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.

Molecular cloning, functional characterization and expression of the β-amyrin synthase gene involved in saikosaponin biosynthesis in Bupleurum chinense DC

Bupleurum chinense DC. is a commonly used plant in traditional Chinese medicine, and saikosaponins(SSs) are the main active oleanane-typetriterpene saponins in B. chinense. β-Amyrin synthase (β-AS) is an important enzyme in oleanane-type triterpenoid saponin synthesis, but its role in saikosaponin synthesis has rarely been studied. Here, the putative β-AS gene BcBAS1(Accession No.ON890382) selected according to metabolomic and transcriptomic analyses was cloned and functionally characterized by heterologous expression in Escherichia coli and Pichia pastoris, and its subcellular localization and expression patterns were examined. The molecular weight of the BcBAS1 recombinant protein was approximately 87 kDa, and this protein could catalyse the production of β-amyrin, the precursor of SSs. Furthermore, BcBAS1 was located in the cytosol, and relative expression in four tissues of the four genotypes was positively correlated with SSa and SSd contents. Our results indicate that BcBAS1 is a β-AS gene and may play an important role in saikosaponin biosynthesis and regulation. This study sheds light on the role of β-AS genes in the synthesis of SSs and provides insights for the metabolic engineering of SSs.

Productive Amyrin Synthases for Efficient α-Amyrin Synthesis in Engineered Saccharomyces cerevisiae

α-Amyrin is a plant-derived pentacyclic triterpenoid, with a lot of important physiological and pharmacological activities. The formation of α-amyrin from (3 S)-2,3-oxidosqualene is catalyzed by α-amyrin synthase (α-AS), a member of the oxidosqualene cyclase (OSC) protein family. However, α-amyrin is not yet commercially developed due to its extremely low productivity in plants. The engineered Saccharomyces cerevisiae with efficient α-amyrin production pathway could be used as an alternative and sustainable solution to produce α-amyrin from renewable raw materials. To efficiently improve α-amyrin production in S. cerevisiae, we identified two α-ASs, EjAS and MdOSC1 from Eriobotrya japonica and Malus × domestica, respectively, through strict bioinformatics screening criteria and phylogenetic analysis. The specific activities of purified EjAS and MdOSC1 were 0.0032 and 0.0293 μmol/min/mg, respectively. EjAS produced α-amyrin and β-amyrin at a ratio of 17:3, MdOSC1 produced α-amyrin, β-amyrin and lupeol at a ratio of 86:13:1, indicating MdOSC1 had significantly higher specific activity and higher ratio of α-amyrin than EjAS. Furthermore, MdOSC1 was introduced into S. cerevisiae combining with the increased supply of (3 S)-2,3-oxidosqualene to achieve the encouraging α-amyrin production, and the titer of α-amyrin achieved 11.97 ± 0.61 mg/L, 5.8 folds of the maximum production reported.

Functional analysis of β-amyrin synthase gene in ginsenoside biosynthesis by RNA interference

KEY MESSAGE

Down-regulation of β-amyrin synthase gene expression by RNA interference led to reduced levels of β-amyrin and oleanane-type ginsenoside as well as up-regulation of dammarane-type ginsenoside level. In the biosynthetic pathway of ginsenosides, β-amyrin synthase catalyzes the reaction from oxidosqualene to β-amyrin, the proposed aglycone of oleanane-type saponins. Here, RNAi was employed to evaluate the role of this gene in ginsenoside biosynthesis of Panax ginseng hairy roots. The results showed that RNAi-mediated down-regulation of this gene led to reduced levels of β-amyrin and oleanane-type ginsenoside Ro as well as increased level of total ginsenosides, indicating an important role of this gene in biosynthesis of ginsenoside. Expression of key genes involved in dammarane-type ginsenoside including genes of dammarenediol synthase and protopanaxadiol and protopanaxatriol synthases were up-regulated in RNAi lines. While expression of squalene synthase genes was not significantly changed, β-amyrin oxidase gene was down-regulated. This work will be helpful for further understanding ginsenoside biosynthesis pathway.

Triterpenoid saponin biosynthetic pathway profiling and candidate gene mining of the Ilex asprella root using RNA-Seq

Ilex asprella, which contains abundant α-amyrin type triterpenoid saponins, is an anti-influenza herbal drug widely used in south China. In this work, we first analysed the transcriptome of the I. asprella root using RNA-Seq, which provided a dataset for functional gene mining. mRNA was isolated from the total RNA of the I. asprella root and reverse-transcribed into cDNA. Then, the cDNA library was sequenced using an Illumina HiSeq™ 2000, which generated 55,028,452 clean reads. De novo assembly of these reads generated 51,865 unigenes, in which 39,269 unigenes were annotated (75.71% yield). According to the structures of the triterpenoid saponins of I. asprella, a putative biosynthetic pathway downstream of 2,3-oxidosqualene was proposed and candidate unigenes in the transcriptome data that were potentially involved in the pathway were screened using homology-based BLAST and phylogenetic analysis. Further amplification and functional analysis of these putative unigenes will provide insight into the biosynthesis of Ilex triterpenoid saponins.