S-adomet

Most methyltransferase reactions use methionine as the source of methyl groups. The actual methyl donor is S-adenosyl methionine, abbreviated S-AdoMet, or more colloquially, SAM. S-Adenosylmethionine is made from methionine and ATP. Note how the reaction consumes all three high-energy phosphate bonds of the ATP as shown in Figure 5-5. 

A more complex series of reactions synthesizes other nonessential amino acids. For example, the carbon chain of cysteine derives from serine and the sulfur derives from homocysteine (which results after methyl donation from S-AdoMet). 

Catechol O-methyltransferase (COMT) is a widespread enzyme that catalyzes the transfer of the methyl group of S-adenosyl-l-methionine (AdoMet) to one of the phenolic group of the catechol substrate (Fig. 1). High COMT activity is found in the liver, kidney and gut wall... 

Catechol-O-Methyltransferase. Figure. 1 The basic function of COMT. Enzymatic O-methylation of the catechol substrate to 3-methoxy (major route) or 4-methoxy (minor route) products in the presence of Mg2+ and S-adenosyl-methionine (AdoMet). 

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. 

S-(5 -Deoxyadenosin-5 -yl)-L-methionine (AdoMet) S-[(1-Adenin-9-yl)-1,5-dideoxy-p-D-ribofuranos-5-yl]-L-methionine [trivial name S-adenosylmethionine (SAM)]... 

In addition to histone deacetylation, histone lysine methylation can also lead to gene silencing which is not blocked by the HDAC inhibitors [6, 51], Several lines of evidence have suggested a connection between cancer and histone lysine methyltrans-ferases (HKMTs) [52], HKMTs catalyze the transfer of methyl group(s) from the cofactor. S -adenosyI-methionine (AdoMet) to some specific lysine residues in the N-terminal histone tails [53, 54], With one exception of Dotl [55], all known HKMTs contain the SET domain which represents a novel structural fold [53, 56], Among SET-domain HKMTs, SET7/9 is one of the best characterized experimentally. It is a... 

Takusagawa F, Fujioka M, Spies A, Schowen RL (1998) S-adenosylmethionine (adomet)-dependent methyltransferases. In Sinnott M., (ed), Comprehensive biological catalysis a mechanistic reference. Academic Press, San Diego, pp 1-30... 

Methylenetetrahydrofolate reductase (MTHFR) catalyzes the NAD(P)H-dependent reduction of 5,10-methylenetetrahydrofolate (CH2-THF) to 5-methyltetrahydrofolate (CH3-THF). CH3-THF then serves as a methyl donor for the synthesis of methionine. The MTHFR proteins and genes from mammalian liver and E. coli have been characterized,12"15 and MTHFR genes have been identified in S. cerevisiae16 and other organisms. The MTHFR of E. coli (MetF) is a homotetramer of 33-kDa subunits that prefers NADH as reductant,12 whereas mammalian MTHFRs are homodimers of 77-kDa subunits that prefer NADPH and are allosterically inhibited by AdoMet.13,14 Mammalian MTHFRs have a two-domain structure the amino-terminal domain shows 30% sequence identity to E. coli MetF, and is catalytic the carboxyterminal domain has been implicated in AdoMet-mediated inhibition of enzyme activity.13,14... 

SMM synthesis is mediated by the enzyme methionine S-methyltransferase (MMT) through the essentially irreversible, AdoMet-mediated methylation of methionine.48"5 Both MMT and SMM are unique to plants 48,50 The opposite reaction, in which SMM is used to methylate homocysteine to yield two molecules of methionine, is catalyzed by the enzyme homocysteine S-methyltransferase (HMT).48 Unlike MMT, HMTs also occur in bacteria, yeast, and mammals, enabling them to catabolize SMM of plant origin, and providing an alternative to the methionine synthase reaction as a means to methylate homocysteine. Plant MMT and HMT reactions, together with those catalyzed by AdoMet synthetase and AdoHcy hydrolase, constitute the SMM cycle (Fig. 2.4).4... 

The best characterized B 12-dependent methyltransferases is methionine synthase (Figure 15.11) from E. coli, which catalyses the transfer of a methyl group from methyltetrahydrofolate to homocysteine to form methionine and tetrahydrofolate. During the catalytic cycle, B12 cycles between CH3-Co(in) and Co(I). However, from time to time, Co(I) undergoes oxidative inactivation to Co(II), which requires reductive activation. During this process, the methyl donor is S-adenosylmethionine (AdoMet) and the electron donor is flavodoxin (Fid) in E. coli, or methionine synthase reductase (MSR) in humans. Methionine synthase... 

S ETdomain containing H KMTs have been classified according to the presence or absence and the nature of the sequences surrounding the SET domain that are conserved within families [71, 72]. The SET domain has a unique structural fold classified as class V AdoMet-dependent methyltransferase (MTase) [69], characterized by four highly-conserved signature sequences, namely motif I... 

According to Challenger (127), arsenate is transformed to trimethyl-arsine by the mold Scopulariopsis brevicaulis, by sequential reduction and oxidative methylation of the arsenic species (Fig. 7). The proposed intermediates in the pathway were MMA, DMA, and TMAO. Although Challenger could not detect these compounds, when they were added to a culture of S. brevicaulis trimethylarsine was formed. Challenger (129) considered that the likely source of methyl groups was S-adeno-sylmethionine (AdoMet), which had previously been identified as an... 

The kinetic mechanism of the methylation reaction of human COMT has been studied exhaustively using recombinant enzymes [19]. The mechanism is sequential ordered AdoMet binding first, then Mg2+ and the catechol substrate as the last ligand. Human S-COMT and MB-COMT have similar kinetic properties. The main difference is the one-order lower Km value of MB-COMT for dopamine as substrate (S-COMT 207 pMand MB-COMT 15 pAT). The COMT enzyme is a rather slow enzyme with a low catalytic number. At saturating substrate levels S-COMT has a double efficiency compared with MB-COMT (kcat=37 and kcat =17, respectively). At low substrate concentrations (<10 iM) the MB-COMT seems to methylate catecholamines more rapidly than S-COMT. 

AASA a-aminoadipic semialdehyde, AdoHcy S-adenosylhomocysteine, AdoMet S-adenosylmethionine, Ala alanine, Arg arginine, alle alloisoleucine, Apo aminopiperidone, ASA argininosuccinate,... 

The loss of a methyl group from AdoMet in each of the reactions yields S-ad-enosylhomocysteine (AdoHcy) and this is subsequently hydrolysed to adenosine and Hey by AdoHcy-hydrolase. Hey sits at a metabolic branch point and can be remethylated to methionine by way of two reactions. One is the 5-methyltetrahydrofo-late dependent reaction catalysed by methionine synthase, which itself is reductively methylated by cobalamin (vitamin B12) and AdoMet, requiring methionine synthase reductase. 5-Methyltetrahydrofolate is generated from 5,10-methylenetetrahydrofo-late (MTHF) by MTHF reductase. The second remethylation reaction is catalysed by betaine methyltransferase, which is restricted to the liver, kidney and brain, while methionine synthase is widely distributed. 

Fig. 2.2.6 Mechanism of the derivatisation of S-adenosylmethionine (AdoMet), S-adenosylho-mocysteine (AdoHcy) and chloroacetaldehyde...

Accurately weigh 1.18 mg of S-adenosylmethionine hydrogen sulphonate (Boeh-ringer Mannheim, cat. no. 102 407, MW 496.2 g/mol) or 1.32 mg of S-adenosylhomocysteine (Sigma cat. no. A 9384, MW 384.4 g/mol). These are dissolved in demineralised water (usually approximately 5 ml) to give a concentration for AdoMet of 0.342 mmol/1 (after correction for the purity declared for the particular batch) and for AdoHcy of 0.686 mmol/1. These solutions are immediately diluted ten-fold with 0.4 mol/1 perchloric acid and can be stored in aliquots at -20 C for at least 6 months. 

Where Cone, is concentration, X is AdoMet or AdoHcy, s is the sample chromatogram, IS is the internal standard and std. is the standard chromatogram. 

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