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

Area of Science:

• Natural Product Biosynthesis
• Enzymology
• Mycology

Background:

• Postmodification of polyketide-nonribosomal peptide (PK-NRP) backbones by oxidation enzymes generates diverse natural products.
• Modification of conjugated polyenes within PK-NRP backbones is a less explored area in natural product biosynthesis.

Purpose of the Study:

• To identify and characterize flavoenzymes involved in the decoration of PK-NRP core backbones.
• To elucidate the enzymatic mechanisms for modifying conjugated polyenes in PK-NRPs.
• To identify novel metabolites and assess their biological activities.

Main Methods:

• Identification of flavoenzymes (FrtD and FrtB) from the frt cluster in Fusarium reticulatum.
• Enzymatic assays to determine the catalytic activities of FrtD (FAD-dependent monooxygenase) and FrtB (berberine bridge enzyme-like oxidase).
• Structural characterization of reaction products, including vicinal diols and cis-2-methyl-2-butene-1,4-dial.

Main Results:

• FrtD catalyzes the dihydroxylation of a five-conjugated polyene PK-NRP precursor, yielding vicinal diols (1a/1b).
• FrtB converts 1a to cis-2-methyl-2-butene-1,4-dial (17) and conjugated polyene α,β-unsaturated ketones (12) through dehydrogenation, C-C bond cleavage, and isomerization.
• cis-2-methyl-2-butene-1,4-dial (17) is identified as a novel carcinogen capable of binding amino acids and inducing DNA damage.

Conclusions:

• The study reveals unusual functions of flavoenzymes in modifying PK-NRPs, particularly on conjugated polyenes.
• A new mycotoxin metabolite, cis-2-methyl-2-butene-1,4-dial, produced by Fusarium sp. has been discovered.
• The identified carcinogen poses potential health risks, necessitating further investigation and attention.