Redox modifications are key complexity-generating steps in the biosynthesis of natural products. The unique structure of rubratoxin A (1), many of which arise through redox modifications, make it a nanomolar inhibitor of protein phosphatase 2A (PP2A). We identified the biosynthetic pathway of 1 and completely mapped the enzymatic sequence of redox reactions starting from the nonadride 5. Six redox enzymes are involved, including four α-ketoglutarate- and iron(II)-dependent dioxygenases that hydroxylate four sp3 carbons; one flavin-dependent dehydrogenase that is involved in formation of the unsaturated lactone; and the ferric-reductase-like enzyme RbtH, which regioselectively reduces one of the maleic anhydride moieties in rubratoxin B to the γ-hydroxybutenolide that is critical for PP2A inhibition. RbtH is proposed to perform sequential single-electron reductions of the maleic anhydride using electrons derived from NADH and transferred through a ferredoxin and ferredoxin reductase pair.
Keywords: biosynthesis; natural products; nonadrides; redox enzymes; rubratoxins.
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