A Flavin Enzyme Catalyzes Epoxidation and Furofuran Formation in the Biosynthesis of Mycotoxin Erythroskyrines

Fused tetrahydrofuran (THF) ring-containing compounds exhibit diverse biological activities. The mycotoxin erythroskyrine ( 1 ) is a polyenoyltetramic acid that features a distinctive furanofuran skeleton. Here, we discover and elucidate the biosynthesis of the diastereomer of 1, designated erythroskyrine B ( 2 ), through heterologous expression and enzymatic assays. We demonstrate that a single flavin-dependent enzyme EtzB, which catalyzes two consecutive rounds of epoxidation–epoxide opening cascade reactions, affords the furanofuran ring. Functional characterization of EtzB counterparts enabled the discovery of another diastereomer designated as erythroskyrine C ( 3 ). Remarkably, this catalytic mode is mechanistically distinct from previously reported biosynthetic pathways for THF-containing natural products, which typically involve a flavin-dependent epoxidase for epoxidation and a specific epoxide hydrolase for ring opening.