Catalyzes the formation of S-adenosylmethionine from methionine and ATP. The reaction comprises two steps that are both catalyzed by the same enzyme: formation of S-adenosylmethionine (AdoMet) and triphosphate, and subsequent hydrolysis of the triphosphate. ATP + H2O + L-methionine = diphosphate + phosphate + S-adenosyl-L-methionine Binds 2 magnesium ions per subunit. The magnesium 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. Heterotrimer; composed of a catalytic MAT2A homodimer that binds one regulatory MAT2B chain. Heterohexamer; composed of a central, catalytic MAT2A homotetramer flanked on either side by a regulatory MAT2B chain. In mammalian tissues, there are three distinct forms of AdoMet synthases designated as alpha, beta, and gamma. Alpha and beta are expressed only in adult liver, while gamma is widely distributed in extrahepatic tissues. In addition, the gamma form predominantly exists in fetal rat liver and is progressively replaced by the alpha and beta forms during development. In the brain, highly expressed at night in pinealocytes (at protein level). Low expression in the medial habenular nucleus, granular layer of the cerebellum, layer II of the neocortex and the pyramidal cells and granular cells of the hippocampal formation. Exhibits night/day variations with a 5-fold increased expression at night in the pineal gland (at protein level). Up-regulation is due to a large degree to the release of norepinephrine from nerve terminals in the pineal gland and cAMP signaling pathway. Belongs to the AdoMet synthase family.