Enzymatic Synthesis of Unnatural Ginsenosides Using a Promiscuous UDP-Glucosyltransferase from Bacillus subtilis

Advances in glycosyltransferase-mediated glycodiversification of small molecules

Currently, numerous glycosides have been synthesized and used in clinical applications, neutraceuticals, cosmetics, and food processing. Structurally, a glycoside is composed of aglycone attaching to one or several sugar moieties so-called glycone. It is found that biochemical or biopharmaceutical properties of glycoside are mainly determined by its sugar part and thereby alternation of this glycone resulting in novel structure and characteristics as well. The use of traditional production methods of glycosides such as direct extraction and purification from plants, animals, or microorganisms is very challenging (laborious, time-consuming, technique, high price, low yield, etc.). Alternatively, the use of enzymatic methods for the biosynthesis of glycosides has become a highly promising tool. Particularly, the diverse structure of glycosides can be obtained using the promiscuous catalytic activity of glycosyltransferases (GT) mined from bioresources (plants, fungi, microorganisms, etc.). In addition, the exploration of GT catalytic promiscuity toward diverse aglycones, and glycones has indeed been interesting and played a key role in the production of novel glycosides. This review described the recent advances in glycosyltransferase-mediated glycodiversification of small molecules (flavonoids, steroids, terpenoids, etc.). Mostly, references were collected from 2014 to 2023.

Hosts engineering and in vitro enzymatic synthesis for the discovery of novel natural products and their derivatives

Novel natural products (NPs) and their derivatives are important sources for drug discovery, which have been broadly applied in the fields of agriculture, livestock, and medicine, making the synthesis of NPs and their derivatives necessarily important. In recent years, biosynthesis technology has received increasing attention due to its high efficiency in the synthesis of high value-added novel products and its advantages of green, environmental protection, and controllability. In this review, the technological advances of biosynthesis strategies in the discovery of novel NPs and their derivatives are outlined, with an emphasis on two areas of host engineering and in vitro enzymatic synthesis. In terms of hosts engineering, multiple microorganisms, including Streptomyces, Aspergillus, and Penicillium, have been used as the biosynthetic gene clusters (BGCs) provider and host strain for the expression of BGCs to discover new compounds over the past years. In addition, the use of in vitro enzymatic synthesis strategy to generate novel compounds such as triterpenoid saponins and flavonoids is also hereby described.

Progress in Identification of UDP-Glycosyltransferases for Ginsenoside Biosynthesis

Ginsenosides, the primary pharmacologically active constituents of the Panax genus, have demonstrated a variety of medicinal properties, including anticardiovascular disease, cytotoxic, antiaging, and antidiabetes effects. However, the low concentration of ginsenosides in plants and the challenges associated with their extraction impede the advancement and application of ginsenosides. Heterologous biosynthesis represents a promising strategy for the targeted production of these natural active compounds. As representative triterpenoids, the biosynthetic pathway of the aglycone skeletons of ginsenosides has been successfully decoded. While the sugar moiety is vital for the structural diversity and pharmacological activity of ginsenosides, the mining of uridine diphosphate-dependent glycosyltransferases (UGTs) involved in ginsenoside biosynthesis has attracted a lot of attention and made great progress in recent years. In this paper, we summarize the identification and functional study of UGTs responsible for ginsenoside synthesis in both plants, such as Panax ginseng and Gynostemma pentaphyllum, and microorganisms including Bacillus subtilis and Saccharomyces cerevisiae. The UGT-related microbial cell factories for large-scale ginsenoside production are also mentioned. Additionally, we delve into strategies for UGT mining, particularly potential rapid screening or identification methods, providing insights and prospects. This review provides insights into the study of other unknown glycosyltransferases as candidate genetic elements for the heterologous biosynthesis of rare ginsenosides.

Evolving the 3-O/6-O regiospecificity of a microbial glycosyltransferase for efficient production of ginsenoside Rh1 and unnatural ginsenoside

Glycosyltransferase is a popular and promising enzyme to produce high-value-added natural products. Rare ginsenoside Rh1 and unnatural ginsenoside 3β-O-Glc-PPT are promising candidates for drugs. Herein, the microbial glycosyltransferase UGTBL1 was able to catalyze the 20(S)-protopanaxatriol (PPT) 3-O/6-O-glycosylation with poor 6-O-regiospecificity. A structure-guided strategy of mutations involving loop engineering, PSPG motif evolution, and access tunnel engineering was proposed to engineer the enzyme UGTBL1. The variant I62R/M320H/P321Y/N170A from protein engineering achieved a great improvement in 6-O regioselectivity which increased from 10.98 % (WT) to 96.26 % and a booming conversion of 95.57 % for ginsenoside Rh1. A single mutant M320W showed an improved 3-O regioselectivity of 84.83 % and an increased conversion of 98.13 % for the 3β-O-glc-PPT product. Molecular docking and molecular dynamics (MD) simulations were performed to elucidate the possible molecular basis of the regiospecificity and catalytic activity. The unprecedented high titer of ginsenoside Rh1 (20.48 g/L) and 3β-O-Glc-PPT (18.04 g/L) was attained with high regioselectivity and yields using fed-batch cascade reactions from UDPG recycle, which was the highest yield reported to date. This work could provide an efficient and cost-effective approach to the valuable ginsenosides.

Advances in steroidal saponins biosynthesis

This work reviews recent advances in the pathways and key enzymes of steroidal saponins biosynthesis and sets the foundation for the biotechnological production of these useful compounds through transformation of microorganisms. Steroidal saponins, due to their specific chemical structures and active effects, have long been important natural products and that are irreplaceable in hormone production and other pharmaceutical industries. This article comprehensively reviewed the previous and current research progress and summarized the biosynthesis pathways and key biosynthetic enzymes of steroidal saponins that have been discovered in plants and microoganisms. On the basis of the general biosynthetic pathway in plants, it was found that the starting components, intermediates and catalysing enzymes were diverse between plants and microorganisms; however, the functions of their related enzymes tended to be similar. The biosynthesis pathways of steroidal saponins in microorganisms and marine organisms have not been revealed as clearly as those in plants and need further investigation. The elucidation of biosynthetic pathways and key enzymes is essential for understanding the synthetic mechanisms of these compounds and provides researchers with important information to further develop and implement the massive production of steroidal saponins by biotechnological approaches and methodologies.

Advance in glycosyltransferases, the important bioparts for production of diversified ginsenosides

Ginsenosides are a series of glycosylated triterpenoids predominantly originated from Panax species with multiple pharmacological activities such as anti-aging, mediatory effect on the immune system and the nervous system. During the biosynthesis of ginsenosides, glycosyltransferases play essential roles by transferring various sugar moieties to the sapogenins in contributing to form structure and bioactivity diversified ginsenosides, which makes them important bioparts for synthetic biology-based production of these valuable ginsenosides. In this review, we summarized the functional elucidated glycosyltransferases responsible for ginsenoside biosynthesis, the advance in the protein engineering of UDP-glycosyltransferases (UGTs) and their application with the aim to provide in-depth understanding on ginsenoside-related UGTs for the production of rare ginsenosides applying synthetic biology-based microbial cell factories in the future.

One-Pot Bi-Enzymatic Cascade Synthesis of Novel Ganoderma Triterpenoid Saponins

Ganoderma lucidum is a medicinal fungus whose numerous triterpenoids are its main bioactive constituents. Although hundreds of Ganoderma triterpenoids have been identified, Ganoderma triterpenoid glycosides, also named triterpenoid saponins, have been rarely found. Ganoderic acid A (GAA), a major Ganoderma triterpenoid, was synthetically cascaded to form GAA-15-O-β-glucopyranoside (GAA-15-G) by glycosyltransferase (BtGT_16345) from Bacillus thuringiensis GA A07 and subsequently biotransformed into a series of GAA glucosides by cyclodextrin glucanotransferase (Toruzyme® 3.0 L) from Thermoanaerobacter sp. The optimal reaction conditions for the second-step biotransformation of GAA-15-G were found to be 20% of maltose; pH 5; 60 °C. A series of GAA glucosides (GAA-G2, GAA-G3, and GAA-G4) could be purified with preparative high-performance liquid chromatography (HPLC) and identified by mass and nucleic magnetic resonance (NMR) spectral analysis. The major product, GAA-15-O-[α-glucopyranosyl-(1→4)-β-glucopyranoside] (GAA-G2), showed over 4554-fold higher aqueous solubility than GAA. The present study demonstrated that multiple Ganoderma triterpenoid saponins could be produced by sequential actions of BtGT_16345 and Toruzyme®, and the synthetic strategy that we proposed might be applied to many other Ganoderma triterpenoids to produce numerous novel Ganoderma triterpenoid saponins in the future.

Endophytes, biotransforming microorganisms, and engineering microbial factories for triterpenoid saponins production

Triterpenoid saponins are structurally diverse secondary metabolites. They are the main active ingredient of many medicinal plants and have a wide range of pharmacological effects. Traditional production of triterpenoid saponins, directly extracted from cultivated plants, cannot meet the rapidly growing demand of pharmaceutical industry. Microorganisms with triterpenoid saponins production ability (especially Agrobacterium genus) and biotransformation ability, such as fungal species in Armillaria and Aspergillus genera and bacterial species in Bacillus and Intestinal microflora, represent a valuable source of active metabolites. With the development of synthetic biology, engineering microorganisms acquired more potential in terms of triterpenoid saponins production. This review focusses on potential mechanisms and the high yield strategies of microorganisms with inherent production or biotransformation ability of triterpenoid saponins. Advances in the engineering of microorganisms, such as Saccharomyces cerevisiae, Yarrowia lipolytica, and Escherichia coli, for the biosynthesis triterpenoid saponins de novo have also been reported. Strategies to increase the yield of triterpenoid saponins in engineering microorganisms are summarized following four aspects, that is, introduction of high efficient gene, optimization of enzyme activity, enhancement of metabolic flux to target compounds, and optimization of fermentation conditions. Furthermore, the challenges and future directions for improving the yield of triterpenoid saponins biosynthesis in engineering microorganisms are discussed.

Biotransformation of celastrol to a novel, well-soluble, low-toxic and anti-oxidative celastrol-29-O-β-glucoside by Bacillus glycosyltransferases

Celastrol is a quinone-methide triterpenoid isolated from the root extracts of Tripterygium wilfordii (Thunder god vine). Although celastrol possesses multiple bioactivities, the potent toxicity and rare solubility in water hinder its clinical application. Biotransformation of celastrol using either whole cells or purified enzymes to form less toxic and more soluble derivatives has been proven difficult due to its potent antibiotic and enzyme-conjugation property. The present study evaluated biotransformation of celastrol by four glycosyltransferases from Bacillus species and found one glycosyltransferase (BsGT110) from Bacillus subtilis with significant activity toward celastrol. The biotransformation metabolite was purified and identified as celastrol-29-O-β-glucoside by mass and nuclear magnetic resonance spectroscopy. Celastrol-29-O-β-glucoside showed over 53-fold higher water solubility than celastrol, while maintained 50% of the free radical scavenging activity of celastrol. When using zebrafish as the in vivo animal model, celastrol-29-O-β-glucoside exhibited 50-fold less toxicity than celastrol. To our knowledge, the present study is not only the first report describing the biotransformation of celastrol, but also the first one detailing a new compound, celastrol-29-O-β-glucoside, that is generated in the biotransformation process. Moreover, celastrol-29-O-β-glucoside may serve as a potential candidate in the future medicine application due to its higher water solubility and lower toxicity.

A Genome-Centric Approach Reveals a Novel Glycosyltransferase from the GA A07 Strain of Bacillus thuringiensis Responsible for Catalyzing 15-O-Glycosylation of Ganoderic Acid A

Strain GA A07 was identified as an intestinal Bacillus bacterium of zebrafish, which has high efficiency to biotransform the triterpenoid, ganoderic acid A (GAA), into GAA-15-O-β-glucoside. To date, only two known enzymes (BsUGT398 and BsUGT489) of Bacillus subtilis ATCC 6633 strain can biotransform GAA. It is thus worthwhile to identify the responsible genes of strain GA A07 by whole genome sequencing. A complete genome of strain GA A07 was successfully assembled. A phylogenomic analysis revealed the species of the GA A07 strain to be Bacillus thuringiensis. Forty glycosyltransferase (GT) family genes were identified from the complete genome, among which three genes (FQZ25_16345, FQZ25_19840, and FQZ25_19010) were closely related to BsUGT398 and BsUGT489. Two of the three candidate genes, FQZ25_16345 and FQZ25_19010, were successfully cloned and expressed in a soluble form in Escherichia coli, and the corresponding proteins, BtGT_16345 and BtGT_19010, were purified for a biotransformation activity assay. An ultra-performance liquid chromatographic analysis further confirmed that only the purified BtGT_16345 had the key biotransformation activity of catalyzing GAA into GAA-15-O-β-glucoside. The suitable conditions for this enzyme activity were pH 7.5, 10 mM of magnesium ions, and 30 °C. In addition, BtGT_16345 showed glycosylation activity toward seven flavonoids (apigenein, quercetein, naringenein, resveratrol, genistein, daidzein, and 8-hydroxydaidzein) and two triterpenoids (GAA and antcin K). A kinetic study showed that the catalytic efficiency (kcat/KM) of BtGT_16345 was not significantly different compared with either BsUGT398 or BsUGT489. In short, this study identified BtGT_16345 from B. thuringiensis GA A07 is the catalytic enzyme responsible for the 15-O-glycosylation of GAA and it was also regioselective toward triterpenoid substrates.

A New Triterpenoid Glucoside from a Novel Acidic Glycosylation of Ganoderic Acid A via Recombinant Glycosyltransferase of Bacillus subtilis

Ganoderic acid A (GAA) is a bioactive triterpenoid isolated from the medicinal fungus Ganoderma lucidum. Our previous study showed that the Bacillus subtilis ATCC (American type culture collection) 6633 strain could biotransform GAA into compound (1), GAA-15-O-β-glucoside, and compound (2). Even though we identified two glycosyltransferases (GT) to catalyze the synthesis of GAA-15-O-β-glucoside, the chemical structure of compound (2) and its corresponding enzyme remain elusive. In the present study, we identified BsGT110, a GT from the same B. subtilis strain, for the biotransformation of GAA into compound (2) through acidic glycosylation. BsGT110 showed an optimal glycosylation activity toward GAA at pH 6 but lost most of its activity at pH 8. Through a scaled-up production, compound (2) was successfully isolated using preparative high-performance liquid chromatography and identified to be a new triterpenoid glucoside (GAA-26-O-β-glucoside) by mass and nuclear magnetic resonance spectroscopy. The results of kinetic experiments showed that the turnover number (k_cat) of BsGT110 toward GAA at pH 6 (k_cat = 11.2 min⁻¹) was 3-fold higher than that at pH 7 (k_cat = 3.8 min⁻¹), indicating that the glycosylation activity of BsGT110 toward GAA was more active at acidic pH 6. In short, we determined that BsGT110 is a unique GT that plays a role in the glycosylation of triterpenoid at the C-26 position under acidic conditions, but loses most of this activity under alkaline ones, suggesting that acidic solutions may enhance the catalytic activity of this and similar types of GTs toward triterpenoids.