Catalyzes the formation of S-adenosylmethionine (AdoMet) from methionine and ATP. The overall synthetic reaction is composed of two sequential steps, AdoMet formation and the subsequent tripolyphosphate hydrolysis which occurs prior to release of AdoMet from the enzyme (PubMed:6251075, PubMed:7629147, PubMed:7629176, PubMed:9753435, PubMed:10551856, PubMed:10660564). Is essential for growth (PubMed:11952912). ATP + H2O + L-methionine = diphosphate + phosphate + S-adenosyl-L-methionine Binds 2 divalent ions per subunit. The ions interact primarily with the substrate. Binds 1 potassium ion per subunit. The potassium ion interacts primarily with the substrate. Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis; S-adenosyl-L-methionine from L-methionine: step 1/1. Homotetramer; dimer of dimers (PubMed:6251075, PubMed:7629147, PubMed:7629176, PubMed:10660564, PubMed:8550549, PubMed:8611562, PubMed:8723769, PubMed:14967023). The active sites are at the interface between subunits; each dimer has two active sites (PubMed:8550549, PubMed:8611562, PubMed:8723769, PubMed:14967023). AdoMet activates the tripolyphosphatase reaction. Cells are resistant to methionine-analogs, such as DL-ethionine(Et). Belongs to the AdoMet synthase family. Was originally thought to differ from MetX, which was assigned as a second AdoMet synthase before being shown to be identical to MetK.