Purification of an exo-beta-(1----3)-D-galactanase of Irpex lacteus (Polyporus tulipiferae) and its action on arabinogalactan-proteins

Characterization of a multi-domain exo-β-1,3-galactanase from Paenibacillus xylanexedens

β-1,3-Galactanases selectively degrade β-1,3-galactan, thus it is an attractive enzyme technique to map high-galactan structure and prepare galactooligosaccharides. In this work, a gene encoding exo-β-1,3-galactanase (PxGal43) was screened form Paenibacillus xylanexedens, consisting of a GH43 domain, a CBM32 domain and α-L-arabinofuranosidase B (AbfB) domain. Using β-1,3-galactan (AG-II-P) as substrate, the recombined enzyme expressed in Escherichia coli BL21 (DE3) exhibited an optimal activity at pH 7.0 and 30 °C. The enzyme was thermostable, retaining >70 % activity after incubating at 50 °C for 2 h. In addition, it showed high tolerance to various metal ions, denaturants and detergents. Substrate specificity indicated that PxGal43 hydrolysis only β-1,3-linked galactosyl oligosaccharides and polysaccharides, releasing galactose as an exo-acting manner. The function of the CBM32 and AbfB domain was revealed by their sequential deletion and suggested that their connection to the catalytic domain was crucial for the oligomerization, catalytic activity, substrate binding and thermal stability of PxGal43. The substrate docking and site-directed mutagenesis proposed that Glu191, Gln244, Asp138 and Glu81 served as the catalytic acid, catalytic base, pKa modulator, and substrate identifier in PxGal43, respectively. These results provide a better understanding and optimization of multi-domain bacterial GH43 β-1,3-galactanase for the degradation of arabinogalactan.

Pectic-AGP is a major form of Arabidopsis AGPs

As a key component in cell walls of numerous organisms ranging from green algae to higher plants, AGPs play principal roles in many biological processes such as cell-cell adhesion and regulating Ca2+ signaling pathway as a Ca2+-capacitor. Consistently, AGP structures vary from species to species and from tissue to tissue. To understand the functions of AGPs, it is vital to know their structural differences relative to their location in the plant. Thus, AGPs were purified from different Arabidopsis tissues. Analyses of these AGPs demonstrated that the AGPs comprised covalently linked pectin and AGP, referred to as pectic-AGPs. Importantly, these pectic-AGPs were glycosylated with a remarkable variety of polysaccharides including homogalacturonan, rhamnogalacturonan-I, and type II arabinogalactan at different ratios and lengths. This result not only suggests that pectic-AGP is a major form of Arabidopsis AGPs, but also supports AGPs serve as crosslinkers covalently connecting pectins with structures tailored for tissue-specific functions.

Plant type II arabinogalactan: Structural features and modification to increase functionality

Type II arabinogalactans (AGs) are a highly diverse class of plant polysaccharides generally encountered as the carbohydrate moieties of certain extracellular proteoglycans, the so-called arabinogalactan-proteins (AGPs), which are found on plasma membranes and in cell walls. The basic structure of type II AG is a 1,3-β-D-galactan main chain with 1,6-β-D-galactan side chains. The side chains are further decorated with other sugars such as α-l-arabinose and β-d-glucuronic acid. In addition, AGs with 1,6-β-D-galactan as the main chain, which are designated as 'type II related AG' in this review, can also be found in several plants. Due to their diverse and heterogenous features, the determination of carbohydrate structures of type II and type II related AGs is not easy. On the other hand, these complex AGs are scientifically and commercially attractive materials whose structures can be modified by chemical and biochemical approaches for specific purposes. In the current review, what is known about the chemical structures of type II and type II related AGs from different plant sources is outlined. After that, structural analysis techniques are considered and compared. Finally, structural modifications that enhance or alter functionality are highlighted.

Arabinogalactan Structures of Repetitive Serine-Hydroxyproline Glycomodule Expressed by Arabidopsis Cell Suspension Cultures

Arabinogalactan-proteins (AGPs) are members of the hydroxyproline-rich glycoprotein (HRGP) superfamily. They are heavily glycosylated with arabinogalactans, which are usually composed of a β-1,3-linked galactan backbone with 6-O-linked galactosyl, oligo-1,6-galactosyl, or 1,6-galactan side chains that are further decorated with arabinosyl, glucuronosyl, rhamnosyl, and/or fucosyl residues. Here, our work with Hyp-O-polysaccharides isolated from (Ser-Hyp)32-EGFP (enhanced green fluorescent protein) fusion glycoproteins overexpressed in transgenic Arabidopsis suspension culture is consistent with the common structural features of AGPs isolated from tobacco. In addition, this work confirms the presence of β-1,6-linkage on the galactan backbone identified previously in AGP fusion glycoproteins expressed in tobacco suspension culture. Furthermore, the AGPs expressed in Arabidopsis suspension culture lack terminal-rhamnosyl residues and have a much lower level of glucuronosylation compared with those expressed in tobacco suspension culture. These differences not only suggest the presence of distinct glycosyl transferases for AGP glycosylation in the two systems, but also indicate the existence of minimum AG structures for type II AG functional features.

In vivo structural modification of type II arabinogalactans with fungal endo-β-1, 6-galactanase in Arabidopsis

Arabinogalactan-proteins (AGPs) are mysterious extracellular glycoproteins in plants. Although AGPs are highly conserved, their molecular functions remain obscure. The physiological importance of AGPs has been extensively demonstrated with β-Yariv reagent, which specifically binds to AGPs and upon introduction into cells, causes various deleterious effects including growth inhibition and programmed cell death. However, structural features of AGPs that determine their functions have not been identified with β-Yariv reagent. It is known that AGPs are decorated with large type II arabinogalactans (AGs), which are necessary for their functions. Type II AGs consist of a β-1,3-galactan main chain and β-1,6-galactan side chains with auxiliary sugar residues such as L-arabinose and 4-O-methyl-glucuronic acid. While most side chains are short, long side chains such as β-1,6-galactohexaose (β-1,6-Gal₆) also exist in type II AGs. To gain insight into the structures important for AGP functions, in vivo structural modification of β-1,6-galactan side chains was performed in Arabidopsis. We generated transgenic Arabidopsis plants expressing a fungal endo-β-1,6-galactanase, Tv6GAL, that degrades long side chains specifically under the control of dexamethasone (Dex). Two of 6 transgenic lines obtained showed more than 40 times activity of endo-β-1,6-galactanase when treated with Dex. Structural analysis indicated that long side chains such as β-1,6-Gal₅ and β-1,6-Gal₆ were significantly reduced compared to wild-type plants. Tv6GAL induction caused retarded growth of seedlings, which had a reduced amount of cellulose in cell walls. These results suggest that long β-1,6-galactan side chains are necessary for normal cellulose synthesis and/or deposition as their defect affects cell growth in plants.

Molecular Cloning and Characterization of a Novel Exo-β-1,3-Galactanase from Penicillium oxalicum sp. 68

Arabinogalactans have diverse biological properties and can be used as pharmaceutical agents. Most arabinogalactans are composed of β-(1→3)-galactan, so it is particularly important to identify β-1,3-galactanases that can selectively degrade them. In this study, a novel exo-β-1,3-galactanase, named PoGal3, was screened from Penicillium oxalicum sp. 68, and hetero-expressed in P. pastoris GS115 as a soluble protein. PoGal3 belongs to glycoside hydrolase family 43 (GH43) and has a 1,356-bp gene length that encodes 451 amino acids residues. To study the enzymatic properties and substrate selectivity of PoGal3, β-1,3-galactan (AG-P-I) from larch wood arabinogalactan (LWAG) was prepared and characterized by HPLC and NMR. Using AG-P-I as substrate, purified PoGal3 exhibited an optimal pH of 5.0 and temperature of 40°C. We also discovered that Zn²⁺ had the strongest promoting effect on enzyme activity, increasing it by 28.6%. Substrate specificity suggests that PoGal3 functions as an exo-β-1,3-galactanase, with its greatest catalytic activity observed on AG-P-I. Hydrolytic products of AG-P-I are mainly composed of galactose and β-1,6-galactobiose. In addition, PoGal3 can catalyze hydrolysis of LWAG to produce galacto-oligomers. PoGal3 is the first enzyme identified as an exo-β-1,3-galactanase that can be used in building glycan blocks of crucial glycoconjugates to assess their biological functions.

Functional metagenomic screening identifies an unexpected β-glucuronidase

The abundance of recorded protein sequence data stands in contrast to the small number of experimentally verified functional annotation. Here we screened a million-membered metagenomic library at ultrahigh throughput in microfluidic droplets for β-glucuronidase activity. We identified SN243, a genuine β-glucuronidase with little homology to previously studied enzymes of this type, as a glycoside hydrolase 3 family member. This glycoside hydrolase family contains only one recently added β-glucuronidase, showing that a functional metagenomic approach can shed light on assignments that are currently 'unpredictable' by bioinformatics. Kinetic analyses of SN243 characterized it as a promiscuous catalyst and structural analysis suggests regions of divergence from homologous glycoside hydrolase 3 members creating a wide-open active site. With a screening throughput of >10⁷ library members per day, picolitre-volume microfluidic droplets enable functional assignments that complement current enzyme database dictionaries and provide bridgeheads for the annotation of unexplored sequence space.

The Role of Arabinogalactan Type II Degradation in Plant-Microbe Interactions

Arabinogalactans (AGs) are structural polysaccharides of the plant cell wall. A small proportion of the AGs are associated with hemicellulose and pectin. Furthermore, AGs are associated with proteins forming the so-called arabinogalactan proteins (AGPs), which can be found in the plant cell wall or attached through a glycosylphosphatidylinositol (GPI) anchor to the plasma membrane. AGPs are a family of highly glycosylated proteins grouped with cell wall proteins rich in hydroxyproline. These glycoproteins have important and diverse functions in plants, such as growth, cellular differentiation, signaling, and microbe-plant interactions, and several reports suggest that carbohydrate components are crucial for AGP functions. In beneficial plant-microbe interactions, AGPs attract symbiotic species of fungi or bacteria, promote the development of infectious structures and the colonization of root tips, and furthermore, these interactions can activate plant defense mechanisms. On the other hand, plants secrete and accumulate AGPs at infection sites, creating cross-links with pectin. As part of the plant cell wall degradation machinery, beneficial and pathogenic fungi and bacteria can produce the enzymes necessary for the complete depolymerization of AGs including endo-β-(1,3), β-(1,4) and β-(1,6)-galactanases, β-(1,3/1,6) galactanases, α-L-arabinofuranosidases, β-L-arabinopyranosidases, and β-D-glucuronidases. These hydrolytic enzymes are secreted during plant-pathogen interactions and could have implications for the function of AGPs. It has been proposed that AGPs could prevent infection by pathogenic microorganisms because their degradation products generated by hydrolytic enzymes of pathogens function as damage-associated molecular patterns (DAMPs) eliciting the plant defense response. In this review, we describe the structure and function of AGs and AGPs as components of the plant cell wall. Additionally, we describe the set of enzymes secreted by microorganisms to degrade AGs from AGPs and its possible implication for plant-microbe interactions.

Cracking the "Sugar Code": A Snapshot of N- and O-Glycosylation Pathways and Functions in Plants Cells

Glycosylation is a fundamental co-translational and/or post-translational modification process where an attachment of sugars onto either proteins or lipids can alter their biological function, subcellular location and modulate the development and physiology of an organism. Glycosylation is not a template driven process and as such produces a vastly larger array of glycan structures through combinatorial use of enzymes and of repeated common scaffolds and as a consequence it provides a huge expansion of both the proteome and lipidome. While the essential role of N- and O-glycan modifications on mammalian glycoproteins is already well documented, we are just starting to decode their biological functions in plants. Although significant advances have been made in plant glycobiology in the last decades, there are still key challenges impeding progress in the field and, as such, holistic modern high throughput approaches may help to address these conceptual gaps. In this snapshot, we present an update of the most common O- and N-glycan structures present on plant glycoproteins as well as (1) the plant glycosyltransferases (GTs) and glycosyl hydrolases (GHs) responsible for their biosynthesis; (2) a summary of microorganism-derived GHs characterized to cleave specific glycosidic linkages; (3) a summary of the available tools ranging from monoclonal antibodies (mAbs), lectins to chemical probes for the detection of specific sugar moieties within these complex macromolecules; (4) selected examples of N- and O-glycoproteins as well as in their related GTs to illustrate the complexity on their mode of action in plant cell growth and stress responses processes, and finally (5) we present the carbohydrate microarray approach that could revolutionize the way in which unknown plant GTs and GHs are identified and their specificities characterized.

Unique active-site and subsite features in the arabinogalactan-degrading GH43 exo-β-1,3-galactanase from Phanerochaete chrysosporium

Arabinogalactan proteins (AGPs) are plant proteoglycans with functions in growth and development. However, these functions are largely unexplored, mainly because of the complexity of the sugar moieties. These carbohydrate sequences are generally analyzed with the aid of glycoside hydrolases. The exo-β-1,3-galactanase is a glycoside hydrolase from the basidiomycete Phanerochaete chrysosporium (Pc1,3Gal43A), which specifically cleaves AGPs. However, its structure is not known in relation to its mechanism bypassing side chains. In this study, we solved the apo and liganded structures of Pc1,3Gal43A, which reveal a glycoside hydrolase family 43 subfamily 24 (GH43_sub24) catalytic domain together with a carbohydrate-binding module family 35 (CBM35) binding domain. GH43_sub24 is known to lack the catalytic base Asp conserved among other GH43 subfamilies. Our structure in combination with kinetic analyses reveals that the tautomerized imidic acid group of Gln263 serves as the catalytic base residue instead. Pc1,3Gal43A has three subsites that continue from the bottom of the catalytic pocket to the solvent. Subsite -1 contains a space that can accommodate the C-6 methylol of Gal, enabling the enzyme to bypass the β-1,6-linked galactan side chains of AGPs. Furthermore, the galactan-binding domain in CBM35 has a different ligand interaction mechanism from other sugar-binding CBM35s, including those that bind galactomannan. Specifically, we noted a Gly → Trp substitution, which affects pyranose stacking, and an Asp → Asn substitution in the binding pocket, which recognizes β-linked rather than α-linked Gal residues. These findings should facilitate further structural analysis of AGPs and may also be helpful in engineering designer enzymes for efficient biomass utilization.

Expression of a fungal exo-β-1,3-galactanase in Arabidopsis reveals a role of type II arabinogalactans in the regulation of cell shape

Arabinogalactan-proteins (AGPs) are a family of plant extracellular proteoglycans implicated in many physiological events. AGP is decorated with type II arabinogalactans (AGs) consisting of a β-1,3-galactan backbone and β-1,6-galactan side chains, to which other sugars are attached. Based on the fact that a type II AG-specific inhibitor, β-Yariv reagent, perturbs growth and development, it has been proposed that type II AGs participate in the regulation of cell shape and tissue organization. However, the mechanisms by which type II AGs participate have not yet been established. Here, we describe a novel system that causes specific degradation of type II AGs in Arabidopsis, by which a gene encoding a fungal exo-β-1,3-galactanase that specifically hydrolyzes β-1,3-galactan backbones of type II AGs is expressed under the control of a dexamethasone-inducible promoter. Dexamethasone treatment increased the galactanase activity, leading to a decrease in Yariv reagent-reactive AGPs in transgenic Arabidopsis. We detected the typical oligosaccharides released from type II AGs by Il3GAL in the soluble fraction, demonstrating that Il3GAL acted on type II AG in the transgenic plants. Additionally, this resulted in severe tissue disorganization in the hypocotyl and cotyledons, suggesting that the degradation of type II AGs affected the regulation of cell shape.

Properties of arabinogalactan-proteins in European pear (Pyrus communis L.) fruits

Arabinogalactans (AGs) and arabinogalactan-proteins (AGPs) were partially purified from an extract of fruits of the European pear (Pyrus communis L.) by DEAE-cellulose ion-exchange and Sepharose 6B gel-filtration chromatography. Among 7 AG(P)-containing fractions, a neutral AGP (SE-1) was confirmed to be highly purified (M_r 67,000) and rich in L-Ara and Gal; this fraction included a small amount (2.6%, w/w) of protein and showed the highest reactivity forming precipitate with β-Glc Yariv reagent among the 7 fractions, the intensity of which was comparable to that of gum arabic, a standard AGP. Another accompanying minor low-M_r neutral AGP (SE-2; M_r approx. 7200) still contained other polysaccharide (starch fragments) and did not show Yariv reactivity. The carbohydrate moieties of SE-1 consisted of consecutive (1 → 3)-linked β-galactosyl backbone chains substituted with side chains of (1 → 6)-linked β-galactosyl residues at O-6, to which mainly single α-l-arabinofuranosyl residues were attached through O-3. This structural feature was also observed for SE-2. Successive digestion of SE-1 with α-l-arabinofuranosidase and exo-β-(1 → 3)-galactanase with the aid of endo-β-(1 → 3)-galactanase released most (more than 98%, w/w) of the carbohydrate moieties as low-M_r fragments. These consisted of free L-Ara and Gal, and a series of β-(1 → 6)-galactooligosaccharides with degree of polymerization (dp) up to at least 17, indicative of attachment of (1 → 6)-linked β-galactosyl side chains of varying length along the (1 → 3)-linked β-galactosyl backbone chains.

Degradation of plant arabinogalactan proteins by intestinal bacteria: characteristics and functions of the enzymes involved

Arabinogalactan proteins (AGPs) are complex plant proteoglycans that function as dietary fiber utilized by human intestinal bacteria such as Bifidobacterium and Bacteroides species. However, the degradative mechanism is unknown because of the complexity of sugar chains of AGPs as well as variation among plant species and organs. Recently, AGP degradative enzymes have been characterized in Bifidobacterium and Bacteroides species. In this review, we summarize the characteristics and functions of AGP degradative enzymes in human intestinal bacteria.

Substrate specificity of novel GH16 endo-β-(1→3)-galactanases acting on linear and branched β-(1→3)-galactooligosaccharides

Arabinogalactan proteins are proteoglycans located in the plant cell wall. Most arabinogalactan proteins are composed of carbohydrate moieties of β-(1→3)-galactan main chains with β-(1→6)-galactan side chains terminated by other glycans. In this study, three novel endo-β-(1→3)-galactanases were identified and the substrate specificity was further studied using well-defined galactan oligomers. Linear and branched β-(1→3)-linked galactans, which resemble the carbohydrate core of the arabinogalactan protein, were used for the characterization of endo-β-(1→3)-galactanases. The identified enzymes required at least three consecutive galactose residues for activity. Non-substituted regions were preferred, but substituents in the -2 and +2 and in some cases also -1 and +1 subsites were tolerated to some extent, depending on the branching pattern, however at a significantly lower rate/frequency.

Arabidopsis thaliana FLA4 functions as a glycan-stabilized soluble factor via its carboxy-proximal Fasciclin 1 domain

Fasciclin-like arabinogalactan proteins (FLAs) are involved in numerous important functions in plants but the relevance of their complex structure to physiological function and cellular fate is unresolved. Using a fully functional fluorescent version of Arabidopsis thaliana FLA4 we show that this protein is localized at the plasma membrane as well as in endosomes and soluble in the apoplast. FLA4 is likely to be GPI-anchored, is highly N-glycosylated and carries two O-glycan epitopes previously associated with arabinogalactan proteins. The activity of FLA4 was resistant against deletion of the amino-proximal fasciclin 1 domain and was unaffected by removal of the GPI-modification signal, a highly conserved N-glycan or the deletion of predicted O-glycosylation sites. Nonetheless these structural changes dramatically decreased endoplasmic reticulum (ER)-exit and plasma membrane localization of FLA4, with N-glycosylation acting at the level of ER-exit and O-glycosylation influencing post-secretory fate. We show that FLA4 acts predominantly by molecular interactions involving its carboxy-proximal fasciclin 1 domain and that its amino-proximal fasciclin 1 domain is required for stabilization of plasma membrane localization. FLA4 functions as a soluble glycoprotein via its carboxy-proximal Fas1 domain and its normal cellular trafficking depends on N- and O-glycosylation.

A protease/peptidase from culture medium of Flammulina velutipes that acts on arabinogalactan-protein

Arabinogalactan-proteins (AGPs) are highly diverse plant proteoglycans found on the plant cell surface. AGPs have large arabinogalactan (AG) moieties attached to a core-protein rich in hydroxyproline (Hyp). The AG undergoes hydrolysis by various glycoside hydrolases, most of which have been identified, whereas the core-proteins is presumably degraded by unknown proteases/peptidases secreted from fungi and bacteria in nature. Although several enzymes hydrolyzing other Hyp-rich proteins are known, the enzymes acting on the core-proteins of AGPs remain to be identified. The present study describes the detection of protease/peptidase activity toward AGP core-proteins in the culture medium of winter mushroom (Flammulina velutipes) and partial purification of the enzyme by several conventional chromatography steps. The enzyme showed higher activity toward Hyp residues than toward proline and alanine residues and acted on core-proteins prepared from gum arabic. Since the activity was inhibited in the presence of Pefabloc SC, the enzyme is probably a serine protease.

L-Fucose-containing arabinogalactan-protein in radish leaves

The carbohydrate moieties of arabinogalactan-proteins (AGPs) have β-(1 → 3)-galactan backbones to which side chains of (1 → 6)-linked β-Gal residues are attached through O-6. Some of these side chains are further substituted with other sugars. We investigated the structure of L-Fuc-containing oligosaccharides released from the carbohydrate moieties of a radish leaf AGP by digestion with α-L-arabinofuranosidase, followed by exo-β-(1 → 3)-galactanase. We detected a series of neutral β-(1 → 6)-galactooligosaccharides branching variously at O-3 of the Gal residues, together with corresponding acidic derivatives terminating in 4-O-methyl-GlcA (4-Me-GlcA) or GlcA at the non-reducing terminals. In neutral oligosaccharides with degree of polymerization (dp) mainly higher than 10, L-Fuc groups were attached through L-Ara residues as the sequence, α-L-Fucp-(1 → 2)-α-L-Araf-(1 →. This sequence was verified by isolation of the pentasaccharide α-L-Fuc-(1 → 2)-α-L-Araf-(1 → 3)-β-Gal-(1 → 6)-β-Gal-(1 → 6)-Gal upon digestion of the higher oligosaccharides with endo-β-(1 → 6)-galactanase. By contrast, in lower polymerized (predominantly dp 4) acidic oligosaccharides, L-Fuc groups were attached directly at the non-reducing terminals through α-(1 → 2)-linkages, resulting in the release of the tetrasaccharides, α-L-Fucp-(1 → 2)-β-GlcA-(1 → 6)-β-Gal-(1 → 6)-Gal and α-L-Fucp-(1 → 2)-β-4-Me-GlcA-(1 → 6)-β-Gal-(1 → 6)-Gal. In long acidic oligosaccharides with dp mainly higher than 13, L-Fuc groups localized on branches were attached to the uronic acids directly and/or L-Ara residues as in the neutral oligosaccharides.

Bifidobacterium longum subsp. longum Exo-β-1,3-Galactanase, an enzyme for the degradation of type II arabinogalactan

Type II arabinogalactan (AG-II) is a suitable carbohydrate source for Bifidobacterium longum subsp. longum, but the degradative enzymes have never been characterized. In this study, we characterized an exo-β-1,3-galactanase, BLLJ_1840, belonging to glycoside hydrolase family 43 from B. longum subsp. longum JCM1217. The recombinant BLLJ_1840 expressed in Escherichia coli hydrolyzed β-1,3-linked galactooligosaccharides but not β-1,4- and β-1,6-linked galactooligosaccharides. The enzyme also hydrolyzed larch wood arabinogalactan (LWAG), which comprises a β-1,3-linked galactan backbone with β-1,6-linked galactan side chains. The kcat/Km ratio of dearabinosylated LWAG was 24-fold higher than that of β-1,3-galactan. BLLJ_1840 is a novel type of exo-β-1,3-galactanase with a higher affinity for the β-1,6-substituted β-1,3-galactan than for nonsubstituted β-1,3-galactan. BLLJ_1840 has 27% to 28% identities with other characterized exo--1,3-galactanases from bacteria and fungi. The homologous genes are conserved in several strains of B. longum subsp. longum and B. longum subsp. infantis but not in other bifidobacteria. Transcriptional analysis revealed that BLLJ_1840 is intensively induced with BLLJ_1841, an endo-β-1,6-galactanase candidate, in the presence of LWAG. This is the first report of exo-β-1,3-galactanase in bifidobacteria, which is an enzyme used for the acquisition of AG-II in B. longum subsp. longum.

Characterization of an Exo-β-1,3-D-galactanase fromStreptomyces avermitilisNBRC14893 Acting on Arabinogalactan-Proteins

A gene belonging to glycoside hydrolase family 43 (GH43) was isolated from Streptomyces avermitilis NBRC14893. The gene encodes a modular protein consisting of N-terminal GH43 module and a family 13 carbohydrate-binding module at the C-terminus. The gene corresponding to the GH43 module was expressed in Escherichia coli, and the gene product was characterized. The recombinant enzyme specifically hydrolyzed only beta-1,3-linkage of two D-galactosyl residues at non-reducing ends of the substrates. The analysis of the hydrolysis products indicated that the enzyme produced galactose from beta-1,3-D-galactan in an exo-acting manner. When the enzyme catalyze hydrolysis of the arabinogalactan-protein, the enzyme produced oligosaccharides together with galactose, suggesting that the enzyme is able to accommodate beta-1,6-linked D-galactosyl side chains. These properties are the same as the other previously reported exo-beta-1,3-D-galactanases. Therefore, we concluded the isolated gene certainly encodes an exo-beta-1,3-D-galactanase. This is the first report of exo-beta-1,3-D-galactanase from actinomycetes.

Mode of Action of β-Glucuronidase fromAspergillus nigeron the Sugar Chains of Arabinogalactan-Protein

A beta-glucuronidase purified from a commercial pectolytic enzyme preparation of Aspergillus niger hydrolyzed about half of the 4-O-methyl-glucuronic acid (4-Me-GlcA) residues located at the nonreducing terminals of (1-->6)-linked beta-galactosyl side chains of the carbohydrate portion of a radish arabinogalactan-protein (AGP) modified by treatment with fungal alpha-L-arabinosidase. Digestion of the alpha-L-arabinosidase-treated AGP with exo-beta-(1-->3)-galactanase released, by exo-fission of beta-(1-->3)-galactosidic bonds in the backbone chains of the AGP, neutral beta-(1-->6)-galactooligosaccharides with various chain lengths and their acidic derivatives substituted at their nonreducing terminals with 4-Me-beta-GlcA groups. In contrast, successive digestion of the alpha-L-arabinosidase-treated AGP with beta-glucuronidase followed by exo-beta-(1-->3)-galactanase liberated much higher amounts of beta-(1-->6)-galactooligomers together with a small portion of short acidic oligomers, mainly 4-Me-beta-GlcA-(1-->6)-Gal and 4-Me-beta-GlcA-(1-->6)-beta-Gal-(1-->6)-Gal. These results indicate that beta-glucuronidase acts upon 4-Me-beta-GlcA residues in long (1-->6)-linked beta-galactosyl side chains of the AGP, whereas short acidic side chains survive the attack of the enzyme.

Enzymatic fragmentation of carbohydrate moieties of radish arabinogalactan-protein and elucidation of the structures

We investigated the structures of L-arabino-galactooligosaccharides released from the sugar moieties of a radish arabinogalactan-protein (AGP) by the action of exo-β-(1→3)-galactanase. We detected a series of neutral β-(1→6)-linked galactooligosaccharides forming branches of one to up to at least 19 consecutive Gal groups, together with corresponding acidic derivatives terminating in 4-O-methyl-glucuronic acid (4-Me-GlcA) at the non-reducing end. Some oligosaccharide chains of degree of polymerization (dp) higher than 3 for neutral, and 4 for acidic oligomers were modified with L-Araf residues. The acidic tetrasaccharide 4-Me-β-GlcA-(1→6)[α-L-Araf-(1→3)]-β-Gal-(1→6)-Gal was detected as an abundant L-Araf-containing oligosaccharide among these neutral and acidic oligomers. A pentasaccharide containing an additional L-Araf group attached to the L-Ara in the tetrasaccharide through an α-(1→5)-linkage was also found. We observed L-arabino-galactooligosaccharides substituted with single or disaccharide L-Araf units at different Gal residues along these neutral and acidic β-(1→6)-galactooligosaccharide chains, indicating that these side chains are highly variable in length and substituted variously with L-Araf residues.

Characterisation of FUT4 and FUT6 α-(1 → 2)-fucosyltransferases reveals that absence of root arabinogalactan fucosylation increases Arabidopsis root growth salt sensitivity

Plant type II arabinogalactan (AG) polysaccharides are attached to arabinogalactan proteins (AGPs) at hydroxyproline residues, and they are very diverse and heterogeneous structures. The AG consists of a β-(1→3)-linked galactan backbone with β-(1→6)-galactan side chains that are modified mainly with arabinose, but they may also contain glucuronic acid, rhamnose or other sugars. Here, we studied the positions of fucose substitutions in AGPs, and we investigated the functions of this fucosylation. Monosaccharide analysis of Arabidopsis leaf AGP extracts revealed a significant reduction in L-Fucose content in the fut4 mutant, but not in the fut6 mutant. In addition, Fucose was reduced in the fut4 mutant in root AGP extracts and was absent in the fut4/fut6 mutant. Curiously, in all cases reduction of fucose was accompanied with a reduction in xylose levels. The fucosylated AGP structures in leaves and roots in wild type and fut mutant plants were characterised by sequential digestion with AG specific enzymes, analysis by Polysaccharide Analysis using Carbohydrate gel Electrophoresis, and Matrix Assisted Laser Desorption/Ionisation (MALDI)-Time of Flight Mass spectrometry (MS). We found that FUT4 is solely responsible for the fucosylation of AGPs in leaves. The Arabidopsis thaliana FUT4 and FUT6 genes have been previously proposed to be non-redundant AG-specific fucosyltransferases. Unexpectedly, FUT4 and FUT6 enzymes both fucosylate the same AGP structures in roots, suggesting partial redundancy to each other. Detailed structural characterisation of root AGPs with high energy MALDI-Collision Induced Dissociation MS and NMR revealed an abundant unique AG oligosaccharide structure consisting of terminal xylose attached to fucose. The loss of this structure in fut4/fut6 mutants explains the reduction of both fucose and xylose in AGP extracts. Under salt-stress growth conditions the fut4/fut6 mutant lacking AGP fucosylation exhibited a shorter root phenotype than wild type plants, implicating fucosylation of AGPs in maintaining proper cell expansion under these conditions.

Arabinogalactan proteins: focus on carbohydrate active enzymes

Arabinogalactan proteins (AGPs) are a highly diverse class of cell surface proteoglycans that are commonly found in most plant species. AGPs play important roles in many cellular processes during plant development, such as reproduction, cell proliferation, pattern formation and growth, and in plant-microbe interaction. However, little is known about the molecular mechanisms of their function. Numerous studies using monoclonal antibodies that recognize different AGP glycan epitopes have shown the appearance of a slightly altered AGP glycan in a specific stage of development in plant cells. Therefore, it is anticipated that the biosynthesis and degradation of AGP glycan is tightly regulated during development. Until recently, however, little was known about the enzymes involved in the metabolism of AGP glycans. In this review, we summarize recent discoveries of carbohydrate active enzymes (CAZy; http://www.cazy.org/) involved in the biosynthesis and degradation of AGP glycans, and we discuss the biological role of these enzymes in plant development.

Intestinal Peyer's patch-immunomodulating glucomannans from rhizomes of Anemarrhena asphodeloides Bunge

During screening for intestinal Peyer's patch-immunomodulating polysaccharides from plant resources including medicinal herbs, a potent modulating activity was observed in a crude polysaccharide fraction (AS-1) from the rhizome of Anemarrhena asphodeloides Bunge. Oral administration of AS-1 (100 mg/kg/day) to aged BALB/c mice enhanced productions of IL-10, IFN-γ and IL-6 from Peyer's patch immunocompetent cells, and its oral administration to ovalbumin (OVA)-fed B10.A mice led to significant suppression on induction of OVA-specific IgE in systemic immune system. Further fractionation of the polysaccharides in the crude polysaccharide fraction, AS-1, yielded 4 polysaccharide fractions that were potently active, and contained glucomannans. Treatment of these polysaccharide fractions with endo-β-D-(1→4)-mannanase significantly decreased their activities. Mannanase digestion of the active glucomannan gave both long and short hexosyl-oligosaccharides, whereas konjac glucomannan, which was inactive, released short oligosaccharides. Structural analysis indicates that the long oligosaccharides from the active glucomannan contain mannanase-resistant complex structure comprising β-D-Man and β-D-Glc.

Biosynthesis of the carbohydrate moieties of arabinogalactan proteins by membrane-bound β-glucuronosyltransferases from radish primary roots

A membrane fraction from etiolated 6-day-old primary radish roots (Raphanus sativus L. var hortensis) contained β-glucuronosyltransferases (GlcATs) involved in the synthesis of the carbohydrate moieties of arabinogalactan proteins (AGPs). The GlcATs transferred [(14)C]GlcA from UDP-[(14)C]GlcA on to β-(1 → 3)-galactan as an exogenous acceptor substrate, giving a specific activity of 50-150 pmol min(-1) (mg protein)(-1). The enzyme specimen also catalyzed the transfer of [(14)C]GlcA on to an enzymatically modified AGP from mature radish root. Analysis of the transfer products revealed that the transfer of [(14)C]GlcA occurred preferentially on to consecutive (1 → 3)-linked β-Gal chains as well as single branched β-(1 → 6)-Gal residues through β-(1 → 6) linkages, producing branched acidic side chains. The enzymes also transferred [(14)C]GlcA residues on to several oligosaccharides, such as β-(1 → 6)- and β-(1 → 3)-galactotrioses. A trisaccharide, α-L-Araf-(1 → 3)-β-Gal-(1 → 6)-Gal, was a good acceptor, yielding a branched tetrasaccharide, α-L-Araf-(1 → 3)[β-GlcA-(1 → 6)]-β-Gal-(1 → 6)-Gal. We report the first in vitro assay system for β-GlcATs involved in the AG synthesis as a step toward full characterization and cloning.

A β-glucuronosyltransferase from Arabidopsis thaliana involved in biosynthesis of type II arabinogalactan has a role in cell elongation during seedling growth

We have characterized a β-glucuronosyltransferase (AtGlcAT14A) from Arabidopsis thaliana that is involved in the biosynthesis of type II arabinogalactan (AG). This enzyme belongs to the Carbohydrate Active Enzyme database glycosyltransferase family 14 (GT14). The protein was localized to the Golgi apparatus when transiently expressed in Nicotiana benthamiana. The soluble catalytic domain expressed in Pichia pastoris transferred glucuronic acid (GlcA) to β-1,6-galactooligosaccharides with degrees of polymerization (DP) ranging from 3-11, and to β-1,3-galactooligosaccharides of DP5 and 7, indicating that the enzyme is a glucuronosyltransferase that modifies both the β-1,6- and β-1,3-galactan present in type II AG. Two allelic T-DNA insertion mutant lines showed 20-35% enhanced cell elongation during seedling growth compared to wild-type. Analyses of AG isolated from the mutants revealed a reduction of GlcA substitution on Gal-β-1,6-Gal and β-1,3-Gal, indicating an in vivo role of AtGlcAT14A in synthesis of those structures in type II AG. Moreover, a relative increase in the levels of 3-, 6- and 3,6-linked galactose (Gal) and reduced levels of 3-, 2- and 2,5-linked arabinose (Ara) were seen, suggesting that the mutation in AtGlcAT14A results in a relative increase of the longer and branched β-1,3- and β-1,6-galactans. This increase of galactosylation in the mutants is most likely caused by increased availability of the O6 position of Gal, which is a shared acceptor site for AtGlcAT14A and galactosyltransferases in synthesis of type II AG, and thus addition of GlcA may terminate Gal chain extension. We discuss a role for the glucuronosyltransferase in the biosynthesis of type II AG, with a biological role during seedling growth.

Differential proteomic analysis of the secretome of Irpex lacteus and other white-rot fungi during wheat straw pretreatment

BACKGROUND

Identifying new high-performance enzymes or enzyme complexes to enhance biomass degradation is the key for the development of cost-effective processes for ethanol production. Irpex lacteus is an efficient microorganism for wheat straw pretreatment, yielding easily hydrolysable products with high sugar content. Thus, this fungus was selected to investigate the enzymatic system involved in lignocellulose decay, and its secretome was compared to those from Phanerochaete chrysosporium and Pleurotus ostreatus which produced different degradation patterns when growing on wheat straw. Extracellular enzymes were analyzed through 2D-PAGE, nanoLC/MS-MS, and homology searches against public databases.

RESULTS

In wheat straw, I. lacteus secreted proteases, dye-decolorizing and manganese-oxidizing peroxidases, and H2O2 producing-enzymes but also a battery of cellulases and xylanases, excluding those implicated in cellulose and hemicellulose degradation to their monosaccharides, making these sugars poorly available for fungal consumption. In contrast, a significant increase of β-glucosidase production was observed when I. lacteus grew in liquid cultures. P. chrysosporium secreted more enzymes implicated in the total hydrolysis of the polysaccharides and P. ostreatus produced, in proportion, more oxidoreductases.

CONCLUSION

The protein pattern secreted during I. lacteus growth in wheat straw plus the differences observed among the different secretomes, justify the fitness of I. lacteus for biopretreatment processes in 2G-ethanol production. Furthermore, all these data give insight into the biological degradation of lignocellulose and suggest new enzyme mixtures interesting for its efficient hydrolysis.

Arabinogalactan protein cluster from Jatropha curcas seed embryo contains fasciclin, xylogen and LysM proteins

An non-GPI-anchored AGP cluster (Y2) was isolated from the seeds of Jatropha curcas L. (Euphorbiaceae) composed of 4.8% polypeptides (mainly Ala, Ser, Gly, Hyp, Glu) and a carbohydrate moiety composed of Gal, Ara, GlcA, Rha, Man and GlcN. Besides the typical structural features of arabinogalactan proteins, typical N-glycan linker of the complex type (GlcNAc4Man3Gal2Fuc1Xyl1) were identified. O-glycosylation occurred mainly via Hyp and to a lesser extent via Thr and Ser. N-glycans from the complex type, carrying at the innermost GlcNAc at position O-3 one α-Fuc-residue, were also present. MS analysis of the tryptic digest assigned peptides of three major protein groups: fasciclin-like arabinogalactan proteins, xylogen-like proteins and LysM domain-containing proteins. They could not be separated further and it is indicated that various homologous protein forms co-exist. Histological investigation of J. curcas seeds revealed the presence of AGPs in the vessels of cotyledons and in the procambium ring of the embryo.

Peculiarities and applications of galactanolytic enzymes that act on type I and II arabinogalactans

Arabinogalactans (AGs) are branched galactans to which arabinose residues are bound as side chains and are widely distributed in plant cell walls. They can be grouped into two types based on the structures of their backbones. Type I AGs have β-1,4-galactan backbones and are often covalently linked to the rhamnogalacturonan-I region of pectins. Type II AGs have β-1,3-galactan backbones and are often covalently linked to proteins. The main enzymes involved in the degradation of AGs are endo-β-galactanases, exo-β-galactanases, and β-galactosidases, although other enzymes such as α-L-arabinofuranosidases, β-L-arabinopyranosidases, and β-D-glucuronidases are required to remove the side chains for efficient degradation of the polysaccharides. Galactanolytic enzymes have a wide variety of potential uses, including the bioconversion of AGs to fermentable sugars for production of commodity chemicals like ethanol, biobleaching of cellulose pulp, modulation of pectin properties, improving animal feed, and determining the chemical structure of AGs. This review summarizes our current knowledge about the biochemical properties and potential applications of AG-degrading enzymes.

β-galactosyl Yariv reagent binds to the β-1,3-galactan of arabinogalactan proteins

Yariv phenylglycosides [1,3,5-tri(p-glycosyloxyphenylazo)-2,4,6-trihydroxybenzene] are a group of chemical compounds that selectively bind to arabinogalactan proteins (AGPs), a type of plant proteoglycan. Yariv phenylglycosides are widely used as cytochemical reagents to perturb the molecular functions of AGPs as well as for the detection, quantification, purification, and staining of AGPs. However, the target structure in AGPs to which Yariv phenylglycosides bind has not been determined. Here, we identify the structural element of AGPs required for the interaction with Yariv phenylglycosides by stepwise trimming of the arabinogalactan moieties using combinations of specific glycoside hydrolases. Whereas the precipitation with Yariv phenylglycosides (Yariv reactivity) of radish (Raphanus sativus) root AGP was not reduced after enzyme treatment to remove α-l-arabinofuranosyl and β-glucuronosyl residues and β-1,6-galactan side chains, it was completely lost after degradation of the β-1,3-galactan main chains. In addition, Yariv reactivity of gum arabic, a commercial product of acacia (Acacia senegal) AGPs, increased rather than decreased during the repeated degradation of β-1,6-galactan side chains by Smith degradation. Among various oligosaccharides corresponding to partial structures of AGPs, β-1,3-galactooligosaccharides longer than β-1,3-galactoheptaose exhibited significant precipitation with Yariv in a radial diffusion assay on agar. A pull-down assay using oligosaccharides cross linked to hydrazine beads detected an interaction of β-1,3-galactooligosaccharides longer than β-1,3-galactopentaose with Yariv phenylglycoside. To the contrary, no interaction with Yariv was detected for β-1,6-galactooligosaccharides of any length. Therefore, we conclude that Yariv phenylglycosides should be considered specific binding reagents for β-1,3-galactan chains longer than five residues, and seven residues are sufficient for cross linking, leading to precipitation of the Yariv phenylglycosides.

Identification of an exo-ß-1,3-D-galactanase from Fusarium oxysporum and the synergistic effect with related enzymes on degradation of type II arabinogalactan

An exo-ß-1,3-D-galactanase (Fo/1,3Gal) was purified from the culture filtrate of Fusarium oxysporum 12S. A cDNA encoding Fo/1,3Gal was isolated by in vitro cloning. Module sequence analysis revealed a "GH43_6" domain and a "CBM35_galactosidase-like" domain in Fo/1,3Gal. The recombinant enzyme (rFo/1,3Gal) expressed in Pichia pastoris degraded ß-1,3-galactan and ß-1,3-galactobiose (Gal2), and released only galactose (Gal). In contrast, the enzyme did not hydrolyze p-nitrophenyl ß-D-galactopyranoside, ß-1,4-Gal2, or ß-1,6-Gal2. The enzyme also showed low activity towards native type II arabinogalactans such as larchwood arabinogalactan (LWAG) and gum arabic. Using LWAG as substrate, rFo/1,3Gal released Gal, ß-1,6-Gal2, ß-1,6-galactotriose (Gal3), and ß-1,6-Gal3 substituted with a single arabinofuranose residue accompanied with unidentified oligosaccharides, indicating that the enzyme can by-pass the branching points of ß-1,3-galactan backbones. A time course analysis of products released by rFo/1,3Gal on LWAG revealed that ß-1,6-Gal2 is the main side chain in LWAG and that the activity of rFo/1,3Gal was decreased when degrees of polymerization of side chains increase. rFo/1,3Gal worked synergistically with three other recombinant F. oxysporum enzymes (ß-1,6-galactanase, ß-L-arabinopyranosidase, and α-L-arabinofuranosidase) that degrade side chains, on the degradation of LWAG. However, the synergism was much lower than anticipated, probably because LWAG have longer side chains than the three enzymes used are able to remove or ß-1,3-galactan main chain is interrupted with glycosidic linkages that are different from the ß-1,3-galactosyl linkage. Affinity gel electrophoresis revealed that rFo/1,3Gal specifically bound to ß-1,3-galactan.

Structural characterization of Arabidopsis leaf arabinogalactan polysaccharides

Proteins decorated with arabinogalactan (AG) have important roles in cell wall structure and plant development, yet the structure and biosynthesis of this polysaccharide are poorly understood. To facilitate the analysis of biosynthetic mutants, water-extractable arabinogalactan proteins (AGPs) were isolated from the leaves of Arabidopsis (Arabidopsis thaliana) plants and the structure of the AG carbohydrate component was studied. Enzymes able to hydrolyze specifically AG were utilized to release AG oligosaccharides. The released oligosaccharides were characterized by high-energy matrix-assisted laser desorption ionization-collision-induced dissociation mass spectrometry and polysaccharide analysis by carbohydrate gel electrophoresis. The Arabidopsis AG is composed of a β-(1→3)-galactan backbone with β-(1→6)-d-galactan side chains. The β-(1→6)-galactan side chains vary in length from one to over 20 galactosyl residues, and they are partly substituted with single α-(1→3)-l-arabinofuranosyl residues. Additionally, a substantial proportion of the β-(1→6)-galactan side chain oligosaccharides are substituted at the nonreducing termini with single 4-O-methyl-glucuronosyl residues via β-(1→6)-linkages. The β-(1→6)-galactan side chains are occasionally substituted with α-l-fucosyl. In the fucose-deficient murus1 mutant, AGPs lack these fucose modifications. This work demonstrates that Arabidopsis mutants in AGP structure can be identified and characterized. The detailed structural elucidation of the AG polysaccharides from the leaves of Arabidopsis is essential for insights into the structure-function relationships of these molecules and will assist studies on their biosynthesis.

An exo-β-(1→3)-D-galactanase from Streptomyces sp. provides insights into type II arabinogalactan structure

An exo-β-(1→3)-D-galactanase (SGalase1) that specifically cleaves the β-(1→3)-D-galactan backbone of arabinogalactan-proteins (AGPs) was isolated from culture filtrates of a soil Streptomyces sp. Internal peptide sequence information was used to clone and recombinantly express the gene in E. coli. The molecular mass of the isolated enzyme was ~45 kDa, similar to the 48.2 kDa mass predicted from the amino acid sequence. The pI, pH and temperature optima for the enzyme were ~7.45, 3.8 and 48 °C, respectively. The native and recombinant enzymes specifically hydrolysed β-(1→3)-D-galacto-oligo- or poly-saccharides from the upstream (non-reducing) end, typical of an exo-acting enzyme. A second homologous Streptomyces gene (SGalase2) was also cloned and expressed. SGalase2 was similar in size (47.9 kDa) and enzyme activity to SGalase1 but differed in its pH optimum (pH 5). Both SGalase1 and SGalase2 are predicted to belong to the CAZy glycosyl hydrolase family GH 43 based on activity, sequence homology and phylogenetic analysis. The K(m) and V(max) of the native exo-β-(1→3)-D-galactanase for de-arabinosylated gum arabic (dGA) were 19 mg/ml and 9.7 μmol D-Gal/min/mg protein, respectively. The activity of these enzymes is well suited for the study of type II galactan structures and provides an important tool for the investigation of the biological role of AGPs in plants. De-arabinosylated gum arabic (dGA) was used as a model to investigate the use of these enzymes in defining type II galactan structure. Exhaustive hydrolysis of dGA resulted in a limited number of oligosaccharide products with a trisaccharide of Gal(2)GlcA(1) predominating.

Beta-D-(1→4)-galactan-containing side chains in RG-I regions of pectic polysaccharides from Biophytum petersianum Klotzsch. contribute to expression of immunomodulating activity against intestinal Peyer's patch cells and macrophages

The aerial parts of the medicinal plant Biophytum petersianum have a long tradition for being used in Mali and other West-African countries against various ailments such as wound healing and malaria. Previous studies on polysaccharides from water extracts of the aerial parts showed the presence of pectic like polymers with an effect on the human complement system as well as the ability to activate macrophages and dendritic cells. The present study shows that pectic polysaccharide fragments (BPII.1 and BPII.2) as well as the original pectic polysaccharide (BPII) expressed immunomodulating activity against Peyer's patch immunocompetent cells. Exo-β-D-(1→3)-galactanase digestion succeeded to decrease IL-6 production enhancing activity against Peyer's patch cells of BPII.2, but the activity of BPII.1 did not decrease. Endo-β-D-(1→4)-galactanase digestion reduced the activities of both BPII.1 and BPII.2. BPII.1 and BPII.2 also stimulated IL-6 production enhancing activity against macrophages, and the activities of both pectic fragments were significantly decreased by either enzymic digestion with exo-β-D-(1→3)-galactanase or endo-β-D-(1→4)-galactanase. Trimming of terminal GlcA by exo-β-D-glucuronidase digestion did not affect IL-6 production enhancing activity against macrophages of both pectic fragments. Methylation analyses of endo-β-D-(1→4)-galactanase digestion products showed the characteristic decrement of 4-linked Gal residues in the pectic fragments. These results suggest that β-D-(1→4)-galactan-containing side chains in BPII.1 and BPII.2 play an important role for expression of immunomodulating activity against both Peyer's patch immunocompetent cells and macrophages in addition to β-D-(1→3,6)-galactan chains.

Endo-beta-1,3-galactanase from winter mushroom Flammulina velutipes

Arabinogalactan proteins are proteoglycans found on the cell surface and in the cell walls of higher plants. The carbohydrate moieties of most arabinogalactan proteins are composed of β-1,3-galactan main chains and β-1,6-galactan side chains, to which other auxiliary sugars are attached. For the present study, an endo-β-1,3-galactanase, designated FvEn3GAL, was first purified and cloned from winter mushroom Flammulina velutipes. The enzyme specifically hydrolyzed β-1,3-galactan, but did not act on β-1,3-glucan, β-1,3:1,4-glucan, xyloglucan, and agarose. It released various β-1,3-galactooligosaccharides together with Gal from β-1,3-galactohexaose in the early phase of the reaction, demonstrating that it acts on β-1,3-galactan in an endo-fashion. Phylogenetic analysis revealed that FvEn3GAL is member of a novel subgroup distinct from known glycoside hydrolases such as endo-β-1,3-glucanase and endo-β-1,3:1,4-glucanase in glycoside hydrolase family 16. Point mutations replacing the putative catalytic Glu residues conserved for enzymes in this family with Asp abolished activity. These results indicate that FvEn3GAL is a highly specific glycoside hydrolase 16 endo-β-1,3-galactanase.

Characterization of an exo-β-1,3-D: -galactanase from Sphingomonas sp. 24T and its application to structural analysis of larch wood arabinogalactan

A type II arabinogalactan-degrading enzyme, termed Exo-1,3-Gal, was purified to homogeneity from the culture filtrate of Sphingomonas sp. 24T. It has an apparent molecular mass of 48 kDa by SDS-PAGE. Exo-1,3-Gal was stable from pH 3 to 10 and at temperatures up to 40 °C. The optimum pH and temperature for enzyme activity were pH 6 to 7 and 50 °C, respectively. Galactose was released from β-1,3-D: -galactan and β-1,3-D: -galactooligosaccharides by the action of Exo-1,3-Gal, indicating that the enzyme was an exo-β-1,3-D: -galactanase. Analysis of the reaction products of β-1,3-galactotriose by high-performance anion-exchange chromatography revealed that the enzyme hydrolyzed the substrate in a non-processive mode. Exo-1,3-Gal bypassed the branching points of β-1,3-galactan backbones in larch wood arabinogalactan (LWAG) to produce mainly galactose, β-1,6-galactobiose, and unidentified oligosaccharides 1 and 2 with the molar ratios of 7:19:62:12. Oligosaccharides 1 and 2 were enzymatically determined to be β-1,6-galactotriose and β-1,6-galactotriose substituted with a single arabinofuranose residue, respectively. The ratio of side chains enzymatically released from LWAG was in good agreement with the postulated structure of the polysaccharide previously determined by chemical methods.

Carbohydrate structural analysis of wheat flour arabinogalactan protein

The water-extractable arabinogalactan protein (AGP) was isolated from bread wheat flour (Triticum aestivum L. variety Cadenza) and the structure of the arabinogalactan (AG) carbohydrate component was studied. Oligosaccharides, released by hydrolysis of the AG with a range of AGP-specific enzymes, were characterised by Matrix Assisted Laser Desorption Ionisation (MALDI)-Time of Flight (ToF)-Mass Spectrometry (MS), MALDI-ToF/ToF high energy collision induced dissociation (CID) and Polysaccharide Analysis by Carbohydrate gel Electrophoresis (PACE). The AG is composed of a β-(1→3)-D-galactan backbone with β-(1→6)-D-galactan side chains. These side chains are highly variable in length, from one to at least 20 Gal residues and are highly substituted with α-L-Araf. Single GlcA residues are also present at the non-reducing termini of some short β-(1→6)-galactan side chains. In addition, the β-(1→6)-galactan side chains are also substituted with β-L-Arap. We propose a polysaccharide structure of the wheat flour AGP that is substantially revised from earlier models.

2,6-Disubstituted benzoates as neighboring groups for enhanced diastereoselectivity in beta-galactosylation reactions: synthesis of beta-1,3-linked oligogalactosides related to arabinogalactan proteins

Arabinogalactan proteins (AGPs) are plant glycoproteins which contain a beta-1,3-linked galactan core. The synthesis of the beta-galactopyranose-1,3-beta-galactopyranose linkage using various 2-O-acyl-protected glycosyl donors has been plagued with poor stereoselectivity and side reactions including orthoester formation and transesterification of the 2-O-acyl group from the donor to the acceptor. We have investigated the use of 2,6-disubstituted benzoyl groups as bulky neighboring groups on the glycosyl donor. A 2,4,6-trimethylbenzoyl group was found to be optimal and enabled the formation of the beta-galactopyranose-1,3-beta-galactopyranose linkage to disarmed ester-protected acceptors, suppressing transesterification and reducing orthoester formation while enhancing the beta-selectivity of galactosylation reactions. A series of beta-1,3-linked oligogalactosides were prepared and elaborated to neoglycoconjugates for the study of AGP biosynthesis and AGP binding proteins.

Different contributions of side-chains in beta-D-(1-->3,6)-galactans on intestinal Peyer's patch-immunomodulation by polysaccharides from Astragalus mongholics Bunge

Thirteen polysaccharides isolated from an extract of the aerial portions of Astragalus mongholics Bunge demonstrated immunomodulating activity against Peyer's patch immunocompetent cells. Nine of the active polysaccharide fractions were composed of either arabinogalactans, pectic arabinogalactans or pectins. The activities of the arabinogalactans and pectic arabinogalactans were associated with beta-D-(1-->3)-galactan moieties branched with beta-D-(1-->6)-galactooligosaccharide side-chains having degrees of polymerization of 8 or less. Degradation of the beta-D-(1-->3)-galactan or beta-D-(1-->6)-galactosyl side-chains in the arabinogalactans significantly decreased immunomodulating activity. Rhamnogalacturonan I (RG-I) with beta-D-(1-->3,6)-galactosyl side-chains having terminal beta-D-GlcA showed activity in the pectin-enriched fractions. Interestingly, the terminal GlcA was not required for activity of the arabinogalactan-enriched fractions, suggesting at least two different immunomodulating structures.

Purification, characterization, and mode of action of a rhamnogalacturonan hydrolase from Irpex lacteus, tolerant to an acetylated substrate

A novel rhamnogalacturonase (RGase) acting on an acetylated substrate was detected in the commercial preparation Driselase, an enzymatic mixture derived from the basidiomycete Irpex lacteus. The activity was isolated by hydrophobic interaction chromatography, gel filtration, and preparative isoelectric focusing, resulting in the isolation of five different rhamnogalacturonan hydrolases exhibiting various isoelectric points from 6.2 to 7.7. Sodium dodecyl sulfate polyacrylamide gel electrophoresis and mass spectrometry analyses after trypsin cleavage of the five fractions revealed that the five rhamnogalacturonases have a molar mass of 55 kDa without any divergences in the identified peptides. The RGase with a pI of 7.2 exhibited a pH optimum between 4.5 and 5 and a temperature optimum between 40 degrees C and 50 degrees C. Its mode of action was analyzed by mass spectrometry of the oligosaccharides produced after hydrolysis of acetylated and nonacetylated rhamnogalacturonan. Oligomers esterified by an acetyl group on the reducing galacturonic acid residue or fully acetylated were detected in the hydrolysate showing that the novel enzyme is able to bind acetylated galacturonic acid in its active site.

Molecular cloning and expression in Pichia pastoris of a Irpex lacteus exo-beta-(1-->3)-galactanase gene

A gene encoding exo-beta-(1-->3)-galactanase from Irpex lacteus was cloned by reverse transcriptase-PCR. The deduced amino acid sequence showed high similarity with exo-beta-(1-->3)-galactanases from other sources. The molecular mass of the mature form was calculated to be 45,520 Da. The gene product expressed in Pichia pastoris specifically hydrolyzed beta-(1-->3)-galactooligosaccharides, as did other exo-beta-(1-->3)-galactanases. The recombinant enzyme showed high activity toward arabinogalactan-proteins (AGPs) from radish as well as beta-(1-->3)-galactan. Product analysis revealed that the enzyme released beta-(1-->6)-galactobiose, beta-(1-->6)-galactotriose, and alpha-L-arabinofuranosyl-(1-->3)-beta-galactosyl-(1-->6)-galactose together with Gal from beta-(1-->3)-galactans attached with and without beta-(1-->6)-galactosyl branches prepared from acacia gum. These results indicate that the exo-beta-(1-->3)-galactanase from I. lacteus efficiently hydrolyzes beta-(1-->3)-galactan main chains of AGPs by bypassing beta-(1-->6)-galactosyl side chains.

Preparation of a new chromogenic substrate to assay for beta-galactanases that hydrolyse type II arabino-3,6-galactans

A chromogenic assay using RB5-dGA, Reactive Black 5 (RB5) dye covalently coupled to de-arabinosylated gum arabic (dGA), was developed for rapid screening of beta-galactanases. dGA was prepared by partial acid hydrolysis (0.25M trifluoroacetic acid for 2h at 90-95 degrees C) of gum Arabic (GA) from Acacia senegal. The dGA exhibited a median molecular mass of approximately 10kDa, corresponding to a degree of polymerisation (DP) approximately 60. It was devoid of Ara residues, and contained mostly Galp (68mol%) together with GlcpA (30mol%). The Galp residues were (1,6)- (34mol%), (1,3)- (3mol%) and (1,3,6)- (26mol%) linked, and the GlcAp residues were primarily terminal (28mol%) together with a small amount of (1,4)-linked (2mol%), as expected for a type II (3,6)-galactan. The new chromogenic assay is simple, cost effective, relatively sensitive, and is specific for either beta-(1-->3)- and/or beta-(1-->6)-d-galactanases. It will enable routine large-scale screening of beta-galactanases from crude enzyme preparations and microorganism cultures, and is suitable for profiling activity during purification processes.