Two Flavoenzymes Convert Fungal Polyketide-Nonribosomal Peptide to cis -2-Butene-1,4-dial-Type Carcinogen via Olefin Dihydroxylation and C–C Bond Cleavage

Postmodification steps by multiple oxidation enzymes are an effective strategy for converting the polyketide-nonribosomal peptide (PK-NRP) core backbone to structurally distinctive natural products, where decoration steps occurring on conjugated polyenes (olefin ≥3) of the PK-NRP core backbones are relatively scarce. In this study, we identified two flavoenzymes from the frt cluster in Fusarium reticulatum: (1) a FAD-dependent monooxygenase FrtD catalyzes olefin dihydroxylation of a five-conjugated polyene PK-NRP precursor (8) to yield vicinal diols (1a/1b); (2) a secretion-type berberine bridge enzyme-like oxidase FrtB is then responsible for converting 1a to cis-2-methyl-2-butene-1,4-dial (17) and three conjugated polyene α,β-unsaturated ketone (12), the process of which includes dehydrogenation, C-C bond cleavage, and isomerization. Most importantly, 17 was characterized as a new cis-2-butene-1,4-dial-type carcinogen from Fusarium sp. due to its ability to bind amino acids and induce DNA damage. Our work reveals the unusual functions of flavoenzymes and discovers a new mycotoxin metabolite from Fusarium sp., which poses potential health risks that require further attention in the future.