Characterization of a Malabaricane-Type Triterpene Synthase from Astragalus membranaceus and Enzymatic Synthesis of Astramalabaricosides

Expansion of the Stereochemical Space of Triterpenes by Mining Noncanonical Oxidosqualene Cyclases Across the Diversity of Green Plants

Triterpenoids and steroids are structurally complex polycyclic natural products with potent biological functions, for example, as hormones. In all eukaryotes, the carbon skeletons of these compounds are generated by oxidosqualene cyclases, which carry out a polycyclization cascade to generate four or five rings with up to nine stereogenic centers in a targeted manner. The tight stereochemical control of this cascade reaction severely limits the stereochemical space accessible by known oxidosqualene cyclases. Considering that naturally occurring hormone stereoisomers have markedly different biological activities, finding ways to produce stereoisomers of triterpenes would be highly desirable to open new avenues for developing triterpenoid and steroid drugs. Here, we present a plant kingdom-wide sequence mining approach based on sequence similarity networks to search for noncanonical oxidosqualene cyclases that might produce triterpene stereoisomers. From 1,891 oxidosqualene cyclase sequences representing the diversity of green plants, six candidates were selected for functional evaluation by heterologous production in Nicotiana benthamiana. Of these six candidates, three produced rare or previously inaccessible triterpene stereoisomers, namely, (3S,13S)-malabarica-17,21-diene-3β,14-diol, 19-epi-lupeol, and a previously unknown hopanoid stereoisomer that we call protostahopenol. Site-directed mutagenesis revealed key residues important for catalytic activity. The sequence similarity network mining strategy employed here will facilitate the targeted discovery of enzymes with unusual activity in higher organisms, which are not amenable to common genome mining approaches. More importantly, our work expands the accessible stereochemical space of triterpenes and represents the first step to the development of new triterpenoid-derived drugs.

Progress on metabolites of Astragalus medicinal plants and a new factor affecting their formation: Biotransformation of endophytic fungi

It is generally believed that the main influencing factors of plant metabolism are genetic and environmental factors. However, the transformation and catalysis of metabolic intermediates by endophytic fungi have become a new factor and resource attracting attention in recent years. There are over 2000 precious plant species in the Astragalus genus. In the past decade, at least 303 high-value metabolites have been isolated from the Astragalus medicinal plants, including 124 saponins, 150 flavonoids, two alkaloids, six sterols, and over 20 other types of compounds. These medicinal plants contain abundant endophytic fungi with unique functions, and nearly 600 endophytic fungi with known identity have been detected, but only about 35 strains belonging to 13 genera have been isolated. Among them, at least four strains affiliated to Penicillium roseopurpureum, Alternaria eureka, Neosartorya hiratsukae, and Camarosporium laburnicola have demonstrated the ability to biotransform four saponin compounds from the Astragalus genus, resulting in the production of 66 new compounds, which have significantly enhanced our understanding of the formation of metabolites in plants of the Astragalus genus. They provide a scientific basis for improving the cultivation quality of Astragalus plants through the modification of dominant fungal endophytes or reshaping the endophytic fungal community. Additionally, they open up new avenues for the discovery of specialized, green, efficient, and sustainable biotransformation pathways for complex pharmaceutical intermediates.

Cutting-edge plant natural product pathway elucidation

Plant natural products (PNPs) play important roles in plant physiology and have been applied across diverse fields of human society. Understanding their biosynthetic pathways informs plant evolution and meanwhile enables sustainable production through metabolic engineering. However, the discovery of PNP biosynthetic pathways remains challenging due to the diversity of enzymes involved and limitations in traditional gene mining approaches. In this review, we will summarize state-of-the-art strategies and recent examples for predicting and characterizing PNP biosynthetic pathways, respectively, with multiomics-guided tools and heterologous host systems and share our perspectives on the systematic pipelines integrating these various bioinformatic and biochemical approaches.

10-Secocycloartane (=9,19-cyclo-9,10-secolanostane) triterpenoid saponins: Huangqiyenins M-X from Astragalus membranaceus (Fisch.) Bge

Phytochemical investigations of the leaves of Astragalus membranaceus (Fisch.) Bge. have led to the isolation of 12 undescribed triterpenoid saponins named huangqiyenins M-X. The structures of the undescribed compounds were determined using NMR and HRESIMS data. The cytotoxicity of these compounds against the RKO and HT-29 colon cancer cell lines was evaluated. Among these compounds, huangqiyenin W exhibited the highest cytotoxic activity against RKO colon cancer cells, whereas huangqiyenin Q and W showed moderate cytotoxic activity against HT-29 colon cancer cells. The network pharmacology results indicated that STAT3, IL-2 and CXCR1 are the correlated targets of huangqiyenin W against colon cancer, with AGE-RAGE and Th17 cell differentiation as the key signaling pathways.

Rhabdastrellosides A and B: Two New Isomalabaricane Glycosides from the Marine Sponge Rhabdastrella globostellata, and Their Cytotoxic and Cytoprotective Effects

Investigation of the Vietnamese marine sponge Rhabdastrella globostellata led to the isolation of two new polar isomalabaricanes: rhabdastrellosides A (1) and B (2). Their structures and stereochemistry were elucidated with the application of 1D and 2D NMR, HRESIMS, and HRESIMS/MS methods, as well as chemical modifications and GC-MS analysis. Metabolites 1 and 2 are the first isomalabaricanes with non-oxidized cyclopentane ring in the tricyclic core system. Moreover, having a 3-O-disaccharide moiety in their structures, they increase a very rare group of isomalabaricane glycosides. We report here a weak cytotoxicity of 1 and 2 toward human neuroblastoma SH-SY5Y cells and normal rat H9c2 cardiomyocytes, as well as the cytoprotective activity of rhabdastrelloside B (2) at 1 µM evaluated using CoCl2-treated SH-SY5Y and H9c2 cells.