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标题：Dissection of the general two-step di-C-glycosylation pathway for the biosynthesis of (iso)schaftosides in higher plants
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作者：Zi-Long Wang ; Hao-Meng Gao ; Shuang Wang 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：48
页码：30816-30823
DOI：10.1073/pnas.2012745117
出版社：The National Academy of Sciences of the United States of America
摘要：Schaftoside and isoschaftoside are bioactive natural products widely distributed in higher plants including cereal crops and medicinal herbs. Their biosynthesis may be related with plant defense. However, little is known on the glycosylation biosynthetic pathway of these flavonoid di- C -glycosides with different sugar residues. Herein, we report that the biosynthesis of (iso)schaftosides is sequentially catalyzed by two C -glycosyltransferases (CGTs), i.e., CGTa for C -glucosylation of the 2-hydroxyflavanone aglycone and CGTb for C -arabinosylation of the mono- C -glucoside. The two enzymes of the same plant exhibit high homology but remarkably different sugar acceptor and donor selectivities. A total of 14 CGTa and CGTb enzymes were cloned and characterized from seven dicot and monocot plants, including Scutellaria baicalensis , Glycyrrhiza uralensis , Oryza sativa ssp. japonica , and Zea mays , and the in vivo functions for three enzymes were verified by RNA interference and overexpression. Through transcriptome analysis, we found homologous genes in 119 other plants, indicating this pathway is general for the biosynthesis of (iso)schaftosides. Furthermore, we resolved the crystal structures of five CGTs and realized the functional switch of SbCGTb to SbCGTa by structural analysis and mutagenesis of key amino acids. The CGT enzymes discovered in this paper allow efficient synthesis of (iso)schaftosides, and the general glycosylation pathway presents a platform to study the chemical defense mechanisms of higher plants.
关键词：flavonoid C -glycoside ; C -glycosyltransferase ; schaftoside ; crystal structure ; catalytic mechanism