SAM-Dependent Enzyme-Catalysed Pericyclic Reactions in Natural Product Biosynthesis

Pericyclic reactions are among the most powerful synthetic transformations to make multiple regioselective and stereoselective carbon-carbon bonds(1). These reactions have been widely applied for the synthesis of biologically active complex natural products containing contiguous stereogenic carbon centers(2–6). Despite the prominence of pericyclic reactions in total synthesis, only three naturally existing enzymatic examples, intramolecular Diels-Alder (IMDA) reaction(7), Cope(8) and Claisen rearrangements(9), have been characterized. Here, we report the discovery of a S-adenosyl-L-methionine (SAM) dependent enzyme LepI that can catalyse stereoselective dehydration, bifurcating IMDA/hetero-DA (HDA) reactions via an ambimodal transition state, and a [3,3]-sigmatropic retro-Claisen rearrangement leading to the formation of dihydopyran core in the fungal natural product leporin(10). Combined in vitro enzymatic characterization and computational studies provide evidence and mechanistic insight about how the O-methyltransferase-like protein LepI regulates the bifurcating biosynthetic reaction pathways (“direct” HDA and “byproduct recycle” IMDA/retro-Claisen reaction pathways) by utilizing SAM as the cofactor in order to converge to the desired biosynthetic end product. This work highlights that LepI is the first example of an enzyme catalysing a (SAM-dependent) retro-Claisen rearrangement. We suggest that more pericyclic biosynthetic enzymatic transformations are yet to be discovered in the intriguing enzyme toolboxes in Nature(11), and propose an ever expanding role of the versatile cofactor SAM in enzyme catalysis.