The enzymatic biosynthesis of acylated steroidal glycosides and their cytotoxic activity

Recent developments in the enzymatic modifications of steroid scaffolds

Steroids are an important family of bioactive compounds. Steroid drugs are renowned for their multifaceted pharmacological activities and are the second-largest category in the global pharmaceutical market. Recent developments in biocatalysis and biosynthesis have led to the increased use of enzymes to enhance the selectivity, efficiency, and sustainability for diverse modifications of steroids. This review discusses the advancements achieved over the past five years in the enzymatic modifications of steroid scaffolds, focusing on enzymatic hydroxylation, reduction, dehydrogenation, cascade reactions, and other modifications for future research on the synthesis of novel steroid compounds and related drugs, and new therapeutic possibilities.

Research of saccharides and related biocomplexes: A review with recent techniques and applications

Saccharides and biocompounds as saccharide (sugar) complexes have various roles and biological functions in living organisms due to modifications via nucleophilic substitution, polymerization, and complex formation reactions. Mostly, mono-, di-, oligo-, and polysaccharides are stabilized to inactive glycosides, which are formed in metabolic pathways. Natural saccharides are important in food and environmental monitoring. Glycosides with various functionalities are significant in clinical and medical research. Saccharides are often studied with the chromatographic methods of hydrophilic interaction liquid chromatography and anion exchange chromatograpy, but also with capillary electrophoresis and mass spectrometry with their on-line coupling systems. Sample preparation is important in the identification of saccharide compounds. The cases discussed here focus on bioscience, clinical, and food applications.

Regioselective O-acetylation of various glucosides catalyzed by Escherichia coli maltose O-acetyltransferase

Escherichia coli, a well-known prokaryotic organism, has been widely employed as a versatile host for heterologous overexpression of proteins/biocatalysts and the production of pharmaceutically important intermediates/small molecules. However, some E. coli endogenous enzymes showing substrate promiscuity may disturb the heterologous metabolic flux, which will result in the reduction of substrates, intermediates, and target products. Here we reported an unexpected E. coli-catalyzed regioselective O-acetylation of various glucosides. The regioselectively O-acetylated products, 6'-O-acetyl-loganin and 6'-O-acetyl-loganic acid, were obtained and characterized from the enzymatic reaction in which the supernatants of E. coli expressing either CaCYP72A565 and CaCPR, the key enzymes involved in camptothecin biosynthesis, or empty vector were used as catalyst and loganin and loganic acid as independent substrate. An alkaloidal glucoside strictosamide was converted into the regioselectively O-acetylated product 6'-O-acetyl-strictosamide, implying substrate promiscuity of the E. coli-catalyzed O-acetylation reaction. Furthermore, 8 glucosides, including 5 iridoid glucosides and 3 flavonoid glucosides, were successfully converted into the regioselectively O-acetylated products by E. coli, indicating the wide substrate range for the unexpected E. coli-catalyzed O-acetylation. E. coli maltose O-acetyltransferase was demonstrated to be responsible for the mentioned regioselective O-acetylation at the 6-OH of the glucopyranosyl group of multiple classes of natural product glucosides through candidate acetyltransferase-encoding gene analysis, gene knock-out, gene complementation, and the relevant enzymatic reaction activity assays. The present study not only provides an efficient biocatalyst for regioselective O-acetylation but also notifies cautions for metabolic engineering and synthetic biology applications in E. coli. KEY POINTS: • 6-OH of glucosyl of multiple glucosides was regioselectively O-acetylated by E. coli. • Endogenous EcMAT is responsible for the regioselective O-acetylation reaction.

A novel sterol glycosyltransferase catalyses steroidal sapogenin 3-O glucosylation from Paris polyphylla var. yunnanensis

BACKGROUND

Paris polyphylla var. yunnanensis is an important medicinal plant, and the main active ingredient of the plant is polyphyllin, which is a steroid saponin with pharmacological activities. The central enzyme genes participating in the biosynthesis of polyphyllin are increasingly being uncovered; however, UGTs are rarely illustrated.

METHODS AND RESULTS

In this study, we cloned a new sterol glycosyltransferase from Paris polyphylla var. yunnanensis and identified its catalytic function in vitro. PpUGT6 showed the ability to catalyse the C-3 glycosylation of pennogenin sapogenin of polyphyllin, and PpUGT6 showed catalytic promiscuity towards steroids at the C-17 position of testosterone and methyltestosterone and the triterpene at the C-3 position of glycyrrhetinic acid. Homology modelling of the PpUGT6 protein and virtual molecular docking of PpUGT6 with sugar acceptors and donors were performed, and we predicted the key residues interacting with ligands.

CONCLUSIONS

Here, PpUGT6, a novel sterol glycosyltransferase related to the biosynthesis of polyphyllin from P. polyphylla, was characterized. PpUGT6 catalysed C-3 glycosylation to pennogenin sapogenin of polyphyllin, which is the first glycosylation step of the biosynthetic pathway of polyphyllins. Interestingly, PpUGT6 demonstrated glycodiversification to testosterone and methyltestosterone at C-17 and triterpene of glycyrrhetinic acid at the C-3 position. The virtual molecular docking of PpUGT6 protein with ligands predicted the key residues interacting with them. This work characterized a novel SGT glycosylating pennogenin sapogenin at C-3 of polyphyllin from P. polyphylla and provided a reference for further elucidation of the phytosterol glycosyltransferases in catalytic promiscuity and key residues interacting with substrates.

Characterization of Three Paris polyphylla Glycosyltransferases from Different UGT Families for Steroid Functionalization

Plant steroid glycosides, such as phytosterol glycosides, steroidal saponins, and steroidal glycoalkaloids, are natural products with great pharmaceutical values. In this study, we characterized three UDP-glycosyltransferases (UGTs) involved in the glycosylation of steroidal sapogenin from Paris polyphylla. Substrate specificity analysis revealed that UGT73CR1 could glycosylate steroidal sapogenins and steroidal alkaloids, with the highest affinity for diosgenin. The residues His27 and Asp129 of UGT73CR1 are conserved in corresponding positions of plant glycosyltransferases, which are crucial for activating the C-3 OH of the receptor substrates. In comparison, UGT80A33 and UGT80A34 exhibited a higher affinity for cholesterol than other steroids. UGT80s have a larger active pocket, which allows them to accommodate the side chain of sterols. In summary, we assessed three P. polyphylla glycosyltransferases from two UGT families for the functionalization of steroidal molecules, which will provide a basis for the future biomanufacturing of diverse bioactive steroid glycosides.

Glycodiversifying Testosterone with a Promiscuous Glycosyltransferase OsSGT2 from Ornithogalum saundersiae

The diversity expansion of testosterone17-O-β-glycosides (TGs) will increase the probability of screening more active molecules from their acetylated derivatives with anticancer activities. Glycosyltransferases (GTs) responsible for the increased diversity of TGs, however, were seldom documented. Herein, a glycosyltransferase OsSGT2 with testosterone glycodiversification capacity was identified from Ornithogalum saundersiae through transcriptome-wide mining. Specifically, OsSGT2 was demonstrated to be reactive with testosterone and eight donors. OsSGT2 displayed both sugar-aglycon and sugar-sugar GT activities. OsSGT2-catalyzed testosterone glycodiversification could be achieved, generating testosterone monoglycosides and disglycosides with varied percentage conversions. Among the eight donors, the conversion of UDP-Glc was the highest, approaching 90%, while the percentage conversions of UDP-GlcNAc, UDP-Gal, helicin, and UDP-Rha were less than 10%. Protein engineering toward F395 was thus performed to improve the conversion of UDP-GlcNAc. Eight variants displayed increased conversions and the mutant F395C got the highest conversion of 72.11 ± 7.82%, eight times more than that of the wild-type. This study provides a promising alternative for diversity expansion of TGs, also significant insights into the molecular basis for the conversion improvement of sugar donors.

Diacerein as a Promising Acyl Donor in Biosynthetic Acetyl-CoA and Glycosyl Esters Mediated by a Multifunctional Maltose O-Acetyltransferase from Escherichia coli

Acetyl-coenzyme A (acetyl-CoA) is an important donor for acetylation modifications of nutritional supplements. The existing enzymatic methods for acetyl-CoA synthesis suffer from cofactor dependence, donor inaccessibility, and biocatalyst instability, leading to its high cost. Hence, a promising alternative is highly desired. Herein, a maltose O-acetyltransferase (MAT) with cofactor independence had been identified as a stable acetyl-CoA-synthesizing biocatalyst in a screen of the Escherichia coli genome. Under the action of MAT, an anthraquinone medicine containing two acetyl groups, diacerein, was screened as an acetyl donor. Saturation mutagenesis at Glu125 was performed to increase the acetyl-CoA-synthesizing capacity of MAT, while decreasing the accompanying hydrolase activities. A mutant MAT-E125F was thus generated and could convert diacerein and CoA into the highest yield of 3892.70 mg/L acetyl-CoA. Moreover, MAT could synthesize puerarin 6″-O-acetate and other glycosyl esters through acetyl-CoA-dependent acetylation or diacerein-based transesterification reaction. To most of the tested glycosides, the transesterification efficiency was higher than that of acetylation. The mutant MAT-E125V acquired the highest conversion of 94.0% to puerarin 6″-O-acetate through transesterification, while MAT-E125N yielded the highest conversion of 68.5% through acetylation. Taking together, the multifunctional MAT displayed a potent acetyl-CoA- and glycosyl ester-synthesizing capacity using diacerein as an acetyl donor.

A Combined Phytochemistry and Network Pharmacology Approach to Reveal Potential Anti-NSCLC Effective Substances and Mechanisms in Marsdenia tenacissima (Roxb.) Moon (Stem)

Marsdeniae tenacissimae Caulis is a traditional Chinese medicine, named Tongguanteng (TGT), that is often used for the adjuvant treatment of cancer. In our previous study, we reported that an ethyl acetate extract of TGT had inhibitory effects against adenocarcinoma A549 cells growth. To identify the components of TGT with anti-tumor activity and to elucidate their underlying mechanisms of action, we developed a technique for isolating compounds, which was then followed by cytotoxicity screening, network pharmacology analysis, and cellular and molecular experiments. We isolated a total of 19 compounds from a TGT ethyl acetate extract. Two novel steroidal saponins were assessed using an ultra-performance liquid chromatography-photodiode array coupled with quadrupole time-of-flight mass (UPLC-ESI-Q/TOF-MS). Then, we screened these constituents for anti-cancer activity against non-small cell lung cancer (NSCLC) in vitro and obtained six target compounds. Furthermore, a compound-target-pathway network of these six bioactive ingredients was constructed to elucidate the potential pathways that controlled anticancer effects. Approximately 205 putative targets that were associated with TGT, as well as 270 putative targets that were related to NSCLC, were obtained from online databases and target prediction software. Protein-protein interaction networks for drugs as well as disease putative targets were generated, and 18 candidate targets were detected based on topological features. In addition, pathway enrichment analysis was performed to identify related pathways, including PI3K/AKT, VEGF, and EGFR tyrosine kinase inhibitor resistance, which are all related to metabolic processes and intrinsic apoptotic pathways involving reactive oxygen species (ROS). Then, various cellular experiments were conducted to validate drug-target mechanisms that had been predicted using network pharmacology analysis. The experimental results showed the four C21 steroidal saponins could upregulate Bax and downregulate Bcl-2 expression, thereby changing the mitochondrial membrane potential, producing ROS, and releasing cytochrome C, which finally activated caspase-3, caspase-9, and caspase-8, all of which induced apoptosis in A549 cells. In addition, these components also downregulated the expression of MMP-2 and MMP-9 proteins, further weakening their degradation of extracellular matrix components and type IV collagen, and inhibiting the migration and invasion of A549 cells. Our study elucidated the chemical composition and underlying anti-tumor mechanism of TGT, which may be utilized in the treatment of lung cancer.

One Pot Use of Combilipases for Full Modification of Oils and Fats: Multifunctional and Heterogeneous Substrates

Lipases are among the most utilized enzymes in biocatalysis. In many instances, the main reason for their use is their high specificity or selectivity. However, when full modification of a multifunctional and heterogeneous substrate is pursued, enzyme selectivity and specificity become a problem. This is the case of hydrolysis of oils and fats to produce free fatty acids or their alcoholysis to produce biodiesel, which can be considered cascade reactions. In these cases, to the original heterogeneity of the substrate, the presence of intermediate products, such as diglycerides or monoglycerides, can be an additional drawback. Using these heterogeneous substrates, enzyme specificity can promote that some substrates (initial substrates or intermediate products) may not be recognized as such (in the worst case scenario they may be acting as inhibitors) by the enzyme, causing yields and reaction rates to drop. To solve this situation, a mixture of lipases with different specificity, selectivity and differently affected by the reaction conditions can offer much better results than the use of a single lipase exhibiting a very high initial activity or even the best global reaction course. This mixture of lipases from different sources has been called “combilipases” and is becoming increasingly popular. They include the use of liquid lipase formulations or immobilized lipases. In some instances, the lipases have been coimmobilized. Some discussion is offered regarding the problems that this coimmobilization may give rise to, and some strategies to solve some of these problems are proposed. The use of combilipases in the future may be extended to other processes and enzymes.

Seven new 1-oxygenated cholestane glycosides from Ornithogalum saundersiae

As the continuous scientific research, seven new 1-oxygenated cholestane glycosides named osaundersiosides 1 A - 1 G were isolated from an EtOH extract of the bulbs of Ornithogalum saundersiae. Their structures were deduced by means of spectroscopic data, chemical evidence and the results of hydrolytic cleavage. The cytotoxicity and anti-inflammatory effects of osaundersiosides 1 A - 1 G were evaluated, but none of them displayed significant activities. [Formula: see text].

OcUGT1-Catalyzing Glycodiversification of Steroids through Glucosylation and Transglucosylation Actions

Steroidal glycosides are important sources of innovative drugs. The increased diversification of steroidal glycosides will expand the probability of discovering active molecules. It is an efficient approach to diversify steroidal glycosides by using steroidal glycosyltransferases. OcUGT1, a uridine diphosphate-d-glucose (UDP-Glc)-dependent glycosyltransferase from Ornithogalum caudatum, is a multifunctional enzyme, and its glycodiversification potential towards steroids has never been fully explored. Herein, the glycodiversification capability of OcUGT1 towards 25 steroids through glucosylation and transglucosylation reactions were explored. Firstly, each of 25 compounds was glucosylated with UDP-Glc. Under the action of OcUGT1, five steroids (testosterone, deoxycorticosterone, hydrocortisone, estradiol, and 4-androstenediol) were glucosylated to form corresponding mono-glucosides and biosides. Next, OcUGT1-mediated transglucosylation activity of these compounds with another sugar donor ortho-nitrophenyl-β-d-glucopyranoside (oNPGlc) was investigated. Results revealed that the same five steroids could be glucosylated to generate mono-glucosides and biosides by OcUGT1 through transglucosylation reactions. These data indicated that OcUGT1-assisted glycodiversification of steroids could be achieved through glucosylation and transglucosylation reactions. These results provide a way to diversify steroidal glycosides, which lays the foundation for the increase of the probability of obtaining active lead compounds.

Structure and bioactivity of cholestane glycosides from the bulbs of Ornithogalum saundersiae Baker

Eight undescribed cholestane glycosides named osaundersioside A-H, along with three previously known compounds named osaundersioside I-K were isolated from Ornithogalum saundersiae Baker bulbs (Asparagaceae). Their structures were elucidated by extensive spectroscopic analysis and chemical methods. All isolates were evaluated for their cytotoxic activity and inhibitory effects on lipopolysaccharide (LPS)-induced nitric oxide (NO) production. Osaundersioside C was thus determined to exhibit specific cytotoxicity towards MCF-7 cell line with an IC₅₀ value of 0.20 μM, Osaundersioside H exhibited inhibitory effect on NO production in macrophages at the concentration of 10^-5 M, with inhibition rate of 56.81%.

Characterization of two flavonol synthases with iron-independent flavanone 3-hydroxylase activity from Ornithogalum caudatum Jacq

BACKGROUND

Flavonol synthase (FLS) is the key enzyme responsible for the biosynthesis of flavonols, the most abundant flavonoids, which have diverse pharmaceutical effects. Flavonol synthase has been previously found in other species, but not yet in Ornithogalum caudatum.

RESULTS

The transcriptome-wide mining and functional characterisation of a flavonol synthase gene family from O. caudatum were reported. Specifically, a small FLS gene family harbouring two members, OcFLS1 and OcFLS2, was isolated from O. caudatum based on transcriptome-wide mining. Phylogenetic analysis suggested that the two proteins showed the closest relationship with FLS proteins. In vitro enzymatic assays indicated OcFLS1 and OcFLS2 were flavonol synthases, catalysing the conversion of dihydroflavonols to flavonols in an iron-dependent fashion. In addition, the two proteins were found to display flavanone 3β-hydroxylase (F3H) activity, hydroxylating flavanones to form dihydroflavonols. Unlike single F3H enzymes, the F3H activity of OcFLS1 and OcFLS2 did not absolutely require iron. However, the presence of sufficient Fe²⁺ was demonstrated to be conducive to successive catalysis of flavanones to flavonols. The qRT-PCR analysis demonstrated that both genes were expressed in the leaves, bulbs, and flowers, with particularly high expression in the leaves. Moreover, their expression was regulated by developmental and environmental conditions.

CONCLUSIONS

OcFLS1 and OcFLS2 from O. caudatum were demonstrated to be flavonol synthases with iron-independent flavanone 3-hydroxylase activity.

Combinatorial mutation on the β-glycosidase specific to 7-β-xylosyltaxanes and increasing the mutated enzyme production by engineering the recombinant yeast

Taxol is a “blockbuster” antitumor drug produced by Taxus species with extremely low amount, while its analogue 7-β-xylosyl-10-deacetyltaxol is generally much higher in the plants. Both the fungal enzymes LXYL-P1−1 and LXYL-P1−2 can convert 7-β-xylosyl-10-deacetyltaxol into 10-deacetyltaxol for Taxol semi-synthesis. Of them, LXYL-P1−2 is twice more active than LXYL-P1−1, but there are only 11 significantly different amino acids in terms of the polarity and acidic-basic properties between them. In this study, single and multiple site-directed mutations at the 11 sites from LXYL-P1−1 to LXYL-P1−2 were performed to define the amino acids with upward bias in activities and to acquire variants with improved catalytic properties. Among all the 17 mutants, E12 (A72T/V91S) was the most active and even displayed 2.8- and 3-fold higher than LXYL-P1−2 on β-xylosidase and β-glucosidase activities. The possible mechanism for such improvement was proposed by homology modeling and molecular docking between E12 and 7-β-xylosyl-10-deacetyltaxol. The recombinant yeast GS115-P1E12-7 was constructed by introducing variant E12, the molecular chaperone gene pdi and the bacterial hemoglobin gene vhb. This engineered yeast rendered 4 times higher biomass enzyme activity than GS115-3.5K-P1−2 that had been used for demo-scale fermentation. Thus, GS115-P1E12-7 becomes a promising candidate to replace GS115-3.5K-P1−2 for industrial purpose.

Transcriptome-Wide Identification of an Aurone Glycosyltransferase with Glycosidase Activity from Ornithogalum saundersiae

Aurone glycosides display a variety of biological activities. However, reports about glycosyltransferases (GTs) responsible for aurones glycosylation are limited. Here, the transcriptome-wide discovery and identification of an aurone glycosyltransferase with glycosidase activity is reported. Specifically, a complementary DNA (cDNA), designated as OsUGT1, was isolated from the plant Ornithogalum saundersiae based on transcriptome mining. Conserved domain (CD)-search speculated OsUGT1 as a flavonoid GT. Phylogenetically, OsUGT1 is clustered as the same phylogenetic group with a putative 5,6-dihydroxyindoline-2-carboxylic acid (cyclo-DOPA) 5-O-glucosyltransferase, suggesting OsUGT1 may be an aurone glycosyltransferase. The purified OsUGT1 was therefore used as a biocatalyst to incubate with the representative aurone sulfuretin. In vitro enzymatic analyses showed that OsUGT1 was able to catalyze sulfuretin to form corresponding monoglycosides, suggesting OsUGT1 was indeed an aurone glycosyltransferase. OsUGT1 was observed to be a flavonoid GT, specific for flavonoid substrates. Moreover, OsUGT1 was demonstrated to display transglucosylation activity, transferring glucosyl group to sulfuretin via o-Nitrophenyl-β-d-glucopyranoside (oNP-β-Glc)-dependent fashion. In addition, OsUGT1-catalyzed hydrolysis was observed. This multifunctionality of OcUGT1 will broaden the application of OcUGT1 in glycosylation of aurones and other flavonoids.