S-adenosylmethionine synthetase, also known as methionine (methionine) adenosyltransferase. S-adenosylmethionine (SAM) carries an activated methyl group and is a coenzyme involved in methyl transfer reactions, present in all eukaryotic cells. SAM is synthesized by adenosine triphosphate (ATP) and methionine in cells through the catalysis of methionine adenosyltransferase (MAT), and loses a methyl group when participating in the methyl transfer reaction as a coenzyme to form S-adenosylhomocysteine. Most of the S-adenosylmethionine is generated in the liver. It plays an important role in the methylation process due to its presence of "active methyl groups". S-adenosylmethionine synthetase, also known as methionine (methionine) adenosyltransferase. S-adenosylmethionine (SAM) carries an activated methyl group and is a coenzyme involved in methyl transfer reactions, present in all eukaryotic cells. SAM is synthesized by adenosine triphosphate (ATP) and methionine in cells through the catalysis of methionine adenosyltransferase (MAT), and loses a methyl group when participating in the methyl transfer reaction as a coenzyme to form S-adenosylhomocysteine. Most of the S-adenosylmethionine is generated in the liver. It plays an important role in the methylation process due to its presence of "active methyl groups". The reactions catalyzed by S-adenosylmethionine synthetase are as follows: ATP+L-Met+H2O=photosphate+diphosphate+S-adenosyl-L-Met