SAM methylation

Now we have seen that the usual reagent for biological methylations is S-adenosylmethionine (SAM) (see Box 6.4). One occasion where SAM is not employed, for fairly obvious reasons, is the regeneration of methionine from homocysteine, after a SAM methylation. For this, A -methyl-FH4 is the methyl donor, with vitamin B12 (see Box 11.4) also playing a role as coenzyme. 

In Section 10.11, we saw that S-adenosylmethionine (SAM) methylates the nitrogen atom of noradrenaline to form adrenaline, a more potent hormone. If SAM methylates an OH group on the benzene ring instead, it completely destroys noradrenaline s activity. Give the mechanism for the methylation of the OH group by SAM. 

The fatty acid 9-methyl-10-hexadecenoic acid (7) is the short-chain analog of 6 which was first identified, in trace amounts, in the marine bacterium Vibrio alginolyticus [21]. Both acids might share a similar biogenesis inasmuch as 7 was postulated to arise from the S-adenosyl methionine (SAM) methylation of (Z)-9-hexadecenoic acid, another acid... 

P. are the intermediates occurring after uroporphyrinogen in the biosynthetic pathway to vitamin B,2. The most important of the as yet discovered P. is P. 2 which can be transformed into other porphinoid natural products such as siroheme and factor F 430. P. 2 is derived from uroporphyrinogen III via P. I, a tetrahydro form of factor I, by methylation with 5-adenosyl-methionine (SAM). Methylation to P. 3A and further reaction steps finally lead to vitamin B,2, with the car-... 

The biosynthesis of marcfortine has been investigated at the Pharmada and Upjohn company [37]. Industrial interest in the biosynthesis of marcfortine A, which does not display the potency that paraherquamide A possesses, is presumably due to the report from the Pharmada and Upjohn group demonstrating that marcfortine A can be semi-synthetically converted into paraherquamide A [38]. Kuo et al. found that marcfortine is derived fi-om L-tryptophan (oxindole moiety), L-methionine (via SAM methylation at the y-N C29), L-lysine (pipecolic acid residue), and acetate (isoprene imits) (Scheme 16) [37]. 

Fig. 3.5 Biosynthesis pathway of saxitoxin as proposed by Shimizu et al. [224]. The reaction steps are (1) Claisen-condensation between acetate and arginine (2) amidino transfer from a second arginine to the a-amino group of intermediate B (3) and (4), cyclization (5), introduction of S-adenosyl methionine (SAM) methyl-derived side chain, involving the loss of one methionine methyl hydride (6) 1,2-H shift (7) epoxidation of methylene side chain (8) opening of epoxide to an aldehyde (9) reduction of the aldehyde to hydroxyl (10) carbamoyl transfer and dihydroxylation of C-12...

In Section 9.9, we saw that 5-adenosylmethionine (SAM) methylates the nitrogen atom of noradrenaline to form adrenaline, a... 

Detoxifica.tlon. Detoxification systems in the human body often involve reactions that utilize sulfur-containing compounds. For example, reactions in which sulfate esters of potentially toxic compounds are formed, rendering these less toxic or nontoxic, are common as are acetylation reactions involving acetyl—SCoA (45). Another important compound is. Vadenosylmethionine [29908-03-0] (SAM), the active form of methionine. SAM acts as a methylating agent, eg, in detoxification reactions such as the methylation of pyridine derivatives, and in the formation of choline (qv), creatine [60-27-5] carnitine [461-06-3] and epinephrine [329-65-7] (50). 

The farnesylation and subsequent processing of the Ras protein. Following farnesylation by the FTase, the carboxy-terminal VLS peptide is removed by a prenyl protein-specific endoprotease (PPSEP) in the ER, and then a prenylprotein-specific methyltransferase (PPSMT) donates a methyl group from S-adenosylmethionine (SAM) to the carboxy-terminal S-farnesylated cysteine. Einally, palmitates are added to cysteine residues near the C-terminus of the protein. 

Uroporphyrinogen I (16c), a constitutional isomer of uroporphyrinogen III, also plays no direct role in porphyrin and corrin biosynthesis, but this unnatural substrate is methylated to give 17c10c f in the presence of SAM by the methyl transferase of some bacteria. A constitutional type I dihydroisobacteriochlorin can be obtained by methylation of uroporphyrinogen I with methylase Ml. Methyltransferase M1 is able to methylate the unnatural precorrin once more to give the trimethylpyrrocorphin type I.IOc 1... 

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... 

S-adenosyl-L-methionine (AdoMet, SAM) is a cofactor and the most important donor of the methyl (CH3-) group for methyltransferases, including COMT. When the methyl-group has been transferred, the remaining demethylated compound is called S-adenosyl-L-homo-cysteine. 

Phenazines — The phenazines are biosynthesized by the shikimic acid pathway, through the intermediate chorismic acid. The process was studied using different strains of Pseudomonas species, the major producers of phenazines. The best-known phenazine, pyocyanine, seems to be produced from the intermediate phenazine-1-carboxylic acid (PCA). Although intensive biochemical studies were done, not all the details and the intermediates of conversion of chorismic acid to PCA are known. In the first step, PCA is N-methylated by a SAM-dependent methyltransferase. The second step is a hydroxylative decarboxylation catalyzed by a flavoprotein monooxygenase dependent on NADH. PCA is also the precursor of phenazine-1-carboxamide and 1-hydroxyphenazine from Pseudomonas species. - - ... 

The possibility that the initial degree of methyl-esterification might be controlled by the properties of the methyltransferase enzymes was examined partial characterisation of these enzymes in suspension-cultured cells of fiax. Pectin methyltransferases beii enzymes characteristic of the Golgi apparatus [22], microsomes were fiactionated daily for ten days from suspension-cultured flax cells and incubated in the presence of C-SAM, the universal donor of methyl groups. 

A)Total activity was measured by monitoring methylation in vitro from SAM. 1 AU 1000 dpm... 

PMT assays were performed as described by Vannier et al. [3] by adding an equal volume of an enzyme preparation to a 0.1 M Tris-HCl buffer containing 3.36 pM of [ C]SAM (1.8 GBq mmol, 740 kBq ml", NEN), 1% (WA ) BSA and 12% sucrose, with or without 0.2% pectic acceptor. The incubation was run at 28°C for 12 h. After precipitation of the reaction product in 70% ethanol, the methylated polymers were selectively extracted with 0.5% ammonium oxalate and radioactivity was measured in a Tricarb 2250 CA Packard scintillation counter. 

Methyl transferases are responsible for methylation of a nucleophile, typically using SAM as the carbon donor. They are known to accept a wide range of nucleophiles such as halides (eq. 1 in Figure 13.22) [64], amines (eq. 2 in Figure 13.22) [65], hydroxyls, and enolates. As expected, the reactivity of methyl transfer to halides follows the order of iodide, bromide, and chloride, with chloride being the poorest acceptor. Methylation of amines in nucleotides and proteins plays important roles in biological activities. 

Despite the higher selectivity of enzymatic methyl transfer over chemical methylation, where toxic or hazardous reagents are often employed, such as methyl sulfonate and diazomethane, the synthetic applications of these enzymes have been largely ignored primarily as a result of high costs associated with the cofactor SAM. Recent efforts have been directed to in vivo methylation, where SAM may be regenerated inside cells. For example, methyl benzoate production was engineered in recombinant Saccharomyces cerevisiae and in vivo... 

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