S-adenosylmethionine synthetase is an enzyme that creates S-adenosylmethionine by reacting methionine and ATP. It is also known as methionine adenosyltransferase. The S-adenosylmethionine synthetase enzyme is found in almost every organism bar parasites, most of them are homo-oligomers and the majority are tetramers. The monomers are organised into three domains formed by nonconsecutive stretches of the sequence, and the subunits interact through a large flat hydrophobic surface to form the dimers.