Molecular Basis of the Unusual Seven-Membered Methylenedioxy Bridge Formation Catalyzed by Fe(II)/α-KG-Dependent Oxygenase CTB9

Methylenedioxy bridges (MDBs) are architecturally important motifs in natural products and bioactive molecules. Cercosporin, a typical perylenequinone pigment, contains an unusual seven-membered MDB, which has versatile biological and photocatalytic activities. Although cercosporin has been isolated, characterized, and studied for several decades, its biosynthetic pathway, especially the formation of the seven-membered MDB, has remained unclear. Here, we show that the formation of the seven-membered MDB is catalyzed by Fe(II)/alpha-ketoglutaric acid (alpha-KG)-dependent dioxygenase CTB9 in cercosporin biosynthesis. Moreover, crystal structures of CTB9 in complex with an a-KG analogue NOG (CTB9 center dot Cu center dot NOG) and its substrate pre-cercosporin with NOG (CTB9 center dot Cu center dot NOG center dot pre-cercosporin) were determined. These structures, together with site-directed mutagenesis studies and quantum mechanics calculations, help define the mechanism of the unique seven-membered MDB in cercosporin biosynthesis. In summary, these results provide molecular insights into other biosynthetic pathways of natural products containing MDBs.

Automated summary

This study reveals the formation mechanism of the seven-membered methylenedioxy bridge in cercosporin biosynthesis, providing molecular insights into the biosynthetic pathways of natural products containing MDBs.