Methylation, acceptors

CH methyl acceptor synthon (from the reagent CHjI)... 

Akashi, T. et al.. New scheme of the biosynthesis of formononetin involving 2,7,4 -trihydroxyiso-flavanone but not daidzein as the methyl acceptor. Biosci. BiotechnoL Biochem., 64, 2276, 2000. 

X Mao, S Shuman. Vaccinia virus mRNA (guanine-7-)methyltransferase mutational effects on cap methylation and AdoHcy-dependent photo-cross-linking of the cap to the methyl acceptor site. Biochemistry 35 6900-6910, 1996. 

SAM) by methionine adenosyltransferase. SAM serves as a methyl donor for a variety of methyl acceptors, including DNA, protein, neurotransmit-ters, and phospholipids. 5-Adenosylhomocysteine (SAH) is produced following methyl donation by SAM, and homocysteine is formed through the liberation of adenosine from SAH by the enzyme SAH hydrolase. Unlike methionine and cysteine, homocysteine is not incorporated into polypeptide chains during protein synthesis. Instead, homocysteine has one of two metabolic fates transsulfuration or remethylation to methionine. 

Another mechanism contributing to the vascular pathogenic effects of homocysteine may be a diminished capacity for methylation reactions. Hyperhomocysteinemia is often accompanied by a decreased intracellular SAM/SAH ratio, which can inhibit the methylation of DNA and other methyl acceptors (Fig. 21-2). DNA hy-pomethylation, with resulting changes in gene expression, has been observed in white blood cells isolated from human subjects with hyperhomocysteinemia (Ingrosso et al., 2003). 

Catalysis appears to proceed simply by positioning an ionized carboxyl group within 3.0 A of a reactive methyl group on SAM. The predominant driving force for transmethylation appears to be proximity rather then the active participation of SAMT active site residues in preparing the methyl acceptor atom. 

Catechol-O-methyltransferase (COMT) is widely distributed throughout the animal and plant kingdom and is primarily associated with the cytosolic fraction of rat liver, skin, kidney, glandular tissue, heart, and brain. In humans, a membrane-bound COMT also has been characterized which is encoded by the same gene as the cytosolic COMT, but with a different transcription start site. Multiple polymorphic forms of COMT may be expressed within a species. COMT catalyzes the transfer of a methyl group from SAM to one of the phenolic groups of a catechol in the presence of Mg2+ ions. With only a few exceptions, all of the methyl acceptor substrates of COMT require the catechol moiety. 

The work of Blaylock and Stadtman has significantly advanced our knowledge of the conversion of methanol to methane in Methanosarcina, A system has been developed in which Bi2s serves as the methyl acceptor in the enzymic activation of methanol. These studies have revealed an unexpected complexity of this methyl transfer reaction the requirements include ferredoxin, a corrinoid protein, an unidentified protein, ATP, Mg, a hydrogen atmosphere, and a heat-stable cofactor for the transfer of the methyl group of methanol to Biog (12). 

HPLC Peak Edman Sequence Predicted Sequence Methyl Acceptor mass (Expected) mass (Observed) mass A ... 

One of the most widely distributed methyltransferases, found in both procaryotic and eucaryotic cells, does not recognize any of the 20 ribosomally synthesized amino acid residues or their enzymatically formed derivatives. Almost all proteins are substrates for this methyltransferase, but the number of methyl groups added is generally very small, on the order of one methyl group per 100 to 1,000,000 polypeptide chains. Both the broad methyl-acceptor specificity and the sub-stoichiometric nature of the methylation reaction can be explained by the observations that this class of enzymes specifically recognizes two degradation products of asparagine and aspartate residues in proteins (Clarke, 1985 Aswad and Johnson, 1987). 

The fungus Neurospora crassa uses a different Hg methylation mechanism. Neurospora methylates Hg by a process involving one or more steps of the methionine biosyntiiesis pathway. This methylation occurs by incorrect transfer of a methyl group to Hg, which had complexed with homocysteine in the cell. Normally, homocysteine (a methyl acceptor) would be methylated to methionine, however, a methylmercury-homocysteine complex is formed. The methyl group is transferred to the Hg atom complexed to homocysteine, then methylmercury is cleaved ... 

Glutathione appears to be the methyl acceptor for this enzyme [155]. 

Glycoproteins of the human erythrocyte membrane, including glycophorin A and band 4.5 have been identified as two major classes of methyl-acceptor polypeptides for the enzyme, protein methylase II (.S -adenosylmethionine protein carboxy 0-methyl transferase, EC 2.1.1.24). ... 

In SAM-dependent alkylation reactions, when the methyl acceptors are carbon atoms, the enzymatic reaction mechanisms are more complicated and depend on the electronic properties of the acceptor molecules. The generation of C-alkyl linkages requires the formation of a nucleophilic carbon. An interesting SAM-dependent... 

C-methylation reaction is the methylation of the C-5 position of cytosine in DNA. In this case, the carbon C-5 of cytosine cannot directly act as a nucleophile. The electron withdrawal by N-3 and the carbonyl, however, makes the C-5—C-6 double bond electron deficient and prone to attack by nucleophiles in a reaction that is similar to a Michael reaction. In DNA methyltransferases (DNMTs), this nucleophile is the thio-late from a Cys residue. The addition product is nucleophilic and reacts with SAM via an Si,j2-like mechanism to capture the methyl group. The resulting intermediate then eliminates the Cys of DNMT to give the methylated cytosine product (Figure 1.9). The methylation of C-5 of cytosine is an example of converting an electron-deficient methyl acceptor to a nucleophile for the methyl-transfer reaction by addition of an active site Cys thiolate. 

For methylations, the vitamer 5-methyltetrahydrofolate transfers its methyl group to homocysteine thus yielding methionine, which can act as a methyl donor via 5 -adenosylmethionine to diverse methyl acceptors such as DNA, phospholipids and various amino acids. However, formyl-tetrahydrofolates are involved in the generation of purines as part of DNA synthesis. 

MnTHFR, methylenetetrahydrofolate reductase MS, methionine synthase MT, methyltransferase R, methyl acceptor SAHH, SAH hydrolase SAMS, SAM synthase SHMT, serine hydroxymethyltransferase THF, tetrahy-drofolate. 

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