S-adenosylmethionine(SAM)is ubiquitous in living organisms and is of great significance in metabolism as a cofactor of various enzymes.Methyltransferases(Mtases),a major group of SAM-dependent enzymes,catalyze methyl transfer from SAM to C,O,N,and S atoms in small-molecule sec-ondary metabolites and macromolecules,including proteins and nucleic acids.Mtases have long been a hot topic in biomedical research because of their crucial role in epigenetic regulation of macromolecules and biosynthesis of natural products with prolific pharmacological moieties.However,another group of SAM-dependent enzymes,sharing similar core domains with Mtases,can catalyze nonmethylation reac-tions and have multiple functions.Herein,we mainly describe the nonmethylation reactions of SAM-dependent enzymes in biosynthesis.First,we compare the structural and mechanistic similarities and dis-tinctions between SAM-dependent Mtases and the non-methylating SAM-dependent enzymes.Second,we summarize the reactions catalyzed by these enzymes and explore the mechanisms.Finally,we discuss the structural conservation and catalytical diversity of class Ⅰ-like non-methylating SAM-dependent en-zymes and propose a possibility in enzymes evolution,suggesting future perspectives for enzyme-mediated chemistry and biotechnology,which will help the development of new methods for drug syn-thesis.