L-methionine is an essential amino acid required for the synthesis of proteins, L-cysteine, and the methyl group donor S-adenosyl-L-methionine (SAM or AdoMet). In the L-methionine cycle, which occurs in the cytosol, L-methionine is converted to SAM which can be catabolised by various methyltransferases, such as Gnmt, to S-adenosyl-L-homocysteine (SAH). SAH is then hydrolysed to L-homocysteine which is either used to synthesize L-cysteine (via the transsulfuration pathway) or re-methylated to produce L-methionine. SAM is the main source of methyl groups for the methylation of proteins, nucleic acids and other substrates. It is also used to generate S-adenosyl 3-(methylsulfanyl)propylamine, a propylamino group donor required for polyamine biosynthesis. (Adapted from PMID:31460700 and FBrf0262689.)