Abstract
The canonical roles for methionine are as the chain-initiating residue in protein biosynthesis and as a metabolic progenitor to S-adenosylmethionine (SAM). SAM synthetase converts the thioether sulfur in methionine to the trigonalized cationic sulfonium center in SAM. This kinetically stable, thermodynamically activated sulfonium can deliver adjacent aminobutyryl, adenosyl and methyl groups as electrophilic fragments to nucleophilic N, O, S and C atoms in cosubstrates of both low and high molecular weight (e.g. C5-methylation of cytidine residues in DNA). Recently SAM has been found to also function as an ylid reagent (CH2−S+) in some enzymatic transformations via a carboxy-SAM metabolite.
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