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.