Insights into the Branched‐Chain Formation of Mycarose: Methylation Catalyzed by an (S)‐Adenosylmethionine‐Dependent Methyltransferase

: A C-methyltransferase involved in methyl-branch formation in sugars has been characterized for the first time. TylC3, an (S)-adenosylmethylthionine(AdoMet)-dependent enzyme, catalyzes the attachment of a methyl branch [Eq. (1)] in the biosynthesis of L-mycarose, an unusual sugar found in tylosin and as its O-3-methyl derivative in erythromycin. The C-3 methylation proceeds with inversion of configuration and does not require the assistance of any cofactors. The turnover rate is 1.4±0.1 min-1 . TDP=thymidine-5'-dihydrogenphosphate.