New mechanism for a methylating enzyme

A team of researchers in China has found a reaction mechanism unprecedented for the many enzymes that add methyl groups to biomolecules. Most of these so-called methyltransferases depend on S-adenosylmethionine (SAM) as a methyl donor, plucking off the molecule's methyl group and relocating it either through an SN2 mechanism or through a radical mechanism. Many scientists thought this type of SAM-dependent enzyme exclusively produced S-adenosylhomocysteine as a coproduct during the transfer. A team led by Qi Zhang of Fudan University reports that's not always the case. NosN, a class C radical SAM methyltransferase involved in the biosynthesis of a thiopeptide antibiotic, uses a different methyl donor and produces thioadenosine as a coproduct (Angew. Chem. Int. Ed. 2017, DOI: 10.1002/anie.201609948). Knowledge of this mechanism could help researchers reengineer the enzyme to produce other compounds, Zhang says. Although NosN uses two SAM molecules, neither of them serves directly as the methyl donor.