Mogrosides, the major bioactive components isolated from the fruits of Siraitia grosvenorii, are a family of cucurbitane-type tetracyclic triterpenoid saponins that are used worldwide as high-potency sweeteners and possess a variety of notable pharmacological activities. Mogrosides are synthesized from 2,3-oxidosqualene via a series of reactions catalyzed by cucurbitadienol synthase (CbQ), Cyt P450s (P450s) and UDP glycosyltransferases (UGTs) in vivo. However, the relevant genes have not been characterized to date. In this study, we report successful identification of SgCbQ and UGT74AC1, which were previously predicted via RNA-sequencing (RNA-seq) and digital gene expression (DGE) profile analysis of the fruits of S. grosvenorii. SgCbQ was functionally characterized by expression in the lanosterol synthase-deficient yeast strain GIL77 and was found to accumulate cucurbitadienol as the sole product. UGT74AC1 was heterologously expressed in Escherichia coli as a His-tag protein and it showed specificity for mogrol by transfer of a glucose moiety to the C-3 hydroxyl to form mogroside IE by in vitro enzymatic activity assays. This study reports the identification of CbQ and glycosyltransferase from S. grosvenorii for the first time. The results also suggest that RNA-seq, combined with DGE profile analysis, is a promising approach for discovery of candidate genes involved in biosynthesis of triterpene saponins.
莫格罗苷是从西瑞亚果中分离出的主要
生物活性成分,属于葫芦烷型四环三萜
皂苷,在全世界范围内被用作高效
甜味剂,并具有多种显著的药理活性。莫格罗苷由2,3-氧代甾烷通过一系列反应合成,这些反应由葫芦烷二烯醇合成酶(CbQ)、细胞色素P450酶(P450s)和
UDP糖基转移酶(UGTs)在体内催化。然而,相关
基因迄今尚未得到鉴定。在这项研究中,我们成功鉴定了SgCbQ和UGT74AC1,此前通过RNA测序(RNA-seq)和西瑞亚果的数字
基因表达谱(DGE)分析预测了这两个
基因。SgCbQ的功能特征是通过在缺乏
羊毛甾醇合酶的酵母菌株GIL77中的表达来鉴定,发现其作为唯一产物积累葫芦烷二烯醇。UGT74AC1作为His标签蛋白在大肠杆菌中异源表达,并通过将
葡萄糖部分转移到C-3羟基上形成莫格罗苷IE,在体外酶促活性测定中显示出对莫格罗苷的特异性。这项研究首次鉴定了西瑞亚果中的CbQ和糖基转移酶。研究