The C1 methyl building unit is supplied from
l-methionine and is introduced by a nucleophilic substitution
reaction. In nature, the leaving group is enhanced
by converting l-methionine into S -adenosylmethionine
(SAM, AdoMet) [Figure 2.4(a)]. This gives a positively
charged sulfur and facilitates the SN2-type nucleophilic
substitution mechanism [Figure 2.4(b)].