Primary structure of the 2-O-methyl-alpha-L-fucose-containing side chain of the pectic polysaccharide, rhamnogalacturonan II

Production and Characterization of a Novel Exopolysaccharide from Ramlibacter tataouinensis

The current study examines the desiccation-resistant Ramlibacter tataouinensis TTB310^T as a model organism for the production of novel exopolysaccharides and their structural features. This bacterium is able to produce dividing forms of cysts which synthesize cell-bound exopolysaccharide. Initial experiments were conducted on the enrichment of cyst biomass for exopolysaccharide production under batch-fed conditions in a pilot-scale bioreactor, with lactate as the source of carbon and energy. The optimized medium produced significant quantities of exopolysaccharide in a single growth phase, since the production of exopolysaccharide took place during the division of the cysts. The exopolysaccharide layer was extracted from the cysts using a modified trichloroacetic acid method. The biochemical characterization of purified exopolysaccharide was performed by gas chromatography, ultrahigh-resolution mass spectrometry, nuclear magnetic resonance, and Fourier-transform infrared spectrometry. The repeating unit of exopolysaccharide was a decasaccharide consisting of ribose, glucose, rhamnose, galactose, mannose, and glucuronic acid with the ratio 3:2:2:1:1:1, and additional substituents such as acetyl, succinyl, and methyl moieties were also observed as a part of the exopolysaccharide structure. This study contributes to a fundamental understanding of the novel structural features of exopolysaccharide from a dividing form of cysts, and, further, results can be used to study its rheological properties for various industrial applications.

Structural characterization and immunoregulatory activity of a novel acidic polysaccharide from Scapharca subcrenata

A novel acidic polysaccharide named SSPA50-1 was isolated from Scapharca subcrenata using a simulated gastric fluid extraction method. SSPA50-1 is a heteropolysaccharide with an average molecular weight of 44.7 kDa that is composed of galacturonic acid, glucose, galactose, mannose, ribose, rhamnose, fucose, xylose and arabinose at a molar ratio of 1.00:5.40:9.04:3.10:1.59:4.01:2.10:2.21:2.28. The structural characterization based on the methylation and 1D/2D NMR analyses indicated that SSPA50-1 is composed of →3)-β-L-Rhap-(1→, →3)-β-L-2-O-Me-Fucp-(1→, →2)-α-D-Xylp-(1→, →5)-α-L-Araf-(1→, →3)-β-D-Galp-(1→, →6)-α-D-Glcp-(1→, →3,4)-β-D-Manp-(1→, →3,4)-β-D-Galp-(1→, β-D-Ribf-(1→, α-D-Glcp-(1→, and α-D-GalAp6Me-(1→. Furthermore, SSPA50-1 possessed potent immunoregulatory activity by enhancing the phagocytosis and NO, iNOS, TNF-α and IL-6 secretion capacity of RAW264.7 cells. Otherwise, SSPA50-1 significantly promoted the proliferation of splenic lymphocytes and RAW264.7 macrophages. These results indicated that SSPA50-1 could be developed as a potential ingredient for immunostimulatory agents.

Protocols for isolating and characterizing polysaccharides from plant cell walls: a case study using rhamnogalacturonan-II

BACKGROUND

In plants, a large diversity of polysaccharides comprise the cell wall. Each major type of plant cell wall polysaccharide, including cellulose, hemicellulose, and pectin, has distinct structures and functions that contribute to wall mechanics and influence plant morphogenesis. In recent years, pectin valorization has attracted much attention due to its expanding roles in biomass deconstruction, food and material science, and environmental remediation. However, pectin utilization has been limited by our incomplete knowledge of its structure. Herein, we present a workflow of principles relevant for the characterization of polysaccharide primary structure using nature's most complex polysaccharide, rhamnogalacturonan-II (RG-II), as a model.

RESULTS

We outline how to isolate RG-II from celery and duckweed cell walls and from red wine using chemical or enzymatic treatments coupled with size-exclusion chromatography. From there, we applied mass spectrometry (MS)-based techniques to determine the glycosyl residue and linkage compositions of the intact RG-II and derived oligosaccharides including special considerations for labile monosaccharides. In doing so, we demonstrated that in the duckweed Wolffiella repanda the arabinopyranosyl (Arap) residue of side chain B is substituted at O-2 with rhamnose. We used electrospray-MS techniques to identify non-glycosyl modifications including methyl-ethers, methyl-esters, and acetyl-esters on RG-II-derived oligosaccharides. We then showed the utility of proton nuclear magnetic resonance spectroscopy (¹H-NMR) to investigate the structure of intact RG-II and to complement the RG-II dimerization studies performed using size-exclusion chromatography.

CONCLUSIONS

The complexity of pectic polysaccharide structures has hampered efforts aimed at their valorization. In this work, we used RG-II as a model to demonstrate the steps necessary to isolate and characterize polysaccharides using chromatographic, MS, and NMR techniques. The principles can be applied to the characterization of other saccharide structures and will help inform researchers on how saccharide structure relates to functional properties in the future.

Chemical characteristic and bioactivity of hemicellulose-based polysaccharides isolated from Salvia miltiorrhiza

Salvia miltiorrhiza roots (SMRs), the main component of cell wall from the residual waste extraction, differ depending on the forming ways of monosaccharides. The extraction from 8% sodium hydroxide solution (H-8) was characterized by gel permeation chromatography (GPC), monosaccharide composition, Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (NMR) spectroscopy. The structure model of hemicellulose-based polysaccharides (HBPs) was derived by combining one-dimensional and two-dimensional NMR. Monosaccharides difference and correlation were performed by partial least square analysis (PLS). Seven H-8s exhibited optimal inhibitory activities, which varied based on different sources of Danshen. The backbone structure indicated that 4-β-D-Xylp served as the main chain connected by 3-α-L-Araf or 5-α-L-Araf-1, 4-β-D-Galp, and β-D-Glcp branch, as well as α-L-Rhap, α-D-GalpA and α-D-GlcpA fragments. The variation of HBPs in terms of the structure and bioactivity of SMRs correlated with different cultivation sites can be a new approach to optimize and utilize the medical materials by chemical and biological aspects of natural macromolecules.

Locating Methyl-Etherified and Methyl-Esterified Uronic Acids in the Plant Cell Wall Pectic Polysaccharide Rhamnogalacturonan II

Rhamnogalacturonan II (RG-II) is a structurally complex pectic polysaccharide that exists as a borate ester cross-linked dimer in the cell walls of all vascular plants. The glycosyl sequence of RG-II is largely conserved, but there is evidence that galacturonic acid (GalA) methyl etherification and glucuronic acid (GlcA) methyl esterification vary in the A sidechain across plant species. Methyl esterification of the galacturonan backbone has also been reported but not confirmed. Here we describe a new procedure, utilizing aq. sodium borodeuteride (NaBD₄)-reduced RG-II, to identify the methyl esterification status of backbone GalAs. Our data suggest that up to two different GalAs are esterified in the RG-II backbone. We also adapted a procedure based on methanolysis and NaBD₄ reduction to identify 3-, 4-, and 3,4-O-methyl GalA in RG-II. These data, together with matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF) MS analysis of sidechain A generated from selected RG-IIs and their NaBD₄-reduced counterparts, suggest that methyl etherification of the β-linked GalA and methyl esterification of the GlcA are widespread. Nevertheless, the extent of these modifications varies between plant species. Our analysis of the sidechain B glycoforms in RG-II from different dicots and nonpoalean monocots suggests that this sidechain has a minimum structure of an O-acetylated hexasaccharide (Ara-[MeFuc]-Gal-AceA-Rha-Api-). To complement these studies, we provide further evidence showing that dimer formation and stability in vitro is cation and borate dependent. Taken together, our data further refine the primary sequence and sequence variation of RG-II and provide additional insight into dimer stability and factors controlling dimer self-assembly.

Chemical structure and rheological studies of arabinoglucuronoxylans from the Cercidium praecox exudate brea gum

The structure of the arabinoglucuronoxylans from brea gum was elucidated through an chemical and NMR spectroscopical analysis. They are composed of xylose, arabinose, glucuronic acid and 4-O-methylglucuronic acid in a molar ratio 1:0.44:0.16:0.22. The structure consists of a central chain of (1→4)-β-d-xylopyranose of which ca.70% are susbstituted in C2 with single stubs of others sugars (β-d-Xylp, α-d-GlcpA and 4-O-Me-α-d-GlcpA), with disaccharides (α-l-Arap-(1→2)-4-O-Me-α-d-GlcpA-(1→, α-l-Arap-(1→2)-α-d-GlcpA-(1→, β-l-Araf-(1→3)-α-l-Araf-(1→ and α-l-Araf-(1→3)-α-l-Araf-(1→5), and possibly with trisaccharides of xylose. The determination of the location of the acetyl groups and their quantification in these arabinoglucuronoxylans has been achieved for the first time. Brea gum presents a higher thickening effect than gum arabic in 5% aqueous solution, demonstrating its potential usefulness for food and pharmaceutical applications.

Changes in the abundance of cell wall apiogalacturonan and xylogalacturonan and conservation of rhamnogalacturonan II structure during the diversification of the Lemnoideae

The diversification of the Lemnoideae was accompanied by a reduction in the abundance of cell wall apiogalacturonan and an increase in xylogalacturonan whereas rhamnogalacturonan II structure and cross-linking are conserved. The subfamily Lemnoideae is comprised of five genera and 38 species of small, fast-growing aquatic monocots. Lemna minor and Spirodela polyrhiza belong to this subfamily and have primary cell walls that contain large amounts of apiogalacturonan and thus are distinct from the primary walls of most other flowering plants. However, the pectins in the cell walls of other members of the Lemnoideae have not been investigated. Here, we show that apiogalacturonan decreased substantially as the Lemnoideae diversified since Wolffiella and Wolffia walls contain between 63 and 88% less apiose than Spirodela, Landoltia, and Lemna walls. In Wolffia, the most derived genus, xylogalacturonan is far more abundant than apiogalacturonan, whereas in Wolffiella pectic polysaccharides have a high arabinose content, which may arise from arabinan sidechains of RG I. The apiose-containing pectin rhamnogalacturonan II (RG-II) exists in Lemnoideae walls as a borate cross-linked dimer and has a glycosyl sequence similar to RG-II from terrestrial plants. Nevertheless, species-dependent variations in the extent of methyl-etherification of RG-II sidechain A and arabinosylation of sidechain B are discernible. Immunocytochemical studies revealed that pectin methyl-esterification is higher in developing daughter frond walls than in mother frond walls, indicating that methyl-esterification is associated with expanding cells. Our data support the notion that a functional cell wall requires conservation of RG-II structure and cross-linking but can accommodate structural changes in other pectins. The Lemnoideae provide a model system to study the mechanisms by which wall structure and composition has changed in closely related plants with similar growth habits.

Preparing rhamnogalacturonan II domains from seven plant pectins using Penicillium oxalicum degradation and their structural comparison

Rhamnogalacturonan II (RG-II) is a complex pectin with diverse pharmaceutical activities. To assess how RG-II functions, the development of methods for its preparation is required. In this paper, pectin from Codonopsis pilosula was used to evaluate the ability of fungi and bacteria to degrade the pectin. We discovered that the fungus Penicillium oxalicum could efficiently lead to the recovery of RG-II domains by degrading the other pectic domains. Further, six pectin fractions from different medical plants were used as the sole carbon source for the growth of Penicillium oxalicum. The major polymeric products remaining after fungus degradation was RG-II domains. Depending of plant source, side chains A differed with respect to their proportion of L-Gal and L-Fuc and to their degree of methyletherification. Side chains B were made of 8-10 sugar residues and up to 2 acetyl groups. Overall, our method provides an effective way to prepare RG-II pectin domains for investigating their structure-function relationships.

Altered lignification in mur1-1 a mutant deficient in GDP-L-fucose synthesis with reduced RG-II cross linking

In the plant cell wall, boron links two pectic domain rhamnogalacturonan II (RG-II) chains together to form a dimer and thus contributes to the reinforcement of cell adhesion. We studied the mur1-1 mutant of Arabidopsis thaliana which has lost the ability to form GDP-fucose in the shoots and show that the extent of RG-II cross-linking is reduced in the lignified stem of this mutant. Surprisingly, MUR1 mutation induced an enrichment of resistant interunit bonds in lignin and triggered the overexpression of many genes involved in lignified tissue formation and in jasmonic acid signaling. The defect in GDP-fucose synthesis induced a loss of cell adhesion at the interface between stele and cortex, as well as between interfascicular fibers. This led to the formation of regenerative xylem, where tissue detachment occurred, and underlined a loss of resistance to mechanical forces. Similar observations were also made on bor1-3 mutant stems which are altered in boron xylem loading, leading us to suggest that diminished RG-II dimerization is responsible for regenerative xylem formation.

How Fucose of Blood Group Glycotopes Programs Human Gut Microbiota

Formation of appropriate gut microbiota is essential for human health. The first two years of life is the critical period for this process. Selection of mutualistic microorganisms of the intestinal microbiota is controlled by the FUT2 and FUT3 genes that encode fucosyltransferases, enzymes responsible for the synthesis of fucosylated glycan structures of mucins and milk oligosaccharides. In this review, the mechanisms of the selection and maintenance of intestinal microorganisms that involve fucosylated oligosaccharides of breast milk and mucins of the newborn's intestine are described. Possible reasons for the use of fucose, and not sialic acid, as the major biological signal for the selection are also discussed.

Complex pectin metabolism by gut bacteria reveals novel catalytic functions

The metabolism of carbohydrate polymers drives microbial diversity in the human gut microbiota. It is unclear, however, whether bacterial consortia or single organisms are required to depolymerize highly complex glycans. Here we show that the gut bacterium Bacteroides thetaiotaomicron uses the most structurally complex glycan known: the plant pectic polysaccharide rhamnogalacturonan-II, cleaving all but 1 of its 21 distinct glycosidic linkages. The deconstruction of rhamnogalacturonan-II side chains and backbone are coordinated to overcome steric constraints, and the degradation involves previously undiscovered enzyme families and catalytic activities. The degradation system informs revision of the current structural model of rhamnogalacturonan-II and highlights how individual gut bacteria orchestrate manifold enzymes to metabolize the most challenging glycan in the human diet.

An Acid-Adapted Endo-α-1,5-L-arabinanase for Pectin Releasing

An arabinanase gene was cloned by overlap-PCR from Penicillium sp. Y702 and expressed in Pichia pastoris. The recombinant enzyme was named AbnC702 with 20 U/mg of endo-arabinanase activity toward linear α-1,5-L-arabinan. The optimal pH and temperature of AbnC702 were 5.0 and 50 °C, respectively. The recombinant AbnC702 was highly stable at pH 5.0-7.0 and 50 °C. It could retain about 72.3 % of maximum specific activity at pH 5.0 after incubation for 2.5 h, which indicated AbnC702 was an acid-adapted enzyme. The K _m and V _max values were 24.8 ± 4.7 mg/ml and 88.5 ± 5.6 U/mg, respectively. A three-dimensional structure of AbnC702 was made by homology modeling, and the counting of acidic/basic amino residues within the region of 10 Å around the active site, as well the hydrogen bonds within the area of 5 Å around the active site, might theoretically interpret the acid adaptability of AbnC702. Analysis of hydrolysis products by thin layer chromatography (TLC) combined with high-performance liquid chromatography (HPLC) verified that the recombinant AbnC702 was an endo-1,5-α-L-arabinanase, which yielded arabinobiose and arabinotriose as major products. AbnC702 was applied in pectin extraction from apple pomace with synergistic action of α-L-arabinofuranosidase.

Structural diversity of xylans in the cell walls of monocots

Xylans in the cell walls of monocots are structurally diverse. Arabinofuranose-containing glucuronoxylans are characteristic of commelinids. However, other structural features are not correlated with the major transitions in monocot evolution. Most studies of xylan structure in monocot cell walls have emphasized members of the Poaceae (grasses). Thus, there is a paucity of information regarding xylan structure in other commelinid and in non-commelinid monocot walls. Here, we describe the major structural features of the xylans produced by plants selected from ten of the twelve monocot orders. Glucuronoxylans comparable to eudicot secondary wall glucuronoxylans are abundant in non-commelinid walls. However, the α-D-glucuronic acid/4-O-methyl-α-D-glucuronic acid is often substituted at O-2 by an α-L-arabinopyranose residue in Alismatales and Asparagales glucuronoxylans. Glucuronoarabinoxylans were the only xylans detected in the cell walls of five different members of the Poaceae family (grasses). By contrast, both glucuronoxylan and glucuronoarabinoxylan are formed by the Zingiberales and Commelinales (commelinids). At least one species of each monocot order, including the Poales, forms xylan with the reducing end sequence -4)-β-D-Xylp-(1,3)-α-L-Rhap-(1,2)-α-D-GalpA-(1,4)-D-Xyl first identified in eudicot and gymnosperm glucuronoxylans. This sequence was not discernible in the arabinopyranose-containing glucuronoxylans of the Alismatales and Asparagales or the glucuronoarabinoxylans of the Poaceae. Rather, our data provide additional evidence that in Poaceae glucuronoarabinoxylan, the reducing end xylose residue is often substituted at O-2 with 4-O-methyl glucuronic acid or at O-3 with arabinofuranose. The variations in xylan structure and their implications for the evolution and biosynthesis of monocot cell walls are discussed.

Expression of fungal acetyl xylan esterase in Arabidopsis thaliana improves saccharification of stem lignocellulose

Cell wall hemicelluloses and pectins are O-acetylated at specific positions, but the significance of these substitutions is poorly understood. Using a transgenic approach, we investigated how reducing the extent of O-acetylation in xylan affects cell wall chemistry, plant performance and the recalcitrance of lignocellulose to saccharification. The Aspergillus niger acetyl xylan esterase AnAXE1 was expressed in Arabidopsis under the control of either the constitutively expressed 35S CAMV promoter or a woody-tissue-specific GT43B aspen promoter, and the protein was targeted to the apoplast by its native signal peptide, resulting in elevated acetyl esterase activity in soluble and wall-bound protein extracts and reduced xylan acetylation. No significant alterations in cell wall composition were observed in the transgenic lines, but their xylans were more easily digested by a β-1,4-endoxylanase, and more readily extracted by hot water, acids or alkali. Enzymatic saccharification of lignocellulose after hot water and alkali pretreatments produced up to 20% more reducing sugars in several lines. Fermentation by Trametes versicolor of tissue hydrolysates from the line with a 30% reduction in acetyl content yielded ~70% more ethanol compared with wild type. Plants expressing 35S:AnAXE1 and pGT43B:AnAXE1 developed normally and showed increased resistance to the biotrophic pathogen Hyaloperonospora arabidopsidis, probably due to constitutive activation of defence pathways. However, unintended changes in xyloglucan and pectin acetylation were only observed in 35S:AnAXE1-expressing plants. This study demonstrates that postsynthetic xylan deacetylation in woody tissues is a promising strategy for optimizing lignocellulosic biomass for biofuel production.

Inhibition of fucosylation of cell wall components by 2-fluoro 2-deoxy-L-fucose induces defects in root cell elongation

Screening of commercially available fluoro monosaccharides as putative growth inhibitors in Arabidopsis thaliana revealed that 2-fluoro 2-l-fucose (2F-Fuc) reduces root growth at micromolar concentrations. The inability of 2F-Fuc to affect an Atfkgp mutant that is defective in the fucose salvage pathway indicates that 2F-Fuc must be converted to its cognate GDP nucleotide sugar in order to inhibit root growth. Chemical analysis of cell wall polysaccharides and glycoproteins demonstrated that fucosylation of xyloglucans and of N-linked glycans is fully inhibited by 10 μm 2F-Fuc in Arabidopsis seedling roots, but genetic evidence indicates that these alterations are not responsible for the inhibition of root development by 2F-Fuc. Inhibition of fucosylation of cell wall polysaccharides also affected pectic rhamnogalacturonan-II (RG-II). At low concentrations, 2F-Fuc induced a decrease in RG-II dimerization. Both RG-II dimerization and root growth were partially restored in 2F-Fuc-treated seedlings by addition of boric acid, suggesting that the growth phenotype caused by 2F-Fuc was due to a deficiency of RG-II dimerization. Closer investigation of the 2F-Fuc-induced growth phenotype demonstrated that cell division is not affected by 2F-Fuc treatments. In contrast, the inhibitor suppressed elongation of root cells and promoted the emergence of adventitious roots. This study further emphasizes the importance of RG-II in cell elongation and the utility of glycosyltransferase inhibitors as new tools for studying the functions of cell wall polysaccharides in plant development. Moreover, supplementation experiments with borate suggest that the function of boron in plants might not be restricted to RG-II cross-linking, but that it might also be a signal molecule in the cell wall integrity-sensing mechanism.

Structural characterization and antioxidant activity in vitro of polysaccharides from angelica and astragalus

In the present study, structural characterization and antioxidant activity of a fraction (AAP-2A) of polysaccharides from angelica and astragalus (AAP) were investigated. Characteriztion assay showed that AAP-2A had molecular weight (Mw), root-mean square (RMS) radius and polydispersity index (Mw/Mn) of 2.252 × 10(3)kDa, 28.4 nm and 1.038, respectively. There were infrared characteristic absorption peaks of polysaccharides in FT-IR spectroscopy. AAP-2A was composed of rhamnose (Rha), galactose (Gal), arabinose (Ara) and glucose (Glc) with a molar ratio of 1:2.13:3.22:6.18 in GC analysis. Methylation analysis combined with NMR spectroscopic analysis demonstrated that a preliminary structure of AAP-2A was proposed as follows: 1,3-linked Rhap, 1,3-linked Galp, 1,3-linked Araf, 1,5-linked Araf, 1,3,5-linked Araf, 1,4-linked Glcp and 1,4,6-linked Glcp interspersed with terminal Glcp. AAP-2A exhibited a surface with a sheet-like appearance in scanning electron microscope and stronger antioxidant capacity compared with AAP.

2-Fluoro-L-Fucose Is a Metabolically Incorporated Inhibitor of Plant Cell Wall Polysaccharide Fucosylation

The monosaccharide L-fucose (L-Fuc) is a common component of plant cell wall polysaccharides and other plant glycans, including the hemicellulose xyloglucan, pectic rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II), arabinogalactan proteins, and N-linked glycans. Mutations compromising the biosynthesis of many plant cell wall polysaccharides are lethal, and as a result, small molecule inhibitors of plant cell wall polysaccharide biosynthesis have been developed because these molecules can be applied at defined concentrations and developmental stages. In this study, we characterize novel small molecule inhibitors of plant fucosylation. 2-fluoro-L-fucose (2F-Fuc) analogs caused severe growth phenotypes when applied to Arabidopsis seedlings, including reduced root growth and altered root morphology. These phenotypic defects were dependent upon the L-Fuc salvage pathway enzyme L-Fucose Kinase/ GDP-L-Fucose Pyrophosphorylase (FKGP), suggesting that 2F-Fuc is metabolically converted to the sugar nucleotide GDP-2F-Fuc, which serves as the active inhibitory molecule. The L-Fuc content of cell wall matrix polysaccharides was reduced in plants treated with 2F-Fuc, suggesting that this molecule inhibits the incorporation of L-Fuc into these polysaccharides. Additionally, phenotypic defects induced by 2F-Fuc treatment could be partially relieved by the exogenous application of boric acid, suggesting that 2F-Fuc inhibits RG-II biosynthesis. Overall, the results presented here suggest that 2F-Fuc is a metabolically incorporated inhibitor of plant cellular fucosylation events, and potentially suggest that other 2-fluorinated monosaccharides could serve as useful chemical probes for the inhibition of cell wall polysaccharide biosynthesis.

The role of the plant-specific ALTERED XYLOGLUCAN9 protein in Arabidopsis cell wall polysaccharide O-acetylation

A mutation in the ALTERED XYLOGLUCAN9 (AXY9) gene was found to be causative for the decreased xyloglucan acetylation phenotype of the axy9.1 mutant, which was identified in a forward genetic screen for Arabidopsis (Arabidopsis thaliana) mutants. The axy9.1 mutant also exhibits decreased O-acetylation of xylan, implying that the AXY9 protein has a broad role in polysaccharide acetylation. An axy9 insertional mutant exhibits severe growth defects and collapsed xylem, demonstrating the importance of wall polysaccharide O-acetylation for normal plant growth and development. Localization and topological experiments indicate that the active site of the AXY9 protein resides within the Golgi lumen. The AXY9 protein appears to be a component of the plant cell wall polysaccharide acetylation pathway, which also includes the REDUCED WALL ACETYLATION and TRICHOME BIREFRINGENCE-LIKE proteins. The AXY9 protein is distinct from the TRICHOME BIREFRINGENCE-LIKE proteins, reported to be polysaccharide acetyltransferases, but does share homology with them and other acetyltransferases, suggesting that the AXY9 protein may act to produce an acetylated intermediate that is part of the O-acetylation pathway.

Recovery and fine structure variability of RGII sub-domains in wine (Vitis vinifera Merlot)

BACKGROUND AND AIMS

Rhamnogalacturonan II (RGII) is a structurally complex pectic sub-domain composed of more than 12 different sugars and 20 different linkages distributed in five side chains along a homogalacturonan backbone. Although RGII has long been described as highly conserved over plant evolution, recent studies have revealed variations in the structure of the polysaccharide. This study examines the fine structure variability of RGII in wine, focusing on the side chains A and B obtained after sequential mild acid hydrolysis. Specifically, this study aims to differentiate intrinsic structural variations in these RGII side chains from structural variations due to acid hydrolysis.

METHODS

RGII from wine (Vitis vinifera Merlot) was sequentially hydrolysed with trifluoroacetic acid (TFA) and the hydrolysis products were separated by anion-exchange chromatography (AEC). AEC fractions or total hydrolysates were analysed by MALDI-TOF mass spectrometry.

KEY RESULTS

The optimal conditions to recover non-degraded side chain B, side chain A and RGII backbone were 0·1 m TFA at 40 °C for 16 h, 0·48 m TFA at 40 °C for 16 h (or 0·1 m TFA at 60 °C for 8 h) and 0·1 m TFA at 60 °C for 16 h, respectively. Side chain B was particularly prone to acid degradation. Side chain A and the RGII GalA backbone were partly degraded by 0·1 m TFA at 80 °C for 1-4 h. AEC allowed separation of side chain B, methyl-esterified side chain A and non-methyl-esterified side chain A. The structure of side chain A and the GalA backbone were highly variable.

CONCLUSIONS

Several modifications to the RGII structure of wine were identified. The observed dearabinosylation and deacetylation were primarily the consequence of acidic treatment, while variation in methyl-esterification, methyl-ether linkages and oxidation reflect natural diversity. The physiological significance of this variability, however, remains to be determined.

Rhamnogalacturonan II structure shows variation in the side chains monosaccharide composition and methylation status within and across different plant species

A paradigm regarding rhamnogalacturonans II (RGII) is their strictly conserved structure within a given plant. We developed and employed a fast structural characterization method based on chromatography and mass spectrometry, allowing analysis of RGII side chains from microgram amounts of cell wall. We found that RGII structures are much more diverse than so far described. In chain A of wild-type plants, up to 45% of the l-fucose is substituted by l-galactose, a state that is seemingly uncorrelated with RGII dimerization capacity. This led us to completely reinvestigate RGII structures of the Arabidopsis thaliana fucose-deficient mutant mur1, which provided insights into RGII chain A biosynthesis, and suggested that chain A truncation, rather than l-fucose to l-galactose substitution, is responsible for the mur1 dwarf phenotype. Mass spectrometry data for chain A coupled with NMR analysis revealed a high degree of methyl esterification of its glucuronic acid, providing a plausible explanation for the puzzling RGII antibody recognition. The β-galacturonic acid of chain A exhibits up to two methyl etherifications in an organ-specific manner. Combined with variation in the length of side chain B, this gives rise to a family of RGII structures instead of the unique structure described up to now. These findings pave the way for studies on the physiological roles of modulation of RGII composition.

Exopolysaccharides of Synechocystis aquatilis are sulfated arabinofucans containing N-acetyl-fucosamine

Cyanobacteria are known to be a rather diverse group of organisms regarding e.g. morphology, metabolism and composition of excreted exopolysaccharides (EPS). Considering the high number of known cyanobacterial species the EPS from only a small percentage are investigated in detail. This work examined EPS from the unicellular strains of Synechocystis aquatilis and S. pevalekii with various methods. The results emphasize the heterogeneity of cyanobacterial EPS. S. pevalekii secrets complex hetero-polysaccharides and acidic proteins as proteoglycan-complexes whereas the protein-free but highly sulfated EPS from S. aquatilis only consist of 4 dominant monosaccharides. Especially remarkable is the composition of these EPS: an arabinofucan with higher amounts of N-acetyl-fucosamine (FucNAc) and only minor quantities of glucose. Both EPS and the newly found component FucNAc in EPS from S. aquatilis extend the possible components of cyanobacterial EPS and the knowledge of heterogeneity of cyanobacterial metabolites.

Structural characterization and antitumor activity of a pectic polysaccharide from Codonopsis pilosula

A pectic polysaccharide (CPP1b) was at first isolated from Codonopsis pilosula. Sugar analysis revealed that CPP1b is composed of rhamnose (Rha), arabinose (Ara), galactose (Gal) and galacturonic acid (GalA) with a molar ratio of 0.25:0.12:0.13:2.51. The result of esterification assay showed that about 46.7±0.4% of carboxylic groups in GalA residues existed as methyl ester. Combined with chemical and spectroscopic analyses, a preliminary structure of CPP1b was proposed as follows: 1,4-linked α-D-GalpA and 1,4-linked α-D-GalpA6Me interspersed with rare 1,2-linked β-L-Rhap, 1,2,6-linked α-D-Galp and terminal α-L-Arap. CPP1b had an average molar mass and root-mean square radius (RMS) of 1.45×10(5) Da and 29.7 nm, respectively, and presented a linear random coil conformation in 0.9% NaCl. The ultrastructure of CPP1b was further investigated by transmission electron microscope (TEM) and scanning electron microscope (SEM). CPP1b exhibited obvious cytotoxicity to human lung adenocarcinoma A549 cells in a dose- and time-dependent manner.

Methylation--an uncommon modification of glycans

A methyl (Me) group on a sugar residue is a rarely reported event. Until now, this type of modification has been found in the animal kingdom only in worms and molluscs, whereas it is more frequently present in some species of bacteria, fungi, algae and plants, but not in mammals. The monosaccharides involved as well as the positions of the Me groups on the sugar vary with species. Methylation appears to play a role in some recognition events, but details are still unknown. This review summarises the current knowledge on methylation of sugars in all types of organism.

Illuminating the wall: using click chemistry to image pectins in Arabidopsis cell walls

Plant cell walls are the most abundant biomaterials on Earth and serve a multitude of purposes in human society. These complex extracellular matrices are mainly composed of polysaccharides, including cellulose, hemicelluloses, and pectins, which cannot be cytologically examined using conventional techniques. Click chemistry, which exploits a bio-orthogonal cycloaddition reaction between alkynyl and azido groups, has proven to be useful for the metabolic incorporation and detection of modified sugars in polysaccharides in animals, fungi, and bacteria, but its use to interrogate the biosynthesis or dynamics of plant cell walls has not been previously reported. Recently, we found that an alkynylated analog of fucose can be metabolically incorporated into Arabidopsis thaliana cell walls and click labeled with fluorescent probes, facilitating imaging of cell wall carbohydrates. Despite the presence of fucose in several classes of wall polysaccharides, fucose-alkyne was primarily incorporated into rhamnogalacturonan-I, a type of pectin. Using timecourse and pulse-labeling experiments, we observed the dynamics of pectin delivery and reorganization in expanding cell walls. The use of click chemistry to investigate plant cell wall architecture should help bridge the gap between biochemical characterization of isolated cell wall components and an understanding of how those components interact in intact cell walls.

Metabolic click-labeling with a fucose analog reveals pectin delivery, architecture, and dynamics in Arabidopsis cell walls

Polysaccharide-rich cell walls are a defining feature of plants that influence cell division and growth, but many details of cell-wall organization and dynamics are unknown because of a lack of suitable chemical probes. Metabolic labeling using sugar analogs compatible with click chemistry has the potential to provide new insights into cell-wall structure and dynamics. Using this approach, we found that an alkynylated fucose analog (FucAl) is metabolically incorporated into the cell walls of Arabidopsis thaliana roots and that a significant fraction of the incorporated FucAl is present in pectic rhamnogalacturonan-I (RG-I). Time-course experiments revealed that FucAl-containing RG-I first localizes in cell walls as uniformly distributed punctae that likely mark the sites of vesicle-mediated delivery of new polysaccharides to growing cell walls. In addition, we found that the pattern of incorporated FucAl differs markedly along the developmental gradient of the root. Using pulse-chase experiments, we also discovered that the pectin network is reoriented in elongating root epidermal cells. These results reveal previously undescribed details of polysaccharide delivery, organization, and dynamics in cell walls.

The Synthesis and Origin of the Pectic Polysaccharide Rhamnogalacturonan II - Insights from Nucleotide Sugar Formation and Diversity

There is compelling evidence showing that the structurally complex pectic polysaccharide rhamnogalacturonan II (RG-II) exists in the primary cell wall as a borate cross-linked dimer and that this dimer is required for the assembly of a functional wall and for normal plant growth and development. The results of several studies have also established that RG-II structure and cross-linking is conserved in vascular plants and that RG-II likely appeared early in the evolution of land plants. Two features that distinguish RG-II from other plant polysaccharides are that RG-II is composed of 13 different glycoses linked to each other by up to 22 different glycosidic linkages and that RG-II is the only polysaccharide known to contain both apiose and aceric acid. Thus, one key event in land plant evolution was the emergence of genes encoding nucleotide sugar biosynthetic enzymes that generate the activated forms of apiose and aceric acid required for RG-II synthesis. Many of the genes involved in the generation of the nucleotide sugars used for RG-II synthesis have been functionally characterized. By contrast, only one glycosyltransferase involved in the assembly of RG-II has been identified. Here we provide an overview of the formation of the activated sugars required for RG-II synthesis and point to the possible cellular and metabolic processes that could be involved in assembling and controlling the formation of a borate cross-linked RG-II molecule. We discuss how nucleotide sugar synthesis is compartmentalized and how this may control the flux of precursors to facilitate and regulate the formation of RG-II.

Small molecule probes for plant cell wall polysaccharide imaging

Plant cell walls are composed of interlinked polymer networks consisting of cellulose, hemicelluloses, pectins, proteins, and lignin. The ordered deposition of these components is a dynamic process that critically affects the development and differentiation of plant cells. However, our understanding of cell wall synthesis and remodeling, as well as the diverse cell wall architectures that result from these processes, has been limited by a lack of suitable chemical probes that are compatible with live-cell imaging. In this review, we summarize the currently available molecular toolbox of probes for cell wall polysaccharide imaging in plants, with particular emphasis on recent advances in small molecule-based fluorescent probes. We also discuss the potential for further development of small molecule probes for the analysis of cell wall architecture and dynamics.

Pectin Rhamnogalacturonan II: On the “Small Stem with Four Branches” in the Primary Cell Walls of Plants

Rhamnogalacturonan II (RG-II) is a type of block copolymer of complex pectins that represents a quantitatively minor component of the primary cell walls of land (vascular) plants. The structural composition of RG-II is almost totally sequenced and appears to be remarkably conserved in all tracheophytes so far examined. The backbone of RG-II, released from complex (cell wall) pectins by endo-polygalacturonase (Endo-PG) treatment, has been found to contain up to 15 (1→4)-linked-α-D-GalpA units, some of which carry four well-defined side chains, often referred to as A-, B-, C-, and D-side chains. Nevertheless, the relative locations on the backbone of these four branches, especially the A chain, remain to be ascertained. A combination of different data suggests that neither the terminal nonreducing GalA nor the contiguous GalA unit is likely to be the branching point of the A chain, but probably the ninth GalA residue from the reducing end, assuming a minimum backbone length of 11 (1→4)-linked-α-d-GalpA. The latest reports on RG-II are here highlighted, with a provided update for the macrostructure and array of functionalities.

Synthesis of apiose-containing oligosaccharide fragments of the plant cell wall: fragments of rhamnogalacturonan-II side chains A and B, and apiogalacturonan

Fragments of pectic polysaccharides rhamnogalacturonan-II (RG-II) and apiogalacturonan were synthesised using p-tolylthio apiofuranoside derivatives as key building blocks. Apiofuranose thioglycosides can be conveniently prepared by cyclization of the corresponding dithioacetals possessing a 2,3-O-isopropylidene group, which is required for preservation of the correct (3R) configuration of the apiofuranose ring. The remarkable stability of this protecting group in apiofuranose derivatives requires its replacement with a more reactive protecting group, such as a benzylidene acetal which was used in the synthesis of trisaccharide β-Rhap-(1→3')-β-Apif-(1→2)-α-GalAp-OMe. The X-ray crystal structure of the protected precursor of this trisaccharide has been elucidated.

Arabidopsis - a powerful model system for plant cell wall research

Plant cell walls are composites of various carbohydrates, proteins and other compounds. Cell walls provide plants with strength and protection, and also represent the most abundant source of renewable biomass. Despite the importance of plant cell walls, comparatively little is known about the identities of genes and functions of proteins involved in their biosynthesis. The model plant Arabidopsis and the availability of its genome sequence have been invaluable for the identification and functional characterization of genes encoding enzymes involved in plant cell-wall biosynthesis. This review covers recent progress in the identification and characterization of genes encoding proteins involved in the biosynthesis of Arabidopsis cell-wall polysaccharides and arabinogalactan proteins. These studies have improved our understanding of both the mechanisms of cell-wall biosynthesis and the functions of various cell-wall polymers, and have highlighted areas where further research is needed.

Structural characterisation of the pectic polysaccharide rhamnogalacturonan II using an acidic fingerprinting methodology

Rhamnogalacturonan II (RG-II) is a structurally complex cell wall pectic polysaccharide. Despite its complexity, both the structure of RG-II and its ability to dimerise via a borate diester are conserved in vascular plants suggesting that RG-II has a fundamental role in primary cell wall organisation and function. The selection and analysis of new mutants affected in RG-II formation represents a promising strategy to unravel these functions and to identify genes encoding enzymes involved in RG-II biosynthesis. In this paper, a novel fingerprinting strategy is described for the screening of RG-II mutants based on the mild acid hydrolysis of RG-II coupled to the analysis of the resulting fragments by mass spectrometry. This methodology was developed using RG-II fractions isolated from citrus pectins and then validated for RG-II isolated from the Arabidopsis mur1 mutant and irx10 irx10-like double mutant.

The structure, function, and biosynthesis of plant cell wall pectic polysaccharides

Plant cell walls consist of carbohydrate, protein, and aromatic compounds and are essential to the proper growth and development of plants. The carbohydrate components make up approximately 90% of the primary wall, and are critical to wall function. There is a diversity of polysaccharides that make up the wall and that are classified as one of three types: cellulose, hemicellulose, or pectin. The pectins, which are most abundant in the plant primary cell walls and the middle lamellae, are a class of molecules defined by the presence of galacturonic acid. The pectic polysaccharides include the galacturonans (homogalacturonan, substituted galacturonans, and RG-II) and rhamnogalacturonan-I. Galacturonans have a backbone that consists of alpha-1,4-linked galacturonic acid. The identification of glycosyltransferases involved in pectin synthesis is essential to the study of cell wall function in plant growth and development and for maximizing the value and use of plant polysaccharides in industry and human health. A detailed synopsis of the existing literature on pectin structure, function, and biosynthesis is presented.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

Synthesis of the Branched-Chain Sugar Aceric Acid: A Unique Component of the Pectic Polysaccharide Rhamnogalacturonan-II

Described herein is the synthesis of 3-C-carboxy-5-deoxy-L-xylose (aceric acid), a rare branched-chain sugar found in the complex pectic polysaccharide rhamnogalacturonan-II. The key synthetic step in the construction of aceric acid was the stereoselective addition of 2-trimethylsilyl thiazole to 5-deoxy-1,2-O-isopropylidene-alpha-L-erythro-pentofuran-3-ulose (2), which was prepared from L-xylose. The thiazole group was efficiently converted into the required carboxyl group via conventional transformations. Aceric acid was also synthesized by dihydroxylation of a 3-C-methylene derivative of 2 followed by oxidation of the resulting hydroxylmethyl group. The C-2 epimer of aceric acid was also synthesized using thiazole addition chemistry, starting from L-arabinose.