S-Adenosylmethionine synthetase (EC 2.5.1.6), also known as methionine adenosyltransferase (MAT), is an enzyme that creates S-adenosylmethionine (also known as AdoMet, SAM or SAMe) by reacting methionine (a non-polar amino acid) and ATP (the basic currency of energy). AdoMet is a methyl donor for transmethylation. It gives away its methyl group and is also the propylamino donor in polyamine biosynthesis. S-adenosylmethionine synthesis can be considered the rate-limiting step of the methionine cycle.