Transcriptome analysis and functional characterization of oxidosqualene cyclases of the arjuna triterpene saponin pathway

UGT73FB1 contributes to scaffold-selective biosynthesis of triterpenoid glucosyl esters in saponin-rich bark of arjuna tree

Plants make structurally diverse triterpenoids for their physiological needs, which have shown numerous therapeutic applications. Arjuna tree (Terminalia arjuna) produces bioactive oleanane (β-amyrin-derived) triterpenoids arjunic acid, arjungenin, and arjunolic acid, and the respective C28-O-glucopyranosyl esters arjunetin, arjunglucoside I, and arjunglucoside II. Arjunic acid and arjunetin are the major oleananes in bark, while arjunolic acid and arjunglucoside II are found in minor levels. Although arjungenin was detected at a considerable level, arjunglucoside I was found only at a trace level, suggesting selective biosynthesis and/or accumulation of triterpenoid glucosyl esters in bark. However, the enzyme contributing to triterpenoid C28-O-glucosylation was not characterized. We mined RNA-sequencing data and identified UDP-glucosyltransferase (UGT) transcripts that were enriched in the bark transcriptome. Further, biochemical screening of UGTs identified UGT73FB1, which catalyzed triterpenoid C28-O-glucosylation in a scaffold-selective manner. Recombinant UGT73FB1 produced in Escherichia coli or Nicotiana benthamiana formed arjunic acid and arjunolic acid C28-O-glucopyranosyl esters arjunetin and arjunglucoside II, but not arjungenin C28-O-glucopyranosyl ester (arjunglucoside I). Interestingly, UGT73FB1 showed better activity using oleananes than ursanes (α-amyrin-derived), but it did not show C28-O-glucosylation activity using various lupane triterpenoids (lupeol-derived). Overall, the spatial patterns of UGT73FB1 transcript expression and triterpenoid accumulation and scaffold-selective activity of UGT73FB1 suggested a major role of UGT73FB1 in the biosynthesis of C28-O-glucopyranosyl esters in arjuna. Moreover, UGT73FB1 co-expression with β-amyrin synthase and triterpenoid C2, C23, and C28 hydroxylases/oxidases led to complete reconstruction of the arjunglucoside II pathway in N. benthamiana, suggesting the utility of arjuna enzymes for the biosynthesis of rare triterpenoid glucopyranosyl esters in heterologous hosts.

Recent trends in the elucidation of complex triterpene biosynthetic pathways in horticultural trees

Triterpene (C30 isoprene compounds) represents the most structurally diverse class of natural products and has been extensively exploited in the food, medicine, and industrial sectors. Decades of research on medicinal triterpene biosynthetic pathways have revealed their roles in stress tolerance and shaping microbiota. However, the biological function and mechanism of triterpenes are not fully identified. Even this scientific window narrows down for horticultural trees. The lack of knowledge and a scalable production system limits the discovery of triterpene pathways. Recent synthetic biology research revealed several important biosynthetic pathways that define their roles and address many societal sustainability challenges. Here, I review the chemical diversity and biosynthetic enzymes involved in triterpene biosynthesis of horticultural trees. This review also outlines the integrated Design-Build-Test-Learn (DBTL) pipelines for the discovery, characterization, and optimization of triterpene biosynthetic pathways. Further, these DBTL components share many fundamental and technical difficulties, highlighting opportunities for interdisciplinary collaboration between researchers worldwide. This advancement opens up unprecedented opportunities for the bioengineering of triterpene compounds toward development and scaleup processes.

Unraveling the role of cytochrome P450 enzymes in oleanane triterpenoid biosynthesis in arjuna tree

Triterpenoids (C30-isoprenoids) represent a major group of natural products with various physiological functions in plants. Triterpenoids and their derivatives have medicinal uses owing to diverse bioactivities. Arjuna (Terminalia arjuna) tree bark accumulates highly oxygenated β-amyrin-derived oleanane triterpenoids (e.g., arjunic acid, arjungenin, and arjunolic acid) with cardioprotective roles. However, biosynthetic routes and enzymes remain poorly understood. We mined the arjuna transcriptome and conducted cytochrome P450 monooxygenase (P450) assays using Saccharomyces cerevisiae and Nicotiana benthamiana to identify six P450s and two P450 reductases for oxidative modifications of oleanane triterpenoids. P450 assays using oleananes revealed a greater substrate promiscuity of C-2α and C-23 hydroxylases/oxidases than C-28 oxidases. CYP716A233 and CYP716A432 catalyzed β-amyrin/erythrodiol C-28 oxidation to produce oleanolic acid. C-2α hydroxylases (CYP716C88 and CYP716C89) converted oleanolic acid and hederagenin to maslinic acid and arjunolic acid. CYP716C89 also hydroxylated erythrodiol and oleanolic aldehyde. However, CYP714E107a and CYP714E107b catalyzed oleanolic acid/maslinic acid/arjunic acid, C-23 hydroxylation to form hederagenin, arjunolic acid and arjungenin, and hederagenin C-23 oxidation to produce gypsogenic acid, but at a lower rate than oleanolic acid C-23 hydroxylation. Overall, P450 substrate selectivity suggested that C-28 oxidation is the first P450-catalyzed oxidative modification in the arjuna triterpenoid pathway. However, the pathway might branch thereafter through C-2α/C-23 hydroxylation of oleanolic acid. Taken together, these results provided new insights into substrate range of P450s and unraveled biosynthetic routes of triterpenoids in arjuna. Moreover, complete elucidation and reconstruction of arjunolic acid pathway in S. cerevisiae and N. benthamiana suggested the utility of arjuna P450s in heterologous production of cardioprotective compounds.

Unraveling the terpene synthase family and characterization of BsTPS2 contributing to (S)-( +)-linalool biosynthesis in Boswellia

Boswellia tree bark exudes oleo-gum resin in response to wounding, which is rich in terpene volatiles. But, the molecular and biochemical basis of wound-induced formation of resin volatiles remains poorly understood. Here, we combined RNA-sequencing (RNA-seq) and metabolite analysis to unravel the terpene synthase (TPS) family contributing to wound-induced biosynthesis of resin volatiles in B. serrata, an economically-important Boswellia species. The analysis of large-scale RNA-seq data of bark and leaf samples representing more than 600 million sequencing reads led to the identification of 32 TPSs, which were classified based on phylogenetic relationship into various TPSs families found in angiosperm species such as TPS-a, b, c, e/f, and g. Moreover, RNA-seq analysis of bark samples collected at 0-24 h post-wounding shortlisted 14 BsTPSs that showed wound-induced transcriptional upregulation in bark, suggesting their important role in wound-induced biosynthesis of resin volatiles. Biochemical characterization of a bark preferentially-expressed and wound-inducible TPS (BsTPS2) in vitro and in planta assays revealed its involvement in resin terpene biosynthesis. Bacterially-expressed recombinant BsTPS2 catalyzed the conversion of GPP and FPP into (S)-( +)-linalool and (E)-(-)-nerolidol, respectively, in vitro assays. However, BsTPS2 expression in Nicotiana benthamiana found that BsTPS2 is a plastidial linalool synthase. In contrast, cytosolic expression of BsTPS2 did not form any product. Overall, the present work unraveled a suite of TPSs that potentially contributed to the biosynthesis of resin volatiles in Boswellia and biochemically characterized BsTPS2, which is involved in wound-induced biosynthesis of (S)-( +)-linalool, a monoterpene resin volatile with a known role in plant defense.

Recent advances in triterpenoid pathway elucidation and engineering

Triterpenoids are among the most assorted class of specialized metabolites found in all the taxa of living organisms. Triterpenoids are the leading active ingredients sourced from plant species and are utilized in pharmaceutical and cosmetic industries. The triterpenoid precursor 2,3-oxidosqualene, which is biosynthesized via the mevalonate (MVA) pathway is structurally diversified by the oxidosqualene cyclases (OSCs) and other scaffold-decorating enzymes such as cytochrome P450 monooxygenases (P450s), UDP-glycosyltransferases (UGTs) and acyltransferases (ATs). A majority of the bioactive triterpenoids are harvested from the native hosts using the traditional methods of extraction and occasionally semi-synthesized. These methods of supply are time-consuming and do not often align with sustainability goals. Recent advancements in metabolic engineering and synthetic biology have shown prospects for the green routes of triterpenoid pathway reconstruction in heterologous hosts such as Escherichia coli, Saccharomyces cerevisiae and Nicotiana benthamiana, which appear to be quite promising and might lead to the development of alternative source of triterpenoids. The present review describes the biotechnological strategies used to elucidate complex biosynthetic pathways and to understand their regulation and also discusses how the advances in triterpenoid pathway engineering might aid in the scale-up of triterpenoid production in engineered hosts.

Molecular and Pharmacokinetic Aspects of the Acetylcholinesterase-Inhibitory Potential of the Oleanane-Type Triterpenes and Their Glycosides

The acetylcholinesterase-inhibitory potential of the oleanane-type triterpenes and their glycosides from thebark of Terminalia arjuna (Combreatceae), i.e.,arjunic acid, arjunolic acid, arjungenin, arjunglucoside I, sericic acid and arjunetin, is presented. The studies are based on in silico pharmacokinetic and biomimetic studies, acetylcholinesterase (AChE)-inhibitory activity tests and molecular-docking research. Based on the calculated pharmacokinetic parameters, arjunetin and arjunglucoside I are indicated as able to cross the blood-brain barrier. The compounds of interest exhibit a marked acetylcholinesterase inhibitory potential, which was tested in the TLC bioautography test. The longest time to reach brain equilibrium is observed for both the arjunic and arjunolic acids and the shortest one for arjunetin. All of the compounds exhibit a high and relatively similar magnitude of binding energies, varying from ca. -15 to -13 kcal/mol. The superposition of the most favorable positions of all ligands interacting with AChE is analyzed. The correlation between the experimentally determined IC50 values and the steric parameters of the molecules is investigated. The inhibition of the enzyme by the analyzed compounds shows their potential to be used as cognition-enhancing agents. For the most potent compound (arjunglucoside I; ARG), the kinetics of AChE inhibition were tested. The Michaelis-Menten constant (Km) for the hydrolysis of the acetylthiocholine iodide substrate was calculated to be 0.011 mM.

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 and Functional Characterization of Oxidosqualene Cyclases from Panax vietnamensis

Panax vietnamensis is a valuable medicinal resource with promising preclinical applications. Ginsenosides, which are triterpenoids, are the primary active components in P. vietnamensis. Oxidosqualene cyclases (OSCs) catalyze the formation of the basic skeleton of triterpenes from 2,3-oxidosqualene, which is a crucial step in the biosynthesis of triterpenoids. The OSCs involved in triterpenoid biosynthesis in P. vietnamensis have not yet been characterized. Four OSC genes (PvOSC1-4) were cloned from P. vietnamensis and functionally characterized via heterologous expression in yeast. Transgenic yeast expressing PvOSC1, PvOSC3, and PvOSC4 produced the corresponding products β-amyrin, cycloartenol, and dammarenediol-II, respectively. PvOSC1, PvOSC3, and PvOSC4 are monofunctional OSCs. In this study, we characterized three PvOSC genes, providing a better understanding of the biosynthesis of triterpenoids in P. vietnamensis and the multiple choices of plant OSCs for metabolic engineering in yeast and other hosts.

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.

Identification of Putative Candidate Genes from Galphimia spp. Encoding Enzymes of the Galphimines Triterpenoids Synthesis Pathway with Anxiolytic and Sedative Effects

Galphimia spp. is popularly used in Mexican traditional medicine. Some populations of Galphimia exert anxiolytic and sedative effects due to the presence of the modified triterpenoids galphimines. However, the galphimine synthesis pathway has not yet been elucidated. Hence, in this study, a comparative transcriptome analysis between two contrasting populations of Galphimia spp., a galphimine-producer, and a non-galphimine-producer, is performed using RNA-Seq in the Illumina Next Seq 550 platform to identify putative candidates genes that encode enzymes of this metabolic pathway. Transcriptome functional annotation was performed using the Blast2GO in levels of gene ontology. For differential expression analysis, edgeR, pheatmap, and Genie3 library were used. To validate transcriptome data, qPCR was conducted. In producer and non-producer plants of both populations of Galphimia spp., most of the transcripts were grouped in the Molecular Function level of gene ontology. A total of 680 differentially expressed transcripts between producer and non-producer plants were detected. In galphimine-producer plants, a larger number of highly expressed transcripts related to acyclic and polycyclic terpene synthesis were identified. As putative candidate genes involved in the galphimine synthesis pathway, P450 family members and enzymes with kinase activity were identified.

BAHD acetyltransferase contributes to wound-induced biosynthesis of oleo-gum resin triterpenes in Boswellia

Triterpenes (30-carbon isoprene compounds) represent a large and highly diverse class of natural products that play various physiological functions in plants. The triterpene biosynthetic enzymes, particularly those catalyzing the late-stage regio-selective modifications are not well characterized. The bark of select Boswellia trees, e.g., B. serrata exudes specialized oleo-gum resin in response to wounding, which is enriched with boswellic acids (BAs), a unique class of C3α-epimeric pentacyclic triterpenes with medicinal properties. The bark possesses a network of resin secretory structures comprised of vertical and horizontal resin canals, and amount of BAs in bark increases considerably in response to wounding. To investigate BA biosynthetic enzymes, we conducted tissue-specific transcriptome profiling and identified a wound-responsive BAHD acetyltransferase (BsAT1) of B. serrata catalyzing the late-stage C3α-O-acetylation reactions in the BA biosynthetic pathway. BsAT1 catalyzed C3α-O-acetylation of αBA, βBA, and 11-keto-βBA in vitro and in planta assays to produce all the major C3α-O-acetyl-BAs (3-acetyl-αBA, 3-acetyl-βBA, and 3-acetyl-11-keto-βBA) found in B. serrata bark and oleo-gum resin. BsAT1 showed strict specificity for BA scaffold, whereas it did not acetylate the more common C3β-epimeric pentacyclic triterpenes. The analysis of steady-state kinetics using various BAs revealed distinct substrate affinity and catalytic efficiency. BsAT1 transcript expression coincides with increased levels of C3α-O-acetyl-BAs in bark in response to wounding, suggesting a role of BsAT1 in wound-induced biosynthesis of C3α-O-acetyl-BAs. Overall, the results provide new insights into the biosynthesis of principal chemical constituents of Boswellia oleo-gum resin.

UGT86C11 is a novel plant UDP-glycosyltransferase involved in labdane diterpene biosynthesis

Glycosyltransferases constitute a large family of enzymes across all domains of life, but knowledge of their biochemical function remains largely incomplete, particularly in the context of plant specialized metabolism. The labdane diterpenes represent a large class of phytochemicals with many pharmacological benefits, such as anti-inflammatory, hepatoprotective, and anticarcinogenic. The medicinal plant kalmegh (Andrographis paniculata) produces bioactive labdane diterpenes; notably, the C19-hydroxyl diterpene (andrograpanin) is predominantly found as C19-O-glucoside (neoandrographolide), whereas diterpenes having additional hydroxylation(s) at C3 (14-deoxy-11,12-didehydroandrographolide) or C3 and C14 (andrographolide) are primarily detected as aglycones, signifying scaffold-selective C19-O-glucosylation of diterpenes in planta. Here, we analyzed UDP-glycosyltransferase (UGT) activity and diterpene levels across various developmental stages and tissues and found an apparent correlation of UGT activity with the spatiotemporal accumulation of neoandrographolide, the major diterpene C19-O-glucoside. The biochemical analysis of recombinant UGTs preferentially expressed in neoandrographolide-accumulating tissues identified a previously uncharacterized UGT86 member (ApUGT12/UGT86C11) that catalyzes C19-O-glucosylation of diterpenes with strict scaffold selectivity. ApUGT12 localized to the cytoplasm and catalyzed diterpene C19-O-glucosylation in planta. The substrate selectivity demonstrated by the recombinant ApUGT12 expressed in plant and bacterium hosts was comparable to native UGT activity. Recombinant ApUGT12 showed significantly higher catalytic efficiency using andrograpanin compared with 14-deoxy-11,12-didehydroandrographolide and trivial activity using andrographolide. Moreover, ApUGT12 silencing in plants led to a drastic reduction in neoandrographolide content and increased levels of andrograpanin. These data suggest the involvement of ApUGT12 in scaffold-selective C19-O-glucosylation of labdane diterpenes in plants. This knowledge of UGT86 function might help in developing plant chemotypes and synthesis of pharmacologically relevant diterpenes.

Nutritional Aspects of Ecologically Relevant Phytochemicals in Ruminant Production

This review provides an update of ecologically relevant phytochemicals for ruminant production, focusing on their contribution to advancing nutrition. Phytochemicals embody a broad spectrum of chemical components that influence resource competence and biological advantage in determining plant species' distribution and density in different ecosystems. These natural compounds also often act as plant defensive chemicals against predatorial microbes, insects, and herbivores. They may modulate or exacerbate microbial transactions in the gastrointestinal tract and physiological responses in ruminant microbiomes. To harness their production-enhancing characteristics, phytochemicals have been actively researched as feed additives to manipulate ruminal fermentation and establish other phytochemoprophylactic (prevent animal diseases) and phytochemotherapeutic (treat animal diseases) roles. However, phytochemical-host interactions, the exact mechanism of action, and their effects require more profound elucidation to provide definitive recommendations for ruminant production. The majority of phytochemicals of nutritional and pharmacological interest are typically classified as flavonoids (9%), terpenoids (55%), and alkaloids (36%). Within flavonoids, polyphenolics (e.g., hydrolyzable and condensed tannins) have many benefits to ruminants, including reducing methane (CH4) emission, gastrointestinal nematode parasitism, and ruminal proteolysis. Within terpenoids, saponins and essential oils also mitigate CH4 emission, but triterpenoid saponins have rich biochemical structures with many clinical benefits in humans. The anti-methanogenic property in ruminants is variable because of the simultaneous targeting of several physiological pathways. This may explain saponin-containing forages' relative safety for long-term use and describe associated molecular interactions on all ruminant metabolism phases. Alkaloids are N-containing compounds with vast pharmacological properties currently used to treat humans, but their phytochemical usage as feed additives in ruminants has yet to be exploited as they may act as ghost compounds alongside other phytochemicals of known importance. We discussed strategic recommendations for phytochemicals to support sustainable ruminant production, such as replacements for antibiotics and anthelmintics. Topics that merit further examination are discussed and include the role of fresh forages vis-à-vis processed feeds in confined ruminant operations. Applications and benefits of phytochemicals to humankind are yet to be fully understood or utilized. Scientific explorations have provided promising results, pending thorough vetting before primetime use, such that academic and commercial interests in the technology are fully adopted.