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. ATP + H2O + L-methionine = diphosphate + phosphate + S-adenosyl-L-methionine Binds 2 divalent ions per subunit. Binds 1 potassium ion per subunit. Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis; S-adenosyl-L-methionine from L-methionine: step 1/1. Homotetramer; dimer of dimers. Belongs to the AdoMet synthase family. Extended N-terminus.