Enzymology Of Pyran Ring Formation In Xiamenmycin Biosynthesis

3-Chromanol ring is a common skeleton of many complex natural products that are widely found in plant,bacterial and fungal sources,including compounds of outstanding potential or main ingredient as pharmaceuticals or traditional medicine,such as bavachromanol and anticancer decursin.Xiamenmycins are a series of 3-Chromanol-containing benzopyrans that were isolated from Streptomyces xiamenensis318 in mangrove forest.But the enzymatic formations of many of them are still unclear.In vitro and in vivo assays were employed to clarify the function of each essential enzymes in the biosynthesis of xiamenmycins.Xim B is a prenyltransferase that exhibits wide selectivity both on the prenyl donors and the prenyl acceptors.The downstream modifying enzymes Xim D and XimE are responsible the epoxidation and cyclization of prenylated aromatics to afford the 3-chromanol skeleton.In which,Xim D is a FADdependent monooxygenase and XimE belongs to the Snoa L-like family.The crystal structures of apo-XimE and XimE-xiamenmycin B complex were determined to illustrate how the extremely unstable epoxide is captured by XimE and to form the pyran ring incorporated in xiamenmycin B.A group of residues Y46-Y90-H102-E136 act as the cooperative roles of general acid and base to deprotonate the phenolic hydroxy and protonate the oxygen atom of epoxide.The enzymatic constraint of XimE leads to the distance between para-OH and carbon atom C2(2.09 ?)is closer than that of between para-OH and C3(2.63 ?)and realize the selectively formation of pyran ring,rather than the spontaneously formed furan ring.Based on the structure of holo-XimE,the substrates scope of Xim D and XimE was succesfully expanded.These two enzymes were proved to be a pair of promising tools for biosynthesis of many other benzoheterocycles,including the pyranocoumarins.The catalytic abilities of Xim B,Xim D and XimE were succesfully exploited to biosynthesize prenylated aromatic and benzopyrans in engineered E.coli.