A highly selective C-rhamnosyltransferase from Viola tricolor and insights into its mechanisms

C-Glycosides are important natural products with various bioactivities. In plant biosynthetic pathways, the C-glycosylation step is usually catalyzed by C-glycosyltransferases (CGTs), and most of them prefer to accept uridine 5'-diphosphate glucose (UDP-Glc) as sugar donor. No CGTs favoring UDP-rhamnose (UDP-Rha) as sugar donor has been reported, thus far. Herein, we report the first selective C-rhamnosyltransferase VtCGTc from the medicinal plant Viola tricolor. VtCGTc could efficiently catalyze C-rhamnosylation of 2-hydroxynaringenin 3-C-glucoside, and exhibited high selectivity towards UDP-Rha. Mechanisms for the sugar donor selectivity of VtCGTc were investigated by molecular dynamics (MD) simulations and molecular mechanics with generalized Born and surface area solvation (MM/GBSA) binding free energy calculations. Val144 played a vital role in recognizing UDP-Rha, and the V144T mutant could efficiently utilize UDP-Glc. This work provides a new and efficient approach to prepare flavonoid C-rhamnosides such as violanthin and iso-violanthin.

Structural mechanism of a dual-functional enzyme DgpA/B/C as both a C-glycoside cleaving enzyme and an O- to C-glycoside isomerase

The C-glycosidic bond that connects the sugar moiety with aglycone is difficult to be broken or made due to its inert nature. The knowledge of C-glycoside breakdown and synthesis is very limited. Recently, the enzyme DgpA/B/C cascade from a human intestinal bacterium PUE was identified to specifically cleave the C-glycosidic bond of puerarin (daidzein-8-C-glucoside). Here we investigated how puerarin is recognized and oxidized by DgpA based on crystal structures of DgpA with or without substrate and biochemical characterization. More strikingly, we found that apart from being a C-glycoside cleaving enzyme, DgpA/B/C is capable of efficiently converting O- to C-glycoside showing the activity as a structure isomerase. A possible mechanistic model was proposed dependently of the simulated complex structure of DgpB/C with 3″-oxo-daidzin and structure-based mutagenesis. Our findings not only shed light on understanding the enzyme-mediated C-glycosidic bond breakage and formation, but also may help to facilitate stereospecific C-glycoside synthesis in pharmaceutical industry.

Study on the separation and identification of flavone C-glycosides from the leaves of Dendrobium officinale and their inhibitory activities to α-gulcosidase / 中国药房

OBJECTIVE To separ ate and identify the flavone C-glycosides from the leaves of Dendrobium officinale ，and to evaluate their in vitro inhibitory activities to α-glucosidase. METHODS The flavone C-glycosides from the leaves of D. officinale were separated and purified by macroporous adsorption resin and preparative high -performance liquid chromatography . The structure of obtained compound was elucidated and identified by spectroscopic methods ，such as ultraviolet spectrum ，nuclear magnetic resonance，high-resolution electrospray ionization mass spectrometry ，etc. The in vitro inhibitory activities of flavone C-glycosides and positive control （acarbose）to α-glucosidase were investigated . RESULTS Five apigenin -6，8-di-C-glycosides were isolated and purified from the leaves of D. officinale，and identified as apigenin -6-C-α-L-rhamnosyl-8-C-β-D-quinovoside（1）， schaftoside（2），isoschaftoside（3），isoviolanthin（4）and violanthin （5）. Half inhibitory concentration of compound 1-5 and acarbose inhibiting α-glucosidase were （1.79±1.27），（2.05±0.72），（1.93±0.67），（1.09±0.46），（1.36±0.58），（18.69±1.24）μmol/L， respectively. CONCLUSIONS Five apigenin -6，8-di-C-glycosides with α-glucosidase inhibitory activity are isolated from the leaves of D. officinale，of which compound 1 is a new compound and compound 2 is isolated from this plant for the first time .

Comprehensive mass spectrum analysis of two flavone-6,8-C-di-glycosides and its application by high resolution electrospray ionization tandem mass spectroscopy in both negative and positive ion modes / 中国中药杂志

The tandem mass spectrum of apigenin-6,8-C-di-glucoside( 1) and apigenin-6-C-glucose-8-C-rhamnoside( 2) were obtained by high resolution electrospray ionization mass spectrometry( HR-ESI-MS/MS) in both positive and negative ion modes. The elemental composition of each ion was determined according to its accurate mass-to-charge,hence,the fragmentation pathways of each compound were proposed in both negative and positive ion modes. Comprehensive analysis of each ion and its proposed fragmentation pathways of the two compounds was initially conducted in both negative and positive ion mode HR-ESI-MS/MS to explore the diagnostic ions for flavone-6,8-C-di-glycosides and the characteristic ions for each compound and their cleavage rules. The results showed that a family of fragmentation ions with m/z 353,325,311,297 in ESI(-)-MS and m/z 355,325,307,295 in ESI( +)-MS could be the diagnostic ions of flavone-6,8-C-di-glycoside,and characteristic neutral loss could be assigned to glycosyl substitution,for example,neutral losses of C_4H_8O_4( 120),C_3H_6O_3( 90),C_2H_4O_2( 60) for glucoside substitution while neutral losses of C_4H_8O_3(104),C_3H_6O_2( 74),C_2H_4O( 44) for rhamnoside substitution. Furthermore,only one H_2O loss from mother ion( [M-H]-) was observed for 1 & 2 in ESI(-)-MS while five to six H2 O loss from mother ion( [M+H]+) was observed for 1 & 2 in ESI( +)-MS to produce a family of ions by subsequent loss of H_2O,which could be applied for glucosyl difference. The flavone-6,8-C-di-glycosides in both ESI( +)-MS and ESI(-)-MS showed the cleavage similarity at sugar substitutions. However,there were much more differences by the fragmentation pathways and neutral losses between ESI( +)-MS and ESI(-)-MS as following,hyperconjugation ions by subsequent loss of H_2O from precursor ions of flavone-6,8-C-di-glycosides in ESI( +)-MS were not observed in ESI(-)-MS; the subsequent neutral loss of CH_2O in ESI( +)-MS were rarely observed in ESI(-)-MS; the loss of CO only happen at C-ring of flavone ESI( +)-MS other than glycosyl position in ESI(-)-MS; the C4-chain neutral loss of flavone-6,8-C-di-glycosides happened at 8-C-glycosyl position other than at 6-C-glycosyl position. The above cleavage rules and diagnostic ions of ESI( +)-MS were successfully applied for the structure identification of 4 flavone-6 C,8 C-diglycosides from the stem extract of Dendrobium officinale as vicenin Ⅱ,vicenin Ⅰ,isoschaftoside,schaftoside as well as one flavone-O-glysoside named rutin,which were supported by ESI(-)-MS data as well.

A new flavonol C-glycoside and a rare bioactive lignanamide from Piper wallichii Miq. Hand.-Mazz / 中国天然药物

This study was conducted to investigate the chemical constituents of Piper wallichii (Miq.) Hand.-Mazz. and evaluate their biological activity. Compounds were isolated by various column chromatographic methods, and their structures were elucidated on the basis of physical characteristics and spectral data. The 1, 1-diphenyl-2-picrylhydrazyl (DPPH)-scavenging activity and acetylcholinesterase (AChE)-inhibitory activity of the compounds were evaluated. Five compounds were obtained and identified as 8-C-β-D-glucopyranosylkaempferol-3-O-β-D-glucopyranoside (1), 1, 2-dihydro-6,8-dimethoxy-7-hydroxy-1-(3, 5-dimethoxy-4-hydroxyphenyl)-N(1), N(2)-bis-[2-(4-hydroxyphenyl)ethyl]-2, 3-naphthalene dicarboxamide (2), goniothalactam (3), aristololactam A IIIa (4) and piperlonguminine (5). Compound 1 was a new flavonol C-glycoside, 2 was a rare lignanamide, which was isolated from the family Piperaceae for the first time, and compound 3 was isolated from this plant for the first time. Among them, 2 showed potent DPPH-scavenging activity, with IC50 of 31.38 ± 0.97 μmol·L(-1); Compounds 2, 3, and 4 showed AChE inhibitory activity at 100 μmol·L(-1), with inhibition rates of 28.57% ± 1.47%, 18.48% ± 2.41% and 17.4% ± 3.03%, respectively.

A new flavone C-glycoside from Ziziphi Spinosae Semen / 中草药

Objective: To study the chemical constituents from 70% ethanol extract of Ziziphi Spinosae Semen. Methods: Five compounds were isolated and purified by macroporous resin D101, silica gel, Sephadex LH-20, Toyopearl HW-40, and ODS column chromatographies. Results: Five compounds (1-5) were isolated from 70% ethanol extract of Ziziphi Spinosae Semen. Their structures were identified by spectroscopic methods, as 6″, 6‴-diferuloylisospinosin (1), kaempferol-3-O-β-D-xylopyranosyl-(1→2)- [α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside (2), spinosin (3), isospinosin (4), and 6‴-feruloylspinosin (5). Conclusion: Compound 1 is a new flavone C-glycoside named 6″, 6‴-diferuloylisospinosin. Compound 2 is reported from the genus Ziziphus Mill. for the first time.