S-adenosylmethionine

S-adenosylmethionine is an activated methionine derivative in which an adenosyl group is bonded to the sulfur atom. The positively charged sulfur bears a transferable methyl group; S-adenosylmethionine therefore couples cellular metabolism to transfer of methyl groups onto other molecules, including DNA and proteins.^[ref]

The same starting molecule can also enter a different reaction. 1-aminocyclopropane-1-carboxylate synthase converts S-adenosylmethionine into 1-aminocyclopropane-1-carboxylate and methylthioadenosine, on the route to plant Ethylene. Here the enzyme directs fragmentation and ring formation rather than simple methyl group transfer. Naming a common substrate does not by itself identify which reaction occurred.^[ref]