Genome-Wide Identification of OSC Gene Family and Potential Function in the Synthesis of Ursane- and Oleanane-Type Triterpene in Momordica charantia

Natural products of pentacyclic triterpenoids: from discovery to heterologous biosynthesis

Covering: up to 2022Pentacyclic triterpenoids are important natural bioactive substances that are widely present in plants and fungi. They have significant medicinal efficacy, play an important role in reducing blood glucose and protecting the liver, and have anti-inflammatory, anti-oxidation, anti-fatigue, anti-viral, and anti-cancer activities. Pentacyclic triterpenoids are derived from the isoprenoid biosynthetic pathway, which generates common precursors of triterpenes and steroids, followed by cyclization with oxidosqualene cyclases (OSCs) and decoration via cytochrome P450 monooxygenases (CYP450s) and glycosyltransferases (GTs). Many biosynthetic pathways of triterpenoid saponins have been elucidated by studying their metabolic regulation network through the use of multiomics and identifying their functional genes. Unfortunately, natural resources of pentacyclic triterpenoids are limited due to their low content in plant tissues and the long growth cycle of plants. Based on the understanding of their biosynthetic pathway and transcriptional regulation, plant bioreactors and microbial cell factories are emerging as alternative means for the synthesis of desired triterpenoid saponins. The rapid development of synthetic biology, metabolic engineering, and fermentation technology has broadened channels for the accumulation of pentacyclic triterpenoid saponins. In this review, we summarize the classification, distribution, structural characteristics, and bioactivity of pentacyclic triterpenoids. We further discuss the biosynthetic pathways of pentacyclic triterpenoids and involved transcriptional regulation. Moreover, the recent progress and characteristics of heterologous biosynthesis in plants and microbial cell factories are discussed comparatively. Finally, we propose potential strategies to improve the accumulation of triterpenoid saponins, thereby providing a guide for their future biomanufacturing.

The triterpenoid saponin content difference is associated with the two type oxidosqualene cyclase gene copy numbers of Pulsatilla chinensis and Pulsatilla cernua

Pulsatilla chinensis is an important medicinal herb, its dried radix is used to treat the inflammation since ancient China. Triterpenoid saponins are proved to be the main active compounds of Pulsatilla genus. The triterpenoid saponin contents vary widely in different Pulsatilla species. But no enzyme involved in the triterpenoid saponin biosynthetic pathway was identified in Pulsitilla genus. This seriously limits the explanation of the triterpene content difference of Pulsatilla species. In this article, we obtained two oxidosqualene cyclase (OSC) genes from P. chinensis and P. cernua by touchdown PCR and anchored PCR. These two OSCs converted 2,3-oxidosqualene into different triterpenoids. The OSC from P. cernua is a monofunctional enzyme for β-amyrin synthesis, while the OSC from P. chinensis is a multifunctional enzyme for lupeol and β-amyrin synthesis, and the lupeol is the main product. Then we identified the 260th amino acid residue was the key site for the product difference by gene fusion and site-directed mutant technology. When the 260th amino acid residue was tryptophan (W260) and phenylalanine (F260), the main catalysate was β-amyrin and lupeol, respectively. Then we found that the expression of these two genes was strongly correlated with the lupeol-type and β-amyrin-type triterpenoid contents in P. cernua and P. chinensis. Finally, we found the gene copy number difference of these two genotypes leaded to the triterpenoid diversity in P. cernua and P. chinensis. This study provides useful information for the molecular breeding and quality improvement of P. chinensis and a molecular marker to identify the P. chinensis decoction pieces.

Characterization of oxidosqualene cyclases from Trichosanthes cucumerina L. reveals key amino acids responsible for substrate specificity of isomultiflorenol synthase

Two key amino acids of isomultiflorenol synthase, Y125 and M254, were first proposed. They could be associated with the production of isomultiflorenol. Oxidosqualene cyclases (OSCs) are the first committed enzymes in the triterpenoid biosynthesis by converting 2,3-oxidosqualene to specific triterpenoid backbones. Thus, these enzymes are potential targets for developing plant-active compounds through the study of triterpenoid biosynthesis. We applied transcriptome information and metabolite profiling from Trichosanthes cucumerina L. to define the diversity of triterpenoids in this plant through OSCs. Isomultiflorenol synthase and cucurbitadienol synthase were previously identified in this plant. Here, three new OSCs, TcBAS, TcLAS, and TcCAS, were cloned and functionally characterized as β-amyrin synthase, lanosterol synthase, and cycloartenol synthase activities, respectively. We also took advantage of the multiple sequence alignment and molecular docking of OSCs exhibiting in this plant and other plant OSCs to identify key residues associated with isomultiflorenol synthase specificity. Two novel key amino acids, referred to the Y125 and M254, were first discovered. These results provide information on a possible catalytic mechanism for plant OSCs that produce specific products.