In enzymology, an adenosylmethionine cyclotransferase () is an enzyme that catalyzes the chemical reaction S-adenosyl-L-methionine 5'-methylthioadenosine + 2-aminobutan-4-olide Hence, this enzyme has one substrate, S-adenosyl-L-methionine, and two products, 5'-methylthioadenosine and 2-aminobutan-4-olide. This enzyme belongs to the family of transferases, specifically those transferring aryl or alkyl groups other than methyl groups. The systematic name of this enzyme class is S-adenosyl-L-methionine alkyltransferase (cyclizing). This enzyme is also called adenosylmethioninase. References EC 2.5.1 Enzymes of unknown structure