Methionine 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... 

JAMES, F., NOLTE, K.D., HANSON A.D., Purification and properties of S-adenosyl-L-methionine L-methionine S-methyltransferase from Wollastonia biflora leaves, J. Biol. Chem., 1995, 270,22343-22350. 

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. ... 

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