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

Abstract

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)/α-ketoglutaric acid (α-KG)-dependent dioxygenase CTB9 in cercosporin biosynthesis. Moreover, crystal structures of CTB9 in complex with an α-KG analogue NOG (CTB9·Cu·NOG) and its substrate pre-cercosporin with NOG (CTB9·Cu·NOG·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.