To consider S-adenosylmethionine (SAM, also known as adomet) a cofactor is to recognize its role in a multitude of reactions that transfer methyl groups to substrates. Thus, SAM is involved in an extensive series of methylation reactions that surpass methylations of either N5-methyl-FH4 or methyl cobalamin combined. The methyl group transferred is the terminal carbon of methionine. To activate the methyl group, methionine reacts with ATP, adding an adenosyl group to the sulfur atom and causing a high-energy methyl-donating species to form (Figure 14). Although SAM is perhaps more of a substrate than a cofactor, its inclusion here is to denote the importance of methionine and its reactive
Figure 14 Synthesis of S-adenosylmethionine from methionine. Note the favorable positioning of the methyl group of methionine (dotted circle) as a result of the condensation with ATP.
form, SAM, in a series of extremely important biosynthetic reactions.
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