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CoFactor: S-adenosylmethionineGeneral information2D representation
Key facts
TagsMolecular functionS-adenosylmethionine (SAM, AdoMet) can act as a methyl-donor [1],[5]. SAM is the „major and most commonly used methyl group donor in all biological systems“ [5]. The coenzyme methylates DNA, RNA, proteins and small molecules [5]. SAM is also involved in radical reactions using the adenosyl-radical [3]. In 7,8-diaminopelargonic acid synthase, SAM uniquely acts as an amino donor. The enzyme also uses PLP as a cofactor and is the second step in biotin biosynthesis [7]. We see SAM not as a cofactor in this reaction. Chemical propertiesSAM contains a reactive sulphur cation, which acts as a methyl-group donor. One product of such a reaction is S-adenosyl-homocysteine (cytotoxic) [1]. SAM is further involved in methionine generation, in which case the cofactor is transformed to a 5’-deoxyadenosyl radical, because it can’t take the electron from the sulphur atom [1]. SAM also catalyses a 1,2-shift of substrate amines [2]. There are only 2 reversible SAM-dependent enzymes known: lysine-2,3-aminomutase and spore photoproduct lyase [3]. In the coenzyme, the methyl group is bound to a charged S atom, which thermodynamically destabilizes the molecule [4]. This results in a high leaving group potential for the methyl-group. The two arms around the S can be found by the protein to optimize conformation and thus catalysis [5]. SAM can be reductively cleaved by an Fe/S cluster [6]. PathwaysSAM occurs in anaerobes, but also in aerobic bacteria, fungi, plants and animals [3]. SAM regulates its own biosynthesis [1]. SAM also takes part in polyamine-biosynthesis[1],[5]. SAM is a co-repressor of methionine biosynthesis [4]. DNA methylation by type I and type III (sometimes also II) restriction enzymes requires SAM [5]. Adenosylmethionine decarboxylase (4.1.1.50) is part of the decay pathway for SAM [KEGG pathways], therefore we consider SAM not to act as a cofactor here. References
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