S-adenosylmethionine synthetase (MAT, EC: 2.5.1.6) is the enzyme that catalyzes the formation of S-adenosylmethionine (AdoMet) from methionine and ATP [PMID:1696256]. AdoMet is an important methyl donor for transmethylation and is also the propylamino donor in polyamine biosynthesis.

In bacteria there is a single isoform of AdoMet synthetase (gene metK), there are two in budding yeast (genes SAM1 and SAM2) and in mammals while in plants there is generally a multigene family.

The sequence of AdoMet synthetase is highly conserved throughout isozymes and species. The active sites of both the Escherichia coli and rat liver MAT reside between two subunits, with contributions from side chains of residues from both subunits, resulting in a dimer as the minimal catalytic entity. The side chains that contribute to the ligand binding sites are conserved between the two proteins. In the structures of complexes with the E. coli enzyme, the phosphate groups have the same positions in the (PPi plus Pi) complex and the (ADP plus Pi) complex, and are located at the bottom of a deep cavity with the adenosyl group nearer the entrance [PMID:1213535].