S-Adenosyl   methionine   serve  as  a  donor  of  methyl  sets  in  numerous biological  reactions instance for in the formation  of creatine phosphate and in the synthesis of nucleic acids. It is formed by the action of the activated methyl cycle. In during donation of its methyl group to another compound, S-adenosyl methionine is altered into S-adenosyl homocysteine.  To reproduce S-adenosyl   methionine,   the adenosyl   group   is deleted   from the S- adenosyl  homocysteine  to form  homocysteine.  This is then methylated by the enzyme   homocysteine    methyltransferase,   only   two   vitamin   B12- holding enzymes found in eukaryotes, to form methionine. The concluded methionine is then activated to S-adenosyl methionine with the release of all three of the phosphates from ATP.