Adenosine triphosphate, amino group methionine

The most common example of this process in living organisms is the reaction of the amino acid methionine with adenosine triphosphate (ATP Section 5.8) to give S-adenosylmethionine. The reaction is somewhat unusual in that the biological leaving group in this SN2 process is the triphosphate ion rather than the more frequently seen rliphosphate ion (Section 11.6). 

SAM is synthesized by the transfer of an adenosyl group from adenosine triphosphate (ATP) to the sulfur atom of the amino acid methionine. The positive charge on the sulfur atom activates the methyl group of methionine, making SAM a potent methyl carbonium ion (Me ) donor. SAM is considered a ubiquitous methylating agent. 

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