Homocysteine S-methyltransferase

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... 

RANOCHA, P, BOURGIS, F., ZEEMAK, M.J., RHODES, D., GAGE, D.A., HANSON, A.D., Characterization and functional expression of cDNAs encoding methionine-sensitive and -insensitive homocysteine. S -methyltransferases from Arabidopsis, J. Biol. Chem., 2000, 275, 15962-15968. 

In contrast to coenzyme Bi2, where the alkyl moiety serves purely in a catalytic role, the alkyl group of methyl cobamides (MeCba s) is utilized as a reagent by MeCba-dependent enzymes it is only the cobamide portion of the coenzyme that is catalytic. The cobamide-dependent methyl transferases have been reviewed [11,24-27,165], Three cobamide-dependent methyl transferases have been studied in some cases, more than one protein is required. The Bi2 proteins include methionine synthase (officially called 5-methyltetrahydrofolate-L-homocysteine-S-methyltransferase [HCM] EC 2.1.1.13) MeCba-dependent enzyme from Meth-anosarcina barkeri (MT 0 and the corrinoid/Fe-S protein from Clostridium ther-moaceticum. 

Tyramine methyltransferase from barley roots Norbelladine-O-mediyltransferase from amarylis bulbs ImidazoIe-N-methyltransferase from guinea pig brain (2.1.1.8) Hydroxyindole-O-methyltransferase from pineal (2.1.1.4) Homocysteine-S-methyltransferase from yeast (2.1.1.10 ) S-Adenosylmethionine cyclotransfeiase from yeast (2.5.1.4) S-Adenosylmethionine decarboxylase from E. coli... 

Methyltransferases that utilize S-adenosyl-L-methionine as the methyl donor (and thus generating S-adenosyl-L-homocysteine) catalyze (a) A-methylation (e.g., norepinephrine methyltransferase, histamine methyltransferase, glycine methyltransferase, and DNA-(adenine-A ) methyltransferase), (b) O-methylation (e.g., acetylsero-tonin methyltransferase, catechol methyltransferase, and tRNA-(guanosine-0 ) methyltransferase), (c) S-methyl-ation (e.g., thiopurine methyltransferase and methionine S-methyltransferase), (d) C-methylation (eg., DNA-(cy-tosine-5) methyltransferase and indolepyruvate methyltransferase), and even (e) Co(II)-methylation during the course of the reaction catalyzed by methionine syn-thase. ... 

Transfer of a methyl group from S-adenosylmethionine yields S-adenosylhomocysteine, which potently inhibits several methyltransferases this may partially explain the pathology of homocystinuria. Tissue levels of S-adenosylhomocysteine ordinarily are very low, since this metabolite is rapidly cleaved by a specific hydrolase to homocysteine and adenosine (Fig. 40-4 reaction 3). 

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... 

Researchers studying the metalloenzyme hydrogenase would like to design small compounds that mimic this enzyme s ability to reversibly reduce protons to H2 and H2 to 2H+, using an active center that contains iron and nickel. Cobalamins (vitamin and its derivatives) contain an easily activated Co-C bond that has a number of biological functions, one of which is as a methyl transferase, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR). This enzyme converts homocysteine (an amino acid that has one more CH2 group in its alkyl side chain than cysteine see Figure 2.2) to methionine as methylcobalamin is converted to cobalamin. 

A coupled enyzmatic assay makes use of S-adenosylhomocysteine hydrolase (SAHH) which hydrolyzes the methyltransfer product SAH to homocysteine and adenosine. The homocysteine concentration can be determined by conjugation of its free sulfhydryl moiety to a thiol-sensitive fluorophore [61]. This could of course also be used for arginine methyltransferases. 

This enzyme [EC 2.1.1.28], also known as phenylethanol-amine A -methyltransferase, catalyzes the reaction of S-adenosyl-L-methionine with phenylethanolamine to produce 5-adenosyl-L-homocysteine and A -methylphenyl-ethanolamine. The enzyme will act on a number of phe-nylethanolamines and will catalyze the conversion of noradrenalin (or norepinephrine) into adrenalin (or epinephrine). 

Enzymatic O-methylation of flavonoids, which is catalyzed by O-methyltransferases (E.C. 2.1.1.6-) involves the transfer of the methyl group of an activated methyl donor, S -adenosyl-L-methionine, to the hydroxyl group of a flavonoid acceptor with the formation of the corresponding methylether and S -adenosyl-L-homocysteine. The latter product is, in... 

Fig. 2.2.1 Outline of homocysteine metabolism in man. BMT Betaine methyltransferase, cblC cobalamin defect type C, cblD cobalamin defect type D, GNMT def glycine N-methyltransferase deficiency, MAT methionine adenosyl transferase, MeCbl methylcobalamin, Met Synth methionine synthase, MTHFR methylenetetrahydrofolate reductase, SAH Hyd dc/S-adenosylhomocys-...

Clarke S, Banfield (2001) S-adenosylmethionine-dependent methyltransferases. In Carmel R, Jacobsen D (eds) Homocysteine in Health and Disease. Cambridge University Press, Cambridge UK, pp 63-78... 

Since preliminary studies showed that 6-hydroxymellein-O-methyl-transferase activity was appreciably inhibited in the presence of the reaction products, the mode of product inhibition of the enzyme was studied in detail in order to understand the regulatory mechanism of in vivo methyltransfer. It is well known that S-adenosyl-Z.-homocysteine (SAH), which is a common product of many O-methyltransferases that use SAM as methyl donor, is usually a potent inhibitor of such enzymes. In the 6-hydroxymellein-Omethyltransferase catalyzing reaction another product of this enzyme, 6-methoxymellein, has pronounced inhibitory activity, in addition to SAH. Since the specific product of the transferase reaction, 6-methoxymellein, is capable of inhibiting transferase activity [88], this observation suggests that activity of the transferase is specifically regulated in response to increases in cellular concentrations of its reaction products in carrot cells. It has been also found that 6-methoxymellein inhibits transferase activity with respect not only to 6-hydroxymellein but also to SAM, competitively. This competitive inhibition was also found in SAH as a function of the co-substrates of the enzyme [89]. It follows that the reaction catalyzed by 6-hydroxymellein-O-methyltransferase proceeds by a sequential bireactant mechanism in which the entry of the co-substrates to form the enzyme-substrate complexes and the release of the co-products to generate free enzyme take place in random order [Fig. (7)]. This result also implies that 6-methoxymellein and SAH have to associate with the free transferase protein to exhibit their inhibitory activities, and cannot work as the inhibitors after the enzyme forms complexes with the the substrate. If, therefore, 6-hydroxymellein-O-methyltransferase activity is controlled in vivo by its specific product 6-methoxymellein, this compound should... 

Hanessian, S. et al. Design and Synthesis of Mimics of S-Adenosyl-L-Homocysteine as Potential Inhibitors of Erythromycin Methyltransferases. 3.1 2000 [123]... 

Substrates of COMT include xenobiotics catechols, catecholamines, and catechol estrogens. Three functional classes of chemicals are known to inhibit COMT. S-Adenosyl-I-homocysteine (SAH) is a potent inhibitor of COMT as well as the other SAM-dependent methyltransferases. Inhibition results from SAH binding to the SAM binding site on the enzyme. Certain divalent ions such as Ca+2 and trivalent metal ions such as the salts of lanthanides, neodymium, and europium are excellent inhibitors of COMT. A number of catechol-type substrates such as pyrogallol, fla-vonoids, pyrones, pyridenes, hydroxyquiolines, 3-mercaptotyramine, and tropolones are irreversible inhibitors of COMT. 

This functional deficiency of folate is exacerbated by the associated low concentrations of methionine and S-adenosyl methioitine, although most tissues (apart from the central nervous system) also have betaine-homocysteine methyltransferase that may be adequate to maintain tissue pools of methionine. Under normal conditions S-adenosyl methioitine inhibits methylene-tetrahydrofolate reductase and prevents the formation of further methyl-tetrahydrofolate. Relief of this inhibition results in increased reduction of one-carbon substituted tetrahydrofolates to methyl-tetrahydrofolate. 

Shapiro, S.K., Yphantis, D.A., Almenas, A., (1964). Biosynthesis of Methionine in Saccha-romyces cerevisiae. Rartial purification and properties of S-adenosylmethionine homocysteine methyltransferase. J. Biol. Chem., 239, 1551-1556. 

FIGURE 6.2 Phosphatidylcholine (PC) synthesis by the CDP-ethanolamine pathway. Structures of ethanolamine and choline. The nucleotide moiety S-adenosyl methionine (SAM) is required to transfer the methyl group of methionine to phosphatidylethanolamine (PE) to form PC. In the process, SAM is converted to S-adenosyl homocysteine (SAH). The nitrogen atom of PC has four covalent bonds and is called a quaternary amine. It bears a positive charge that is not influenced by changes in the pH of the surroimding fluids. The PE methyltransferase pathway of PC synthesis occurs only in the liver. 

The crystal structure of 5-8-azaadenosyl-L-homocysteine has been examined because this substance inhibits a methyltransferase that depends on S -adenosyl-L-methionine as a cofactor. ... 

Figure 8 Extended folate metabolism, including compartmentation. MTHFR, methylenetetrahydrofolate reductase SHMT, serine hydroxymethyltransferase BHMT, betaine homocysteine methyltransferase, MAT, methionine adenosyltransferase SAH-hydrolase, S-adenosylhomocysteine hydrolase MT, methyltransferase CBS, cystathionine /i-synthase SAM, S-adenosylmethionine SAH, S-aden-osylhomocysteine THF, tetrahydrofolate and 5-MeTHF, 5-methyltetrahydrofolate. (Reproduced from Van der Put etal. (2001) Folate, homocysteine and neural tube defects An overview. Experimental Biology and Medicine 226 243-270.)...

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