is... 

It is the role of jV5-methyl THF which is key to understanding the involvement of cobalamin in megaloblastic anaemia. The metabolic requirement for N-methyl THF is to maintain a supply of the amino acid methionine, the precursor of S-adenosyl methionine (SAM), which is required for a number of methylation reactions. The transfer of the methyl group from jV5-methyl THF to homocysteine is cobalamin-dependent, so in B12 deficiency states, the production of SAM is reduced. Furthermore, the reaction which brings about the formation of Ns-methyl THF from N5,N10-methylene THF is irreversible and controlled by feedback inhibition by SAM. Thus, if B12 is unavailable, SAM concentration falls and Ah -methyl THF accumulates and THF cannot be re-formed. The accumulation of AT-methyl THF is sometimes referred to as the methyl trap because a functional deficiency of folate is created. 

S-Adenosyl Methionine S-adenosyl methionine (SAM) (Fig. Ip) is the main donor of activated methyl groups to biological compounds, including metabolites and N-, C-, and S-nucleophiles contained in DNA, RNA, and proteins (88). SAM is the cofactor with the second higher cellular concentration, with ATP being the highest. The SAM-dependent enzymes represent about 3% of cellular proteins (89). The catalyzed enzyme reaction is S- adenosyl methionine + substrate => methylated substrate + S - adenosyl... 

The cooperation between the coenzyme and the residues within the active sites dictates the catalytic reaction and the substrate specificity, leading to the well-known versatility of PLP chemistry, unparalleled by any other coenzyme. Recently, the catalytic versatility of the PLP-dependent enzymes has been found to be expanded and modulated by the copresence of other coenzymes, as heme, S-adenosyl methionine (SAM), and cobalamine. 

S-adenosyl—methionine (SAM). At this point, the vinyl group at C-8 may or may not be reduced to an ethyl group, depending on the species of plant and whether it is day or night. Alternatively, this reduction can happen later on at the chlorphyllide stage. 

Methionine synthase (MetH) of E. coli represents the most thoroughly studied B12-dependent methyl transferase and is one of the essential roles of B12 in mammalian metabolism [125,153,154]. It is a modular enzyme containing separate binding domains for homocysteine, N -methyltetrahydrofolate, S-adenosyl-methionine (SAM) and the Bi2-cofactor [125,153-155]. The B12-binding domain in its different oxidation states must interact punctually and specifically with each of the other three domains The Co(I) form with the N -methyltetrahydrofolate binding domain, the Co(II) form with the SAM binding domain, and the CH3 - Co (III) form with the homocysteine binding domain [153,155]. 

According to the above-mentioned hypothesis, the caffeic acid moiety is retransferred to coenzyme A for further modification reactions. Methylation of the caffeoyl moiety in position 3 is achieved by S-adenosyl-L-methionine (SAM)-dependent 0-methyltransferases (OMTs) either acting on the level of the free acid or the coenzyme A thioester. Hydroxylation in position 5 is catalysed by a cytochrome P450 of the CYP84 family which will be described in more detail. Establishment of the sinapoyl substitution pattern by adding another methyl group will be depicted below. 

Martin JL, McMillan FM. SAM (dependent) I AM the S-adenosyl-methionine-dependent methyltransferase fold. Curr. Opin. Struct. Biol. 2002 12 783-793. 

S-adenosyl-L-methionine (SAM)-dependent methyl-ation was briefly discussed under Thiomethylation (see Figure 14). Other functional groups that are methylated by this mechanism include aliphatic and aromatic amines, N-heterocyclics, monophenols, and polyphenols. The most important enzymes involved in these methylation reactions with xenobiotics are catechol O-methyltransferase, histamine N-methylt-ransferase, and indolethylamine N-methyltransferase - each catalyzes the transfer of a methyl group from SAM to phenolic or amine substrates (O- and N-methyltransferases, respectively). Methylation is not a quantitatively important metabolic pathway for xenobiotics, but it is an important pathway in the intermediary metabolism of both N- and O-contain-ing catechol and amine endobiotics. 

The most widely used methyl donor for enzymatic methyl transfer is the cofactor S-adenosyl-L-methionine (SAM). The methyl moiety on the L-methionine is supplied by another known methyl donor, N5-methyl tetrahydrofolate.30 To date, numerous enzymes that perform SAM- dependent methylation reactions have been described in plants, and several reports attempting to sort out their evolutionary relationships have been published.31- 3... 

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