S-adenosylhomocysteine as a methyl transfer catalyst in biocatalytic methylation reactions

S-adenosylmethionine-dependent methyltransferases form a large family of enzymes that can catalyse regio-, chemo- and stereospecific methylation of complex natural products. These enzymes could be very useful tools for the chemoenzymatic production and diversification of natural or artificial compounds. Despite this potential, in vitro applications of methyltransferases are limited by their requirement for S-adenosylmethionine as a stoichiometric methyl donor. The chemical complexity, instability, high cost and poor atom economy of this reagent prevent preparative in vitro methylation reactions from becoming routine protocols in natural product research and viable options for process development. In this Article we demonstrate that C-, N- and O-specific methyltransferases can be combined with halide methyltransferases to form enzyme cascades that require only catalytic concentrations of S-adenosylmethionine and use methyl iodide as the stoichiometric methyl donor. Regenerating expensive S-adenosylmethionine (SAM) in enzymatic in vitro reactions is challenging—but important for the commercial scope of SAM-dependent enzymes. This work reports a simple two-enzyme cascade for the in vitro regeneration of SAM for the enzymatic methylation of diverse substrates.