New N-glycans in horseradish peroxidase

Glycosylation increases active site rigidity leading to improved enzyme stability and turnover

Glycosylation is the most prevalent protein post-translational modification, with a quarter of glycosylated proteins having enzymatic properties. Yet, the full impact of glycosylation on the protein structure-function relationship, especially in enzymes, is still limited. Here, we show that glycosylation rigidifies the important commercial enzyme horseradish peroxidase (HRP), which in turn increases its turnover and stability. Circular dichroism spectroscopy revealed that glycosylation increased holo-HRP's thermal stability and promoted significant helical structure in the absence of haem (apo-HRP). Glycosylation also resulted in a 10-fold increase in enzymatic turnover towards o-phenylenediamine dihydrochloride when compared to its nonglycosylated form. Utilising a naturally occurring site-specific probe of active site flexibility (Trp117) in combination with red-edge excitation shift fluorescence spectroscopy, we found that glycosylation significantly rigidified the enzyme. In silico simulations confirmed that glycosylation largely decreased protein backbone flexibility, especially in regions close to the active site and the substrate access channel. Thus, our data show that glycosylation does not just have a passive effect on HRP stability but can exert long-range effects that mediate the 'native' enzyme's activity and stability through changes in inherent dynamics.

Unidentified N-glycans by N-glycosidase A were Identified by Nglycosidase F under Denaturing Conditions in Plant Glycoprotein

Background:

The identification of N-glycans in plant glycoproteins or plant-made pharmaceuticals is essential for understanding their structure, function, properties, immunogenicity, and allergenicity (induced by plant-specific core-fucosylation or xylosylation) in the applications of plant food, agriculture, and plant biotechnology. N-glycosidase A is widely used to release the N-glycans of plant glycoproteins because the core-fucosylated N-glycans of plant glycoproteins are hydrolyzed by N-glycosidase A but not by N-glycosidase F. However, the efficiency of N-glycosidase A activity on plant glycoproteins remains unclear.

Objective:

To elucidate the efficient use of N-glycosidases to identify and quantify the N-glycans of plant glycoproteins, the identification of released N-glycans by N-glycosidase F and their relative quantities with a focus on unidentified N-glycans by N-glycosidase A in plant glycoproteins, Phaseolus vulgaris lectin (PHA) and horseradish peroxidase (HRP), were investigated.

Methods:

Liquid chromatography–tandem mass spectrometry was used to analyze and compare the N-glycans of PHA and HRP treated with either N-glycosidase A or F under denaturing conditions. The relative quantities (%) of each N-glycan (>0.1%) to the total N-glycans (100%) were determined.

Results:

N-glycosidase A and F released 9 identical N-glycans of PHA, but 2 additional core-fucosylated N-glycans were released by only N-glycosidase A, as expected. By contrast, in HRP, 8 N-glycans comprising 6 core-fucosylated N-glycans, 1 xylosylated N-glycan, and 1 mannosylated N-glycan were released by N-glycosidase A. Moreover, 8 unexpected N-glycans comprising 1 core-fucosylated N-glycan, 4 xylosylated N-glycans, and 3 mannosylated N-glycans were released by N-glycosidase F. Of these, 3 xylosylated and 2 mannosylated N-glycans were released by only N-glycansodase F.

Conclusion:

These results demonstrated that N-glycosidase A alone is insufficient to release the N-glycans of all plant glycoproteins, suggesting that to identify and quantify the released N-glycans of the plant glycoprotein HRP, both N-glycosidase A and F treatments are required.

The Structural Characterization and Antipathogenic Activities of Quinoin, a Type 1 Ribosome-Inactivating Protein from Quinoa Seeds

Quinoin is a type 1 ribosome-inactivating protein (RIP) we previously isolated from the seeds of pseudocereal quinoa (Chenopodium quinoa) and is known as a functional food for its beneficial effects on human health. As the presence of RIPs in edible plants could be potentially risky, here we further characterised biochemically the protein (complete amino acid sequence, homologies/differences with other RIPs and three-dimensional homology modeling) and explored its possible defensive role against pathogens. Quinoin consists of 254 amino acid residues, without cysteinyl residues. As demonstrated by similarities and homology modeling, quinoin preserves the amino acid residues of the active site (Tyr75, Tyr122, Glu177, Arg180, Phe181 and Trp206; quinoin numbering) and the RIP-fold characteristic of RIPs. The polypeptide chain of quinoin contains two N-glycosylation sites at Asn115 and Asp231, the second of which appears to be linked to sugars. Moreover, by comparative MALDI-TOF tryptic peptide mapping, two differently glycosylated forms of quinoin, named pre-quinoin-1 and pre-quinoin-2 (~0.11 mg/100 g and ~0.85 mg/100 g of seeds, respectively) were characterised. Finally, quinoin possesses: (i) strong antiviral activity, both in vitro and in vivo towards Tobacco Necrosis Virus (TNV); (ii) a growth inhibition effect on the bacterial pathogens of plants; and (iii) a slight antifungal effect against two Cryphonectria parasitica strains.

Development of a colorimetric PNGase activity assay

PNGases are crucial targets and valuable tools in analyzing asparagine-linked carbohydrate moieties (N-glycans) of glycoproteins. Activity tests of PNGases have been little improved since their discovery four decades ago, and still rely on observing deglycosylation patterns of glycoproteins or glycopeptides using SDS-PAGE or HPLC analysis. These techniques cannot be easily adapted for automated sampling and high-throughput procedures. Herein, we describe a PNGase activity assay which relies on the conversion of WST-1, a yellowish, water-soluble tetrazolium dye (sodium 2-(4-Iodophenyl)-3-(4-nitro-phenyl)-5-(2,4-disulfophenyl)-2H-tetrazolate), into a blue formazan dye. In this work, we showed that WST-1 could be reduced by N-glycans, which were enzymatically released from glycoprotein substrates. After optimization of the assay conditions, the robustness of the method was challenged by quantifying the activity of various PNGase isoforms at different purification stages using a microwell plate reader. Furthermore, the assay could be used to obtain steady-state kinetics of PNGase H⁺ wild-type and mutant variants, which showed significant differences in their enzymatic reaction rates. The simplicity and robustness of this method might be of benefit for the detection of PNGase activity in routine applications of large amounts of samples.

Rapid Sample Preparation Methodology for Plant N-Glycan Analysis Using Acid-Stable PNGase H+

The quantification of potentially allergenic carbohydrate motifs of plant and insect glycoproteins is increasingly important in biotechnological and agricultural applications as a result of the use of insect cell-based expression systems and transgenic plants. The need to analyze N-glycan moieties in a highly parallel manner inspired us to develop a quick N-glycan analysis method based on a recently discovered bacterial protein N-glycanase (PNGase H(+)). In contrast to the traditionally used PNGase A, which is isolated from almond seeds and only releases N-glycans from proteolytically derived glycopeptides, the herein implemented PNGase H(+) allows for the release of N-glycans directly from the glycoprotein samples. Because PNGase H(+) is highly active under acidic conditions, the consecutive fluorescence labeling step using 2-aminobenzamide (2AB) can be directly performed in the same mixture used for the enzymatic deglycosylation step. All sample handling and incubation steps can be performed in less than 4 h and are compatible with microwell-plate sampling, without the need for tedious centrifugation, precipitation, or sample-transfer steps. The versatility of this methodology was evaluated by analyzing glycoproteins derived from various plant sources using ultra-performance liquid chromatography (UPLC) analysis and further demonstrated through the activity analysis of four PNGase H(+) mutant variants.

Analysis of the glycosylation pattern of plant copper amine oxidases by MALDI-TOF/TOF MS coupled to a manual chromatographic separation of glycans and glycopeptides

The N-glycosylation in pea seedling amine oxidase and lentil seedling amine oxidase was analyzed in the present work. For that purpose, the enzymes were purified as native proteins from their natural sources. An enzymatic deglycosylation of pea seedling amine oxidase by endoglycosidase H under denaturing conditions combined with its proteolytic digestion by trypsin was carried out in order to analyze both N-glycans and "trimmed" N-glycopeptides with a residual N-acetylglucosamine attached at the originally occupied N-glycosylation sites. The released N-glycans were subjected to a manual chromatographic purification followed by MALDI-TOF/TOF MS. MS and MS/MS analyses were also performed directly on peptides and N-glycopeptides generated by proteolytic digestion of the studied enzymes. Sequencing of glycopeptides by MALDI-TOF/TOF MS/MS after their separation on a RP using a microgradient chromatographic device clearly demonstrated binding of paucimannose and hybrid N-glycan structures at Asn558. Such carbohydrates have been reported to exist in many plant N-glycoproteins, e.g. in peroxidases. Although high-mannose glycan structures were identified after the enzymatic deglycosylation, they could not be assigned to a particular N-glycosylation site. The presence of unoccupied glycosylation sites in several peptides was also confirmed from MS/MS results.

Sub 2-μm macroporous silica particles derivatized for enhanced lectin affinity enrichment of glycoproteins

A new, mechanically stable silica microparticle with macrosized internal pores (1.6 μm particles with 100 nm pores) has been developed for chromatography. The particles are characterized by an extensive network of interconnected macropores with a high intraparticle void volume, as observed by transmission electron microscopy (TEM). They are synthesized by an aerosol assembly technique called ultrasonic spray pyrolysis (USP). The particles have a high surface area for a macroporous material, ∼200 m(2)/g, making them suitable for large biomolecular separations. To demonstrate their potential for bioseparations, they have been functionalized with lectins for affinity enrichment of glycoproteins. The material was derivatized with two lectins, Concanavalin A (Con A) and Aleuria aurantia lectin (AAL), and binding properties were tested with standard glycoproteins. The columns exhibited excellent binding capacities for microaffinity enrichment: Con A was able to bind 75 μg of a standard glycoprotein in a 50 × 1 mm column. Following initial tests, the lectin microcolumns were utilized for enrichment of glycoproteins from 1 μL volumes of blood serum samples, performed in triplicate for each lectin. The enriched serum fractions were subjected to side-by-side glycomic and glycoproteomic profiling analyses with mass spectrometry to show that the new particles offer excellent sensitivity for microscale analyses of precious biological sample materials. The unique combination of the macroporous architecture and small particle diameter suggests the material may have advantages for conventional modes of chromatographic separation of macromolecules in an ultra-high-pressure liquid chromatography (UHPLC) format.

Generation of a natural glycan microarray using 9-fluorenylmethyl chloroformate (FmocCl) as a cleavable fluorescent tag

Glycan microarray technology has become a successful tool for studying protein-carbohydrate interactions, but a limitation has been the laborious synthesis of glycan structures by enzymatic and chemical methods. Here we describe a new method to generate quantifiable glycan libraries from natural sources by combining widely used protease digestion of glycoproteins and Fmoc chemistry. Glycoproteins including chicken ovalbumin, bovine fetuin, and horseradish peroxidase (HRP) were digested by Pronase, protected by FmocCl, and efficiently separated by 2D-HPLC. We show that glycans from HRP glycopeptides separated by HPLC and fluorescence monitoring retained their natural reducing end structures, mostly core alpha1,3-fucose and core alpha1,2-xylose. After simple Fmoc deprotection, the glycans were printed on NHS-activated glass slides. The glycans were interrogated using plant lectins and antibodies in sera from mice infected with Schistosoma mansoni, which revealed the presence of both IgM and IgG antibody responses to HRP glycopeptides. This simple approach to glycopeptide purification and conjugation allows for the development of natural glycopeptide microarrays without the need to remove and derivatize glycans and potentially compromise their reducing end determinants.

Extracting both peptide sequence and glycan structural information by 157 nm photodissociation of N-linked glycopeptides

The 157 nm photodissociation of N-linked glycopeptides was investigated in MALDI tandem time-of-flight (TOF) and linear ion trap mass spectrometers. Singly charged glycopeptides yielded abundant peptide and glycan fragments. The peptide fragments included a series of x-, y-, v-, and w- ions with the glycan remaining intact. These provide information about the peptide sequence and the glycosylation site. In addition to glycosidic fragments, abundant cross-ring glycan fragments that are not observed in low-energy CID were detected. These fragments provide insight into the glycan sequence and linkages. Doubly charged glycopeptides generated by nanospray in the linear ion trap mass spectrometer also yielded peptide and glycan fragments. However, the former were dominated by low-energy fragments such as b- and y- type ions while glycan was primarily cleaved at glycosidic bonds.

High-throughput quantitative analysis of plant N-glycan using a DNA sequencer

High-throughput quantitative analytical method for plant N-glycan has been developed. All steps, including peptide N-glycosidase (PNGase) A treatment, glycan preparation, and exoglycosidase digestion, were optimized for high-throughput applications using 96-well format procedures and automatic analysis on a DNA sequencer. The glycans of horseradish peroxidase with plant-specific core alpha(1,3)-fucose can be distinguished by the comparison of the glycan profiles obtained via PNGase A and F treatments. The peaks of the glycans with (91%) and without (1.2%) alpha(1,3)-fucose could be readily quantified and shown to harbor bisecting beta(1,2)-xylose via simultaneous treatment with alpha(1,3)-mannosidase and beta(1,2)-xylosidase. This optimized method was successfully applied to analyze N-glycans of plant-expressed recombinant antibody, which was engineered to contain a minor amount of glycan harboring beta(1,2)-xylose. These results indicate that our DNA sequencer-based method provides quantitative information for plant-specific N-glycan analysis in a high-throughput manner, which has not previously been achieved by glycan profiling based on mass spectrometry.

Change in glycosylation pattern with extension of endoplasmic reticulum retention signal sequence of mouse antibody produced by suspension-cultured tobacco BY2 cells

The production of antibodies using plants as bioreactors has been realized. Because sugar chain structures on recombinant proteins are a cause of concern, remodeling technology is highly promising. Localizing recombinant proteins in the endoplasmic reticulum (ER) affects the glycosylation pattern in transgenic plants. Previously, a mouse antibody produced by suspension-cultured tobacco BY2 cells has sugar chains with possible glycoepitopes as the predominant structures. In this study, we extended the Lys-Asp-Glu-Leu (KDEL) ER retention signal sequence over the heavy (H) and light (L) chains of the antibody and expressed the altered antibody in tobacco BY2 cells to study the effect of the KDEL sequence on glycosylation. For the antibody with the KDEL-extended H-chains, glycans with beta(1,2)-xylose or alpha(1,3)-fucose residues accounted for 49% of the total glycans. Meanwhile, for the antibody with the KDEL-extended H- and L-chains, glycans with xylose or fucose accounted for 38% of the total glycans. Although the addition of an ER retention signal shifted the dominant glycan structures of the KDEL-extended antibody to high-mannose-type structures, some of the antibodies escaped the retrieval system during intracellular traffic and were then modified by xylosylation or fucosylation.

Production of mouse monoclonal antibody with galactose-extended sugar chain by suspension cultured tobacco BY2 cells expressing human beta(1,4)-galactosyltransferase

Previously, we developed a transgenic tobacco BY2 cell line (GT6) in which glycosylation was modified by expressing human beta(1,4)-galactosyltransferase (betaGalT). In this study, we produced a mouse monoclonal antibody in GT6 cells, and determined the sugar chain structures of plant-produced antibodies. Galactose-extended N-linked glycans comprised 16.7%, and high-mannose-type and complex-type glycans comprised 38.5% and 35.0% of the total number of glycans, respectively. N-linked glycans with the plant-specific sugars beta(1,2)-xylose and alpha(1,3)-fucose comprised 9.8%. The introduction of human betaGalT into suspension cultured tobacco cells resulted in the production of recombinant proteins with galactose-extended glycans and decreased contents of beta(1,2)-xylose and alpha(1,3)-fucose.

Utility of pentose colorimetric assay for the purification of potato lectin, an arabinose-rich glycoprotein

Potato lectin (Solanum tuberosum agglutinin, STA) is an unusual glycoprotein containing approximately 50% carbohydrates by weight. Of the total carbohydrates, 92% is contributed by L: -arabinose, which are O-linked to hydroxyproline residues. The ferric chloride-orcinol assay (Bial's test), which is specific for pentoses has so far been used only for the determination of free pentoses in biological samples. However, this colorimetric assay has not been used for the detection of pentoses in bound form as it occurs in Solanaceae lectins (potato, tomato, and Datura lectins). Utilizing the pentose colorimetric assay for monitoring the presence of potato lectin, a simpler and shorter procedure for the purification of this lectin from potato tubers has been developed. The yield of potato lectin (1.73 mg per 100 g potato tuber) is twice compared to the yields reported in earlier procedures. Although potato lectin is well known for its specificity to free trimers and tetramers of N-acetyl-D: -glucosamine (GlcNAc), it possesses a similar specificity to the core (GlcNAc)(2) of N-linked glycoproteins. The utilization of the pentose assay in the purification of arabinose-rich lectins/agglutinins obviates the necessity for the use of agglutination assay in the various purification steps. The pentose assay appears to be a simple and convenient colorimetric assay for detecting any pentose-rich glycoprotein in plant extracts. The utility of the pentose assay appears to have a significant potential in the detection of hydroxyproline-rich glycoproteins (HRGPs), which are generally O-arabinosylated.

Functional characterization of the FimH adhesin from Salmonella enterica serovar Enteritidis

Salmonella entericaserovar Enteritidis has emerged during the last 20 years as the major causative agent of food-borne gastroenteritis in humans and as the major infectious agent on poultry farms, replacingSalmonella entericaserovar Typhimurium as the dominant pathogenic serovar. Because adhesion to gut tissues and colonization of the alimentary tract, mediated in large part by the FimH adhesins located on type 1 fimbriae, is an important stage in the pathogenesis of both serovars, the binding properties of the FimH adhesins from these two enteropathogens were compared.SalmonellaEnteritidis FimH protein and theSalmonellaTyphimurium low-adhesive variant of this adhesin were expressed inEscherichia coliand the recombinant proteins were analysed for their ability to bind glycoproteins carrying different oligomannosidic structures and different types of eukaryotic cells. In static binding assays (ELISA and Western blotting) both FimH proteins bound equally well to all three tested glycoproteins (RNase B, horseradish peroxidase and mannan-BSA). In addition, no differences were found in the binding specificity of the FimH proteins and intact cells ofSalmonellaEnteritidis andSalmonellaTyphimurium to human colon carcinoma or bladder cancer cells. The presence of the same amino acid residues at positions 61 (glycine) and 118 (phenylalanine) and the similar binding properties of these two adhesins suggest that the newly described FimH protein ofSalmonellaEnteritidis represents the low-adhesive variant found inSalmonellaTyphimurium. To study the binding specificity ofSalmonellaEnteritidis FimH protein further, direct kinetic analysis using surface plasmon resonance was performed. With this method it was found thatSalmonellaEnteritidis FimH adhesin bound with the highestKdvalue to high-mannose typeN-glycans carried by RNase B; about 100 times lowerKdvalues were obtained in the interactions with mannan-BSA and horseradish peroxidase.

N-linked glycan structures of a mouse monoclonal antibody produced from tobacco BY2 suspension-cultured cells

cDNA encoding H- and L-chains from a mouse monoclonal antibody was introduced into tobacco BY2 cells, and the resulting sugar chain structures of plant-produced antibodies were analyzed by a combination of HPLC, exoglucosidase digestion and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The glycan structures determined were Man(5-6)GlcNAc2 (22.3%), GlcNAcMan5GlcNAc2 (3.1%), GlcNAcMan3FucXylGlcNAc2 (24.4%), GlcNAcMan3XylGlcNAc2 (17.8%), Man3FucXylGlcNAc2 (24.3%), and Man3XylGlcNAc2 (8.1%). The major glycan structures of the antibodies produced by transgenic suspension-cultured cells contain typical plant bisecting beta(1,2)-xylose and alpha(1,3)-fucose residues, suggesting the posttranslational modification of a recombinant antibody in the late Golgi apparatus.

Functionalization of thioctic acid-capped gold nanoparticles for specific immobilization of histidine-tagged proteins

This paper presents an efficient strategy for the specific immobilization of fully functional proteins onto the surface of nanoparticles. Thioctic acid-derivatized gold clusters are used as a scaffold for further stepwise modification, leading to a cobalt(II)-terminated ligand shell. A histidine tag introduced by genetic engineering into a protein is coordinated to this transition metal ion. The specific immobilization has been demonstrated for the cases of a genetically engineered horseradish peroxidase and ferredoxin-NADP(+) reductase, confirming the attachment of the fully functional proteins to the Co(II)-terminated nanointerface. The absence of nonspecific protein adsorption and the specificity of the binding site have been verified using several analogues of the enzymes without the histidine tag.

Protein glycosylation analyzed by normal-phase nano-liquid chromatography--mass spectrometry of glycopeptides

A new method for the mass spectrometric characterization of site-specific protein glycosylation is presented. Glycoprotein samples were subjected to unspecific proteolysis by Pronase, resulting in glycopeptides with peptide moieties of mostly two to eight amino acids. Resulting (glyco-)peptide samples were resolved by nanoscale normal-phase liquid chromatography (LC)-online mass spectrometry (MS). Retention depended on the size of the glycan chain and allowed the separation of identical peptide moieties containing different N-glycan structures. Glycopeptides were analyzed in an ion trap instrument performing repetitive ion isolation/fragmentation cycles. While the MS/MS spectra were dominated by fragmentations of glycosidic linkages, MS(3) spectra exhibited cleavages of the peptide backbone and provided information on the peptide sequence and glycan attachment site. When applied to the model glycoproteins ribonuclease B and horseradish peroxidase (HRP), the method provided detailed insights into protein glycosylation and revealed some new features of site-specific glycosylation of HRP. Application of the method to Dolichos biflorus lectin, which has hitherto not been studied with respect to its glycosylation, identified two glycans attached alternatively to its single glycosylation site. Thus, the presented, unique combination of Pronase digestion of glycoproteins, normal-phase nano-LC, and multistage MS provides a method for the facile characterization of site-specific protein glycosylation.

New features of site-specific horseradish peroxidase (HRP) glycosylation uncovered by nano-LC-MS with repeated ion-isolation/fragmentation cycles

Horseradish peroxidase (HRP) is widely used in biomedical research as a reporter enzyme in diagnostic assays. In addition, it is of considerable interest as a model glycoprotein with core-xylosylated and -(alpha1-3)-fucosylated N-glycans that form antigenic elements of plant allergens and parasitic helminths. Using a combination of techniques comprising (1) nano-liquid chromatography (LC)-mass spectrometry (MS)/MS with multiple selection/fragmentation cycles of HRP tryptic (glyco-)peptides, (2) nano-electrospray MS of intact HRP, and (3) carbohydrate linkage analysis, it was revealed that most of the HRP N-glycosylation sites can be occupied with an alternative Fuc(1-3)GlcNAc-disaccharide. Two main variants of HRP occur: The major population (approximately 60%) has eight glycosylation sites carrying core(1-3)fucosylated, xylosylated, trimannosyl N-glycans, with the ninth potential N-glycosylation site Asn316 not occupied. Another group of HRP carries seven of the above-mentioned N-glycans, with an eighth N-glycosylation site carrying the alternative Fuc(1-3)GlcNAc-unit (approximately 35%). In addition, minor subsets of HRP were found to contain a xylosylated, trimannosyl N-glycan lacking core-fucosylation as a ninth N-glycan attached to Asn316, which has hitherto been assumed to be unoccupied. The finding of these new features of glycosylation of an already exceptionally well-studied glycoprotein underscores the potential of the nano-LC-MS(n) based analytical approach followed.

Horseradish peroxidase: a valuable tool in biotechnology

Peroxidases have conquered a prominent position in biotechnology and associated research areas (enzymology, biochemistry, medicine, genetics, physiology, histo- and cytochemistry). They are one of the most extensively studied groups of enzymes and the literature is rich in research papers dating back from the 19th century. Nevertheless, peroxidases continue to be widely studied, with more than 2000 articles already published in 2002 (according to the Institute for Scientific Information). The importance of peroxidases is emphasised by their wide distribution among living organisms and by their multiple physiological roles. They have been divided into three superfamilies according to their source and mode of action: plant peroxidases, animal peroxidases and catalases. Among all peroxidases, horseradish peroxidase (HRP) has received a special attention and will be the focus of this review. A brief description of the three super-families is included in the first section of this review. In the second section, a comprehensive description of the present state of knowledge of the structure and catalytic action of HRP is presented. The physiological role of peroxidases in higher plants is described in the third section. And finally, the fourth section addresses the applications of peroxidases, especially HRP, in the environmental and health care sectors, and in the pharmaceutical, chemical and biotechnological industries.

Immobilization of peroxidase glycoprotein on gold electrodes modified with mixed epoxy-boronic Acid monolayers

The development of bioelectronic enzyme applications requires the immobilization of active proteins onto solid or colloidal substrates such as gold. Coverage of the gold surface with alkanethiol self-assembled monolayers (SAMs) reduces nonspecific adsorption of proteins and also allows the incorporation onto the surface of ligands with affinity for complementary binding sites on native proteins. We present in this work a strategy for the covalent immobilization of glycosylated proteins previously adsorbed through weak, reversible interactions, on tailored SAMs. Boronic acids, which form cyclic esters with saccharides, are incorporated into SAMs to weakly adsorb the glycoprotein onto the electrode surface through their carbohydrate moiety. To prevent protein release from the electrode surface, we combine the affinity motif of boronates with the reactivity of epoxy groups to covalently link the protein to heterofunctional boronate-epoxy SAMs. The principle underlying our strategy is the increased immobilization rate achieved by the weak interaction-induced proximity effect between slow reacting oxyrane groups in the SAM and nucleophilic residues from adsorbed proteins, which allows the formation of very stable covalent bonds. This approach is exemplified by the use of phenylboronates-oxyrane mixed monolayers as a reactive support and redox-enzyme horseradish peroxidase as glycoprotein for the preparation of peroxidase electrodes. Quartz crystal microbalance, atomic force microscopy, and electrochemical measurements are used to characterize these enzymatic electrodes. These epoxy-boronate functional monolayers are versatile, stable interfaces, ready to incorporate glycoproteins by incubation under mild conditions.

In vivo conversion of a glycan to human compatible type by transformed tobacco cells

Horseradish peroxidase isozyme C (HRP; EC 1.11.1.7) was used as a model protein to evaluate the capacity of tobacco cells transformed with human beta 1,4-galactosyltransferase (GT6) to modify and galactosylate a foreign glycoprotein. Cells transformed with the HRP gene are designated as BY2-HRP and GT6-HRP, for wild type BY2 and GT6 transformed cells, respectively. Expression of HRP cells was confirmed by isoelectric focusing, peroxidase activity staining, Western blotting, and enzymatic assays. The presence of HRP galactosylated N-glycans in GT6-HRP cells was analyzed by lectin staining, affinity chromatography, and structural analyses of pyridylamino-labeled RCA(120)-bound sugar chains. The structure of Gal(1)GlcNAc(1)Man(5)GlcNAc(2) was proposed based from the results of exoglycosidase digestions and two-dimensional sugar chain mapping. Unlike the HRP produced in BY2-HRP cells, the HRP from GT6-HRP cells has galactosylated glycoproteins that did not bind to the xylose-specific antiserum, suggesting the absence of the beta 1,2-xylose residue in the sugar chain.

Analysis of glycoprotein-derived glycopeptides

Glycosylation of proteins represents one of the most important post-(co-)translational events in view of the ubiquity of the phenomenon. In most cases, the covalently linked glycans are involved in the functioning of these biomolecules in biological systems. Detailed information on the carbohydrate moieties including monosaccharide composition, anomeric configurations, type of glycosidic linkages and attachment sites at the protein is indispensable in describing the ultimate structure of a specific glycoprotein. This chapter presents a general strategy for the structural characterization of glycoproteins/glycopeptides focussed on the glycan part. Some of the techniques commonly used, like enzyme treatments, separation methods, chemical analyses, mass spectrometry and nuclear magnetic resonance spectroscopy are briefly reviewed.

Beta(1,2)-xylose and alpha(1,3)-fucose residues have a strong contribution in IgE binding to plant glycoallergens

Primary structures of the N-glycans of two major pollen allergens (Lol p 11 and Ole e 1) and a major peanut allergen (Ara h 1) were determined. Ole e 1 and Ara h 1 carried high mannose and complex N-glycans, whereas Lol p 11 carried only the complex. The complex structures all had a beta(1,2)-xylose linked to the core mannose. Substitution of the proximal N-acetylglucosamine with an alpha(1, 3)-fucose was observed on Lol p 11 and a minor fraction of Ole e 1 but not on Ara h 1. To elucidate the structural basis for IgE recognition of plant N-glycans, radioallergosorbent test analysis with protease digests of the three allergens and a panel of glycoproteins with known N-glycan structures was performed. It was demonstrated that both alpha(1,3)-fucose and beta(1,2)-xylose are involved in IgE binding. Surprisingly, xylose-specific IgE antibodies that bound to Lol p 11 and bromelain did not recognize closely related xylose-containing structures on horseradish peroxidase, phytohemeagglutinin, Ole e 1, and Ara h 1. On Lol p 11 and bromelain, the core beta-mannose is substituted with just an alpha(1,6)-mannose. On the other xylose-containing N-glycans, an additional alpha(1,3)-mannose is present. These observations indicate that IgE binding to xylose is sterically hampered by the presence of an alpha(1,3)-antenna.

Differential potency of two crosslinking plant lectins to induce formation of haptenic-sugar-resistant aggregates of rat thymocytes by post-binding signaling

To evaluate the significance of post-binding events for stable aggregate formation, the aggregation/dissociation of rat thymocytes initiated by two crosslinking plant lectins, namely concanavalin A (Con A) and Solanum tuberosum agglutinin (STA), were comparatively studied. Despite intimate cell contacts in the aggregates only Con A led to establishment of haptenic-sugar-resistant (HSR) complexes. The presence of inhibitor II of diacylglycerol kinase, a dual calmodulin antagonist/protein kinase C inhibitor (trifluoperazine), and a sulfhydryl group reagent (N-ethylmaleimide) impaired this process. The obtained results indicate that the formation of HSR cellular contacts is not an automatic response to lectin-dependent cell association. In contrast to STA, Con A binding elicits this reaction with involvement of diacylglycerol kinase, protein kinase C and/or calmodulin as well as thiol level perturbation, as inferred by the application of target-selective inhibitors.

Oligosaccharide and polypeptide homology of lupin (Lupinus luteus L.) acid phosphatase subunits

Peptide mapping of lupin acid phosphatase clearly demonstrated the homology between its two subunits. Sequenced tryptic peptides also showed 78% identity (92% similarity) to the red bean acid phosphatase. Peptides exclusive for the 50-kDa subunit are homologous to N-terminally located sequences in red bean acid phosphatase, leading to the assumption that the shorter subunit of lupin acid phosphatase is generated by the deletion of the N-terminal part of the longer subunit. Carbohydrate moiety was found to be identical in both subunits. Oligosaccharide chains released by hydrazinolysis from the both subunits were fluorescently labeled and separated by HPLC. The structure of oligosaccharides was elucidated by exoglycosidase sequencing. Seventeen percent of isolated glycans were found to be of the high-mannose type, while the rest belonged to plant complex-type structures. Most of the complex glycans were fucosylated and xylosylated; some were fucosylated or xylosylated only.

Heterogeneity of glycans at each N-glycosylation site of horseradish peroxidase

The tryptic glycopeptides of horseradish peroxidase isozyme c (HRPc) were studied by methylation linkage analysis, exoglycosidase degradation, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOFMS). Over 90% of the predicted tryptic peptides and glycopeptides of HRPc could be identified in the unfractionated digest. Four glycans, namely (Xyl)Man3(Fuc)GlcNAc2 (major species), (Xyl)Man2(Fuc)GlcNAc2, (Xyl)Man3GlcNAc2, and Man3(Fuc)GlcNAc2 (minor species), were observed at all of the N-glycosylation sites and account for greater than 95% of the carbohydrate. Other members of this glycan family, namely (Xyl)xManm(Fuc)f GlcNAc2 (x = 0 or 1, f = 0 or 1, m = 4, 5, 6, or 7), account for the rest of the glycans. Only traces of high mannose-type glycans were detected in HRPc. Two sites, namely those at Asn-57 and Asn-267, were found to be more heterogeneous than the sites at Asn-13, Asn-158, Asn-186, 198 (doubly glycosylated peptide), Asn-214, and Asn-255. Two of the glycopeptides were observed as part of disulfide-linked species. MALDITOFMS confirmed the N-glycosylation sites previously reported [K.G. Welinder, Eur. J. Biochem., 96 (1979) 483-502] and was used to determine the heterogeneity of the glycan pool at each site.

Beyond plant lectin histochemistry: preparation and application of markers to visualize the cellular capacity for protein-carbohydrate recognition

Oligosaccharides can store biological information. In this respect, their capacity even outmatches that of oligo- and polymeric structures of nucleotides and amino acids. Protein-carbohydrate interactions are thus considered to be involved in the regulation of diverse cellular activities. Over decades, plant lectins have proven valuable for assessing structural aspects of the enormous variety of carbohydrate epitopes and for monitoring spatially and/or temporally restricted patterns of expression. If the presence of these epitopes and the alterations in their occurrence bear physiological relevance, one reasonable possibility is that the visualized saccharides serve as ligands in an operative protein-carbohydrate recognition system. To support the validity of this hypothesis, receptor sites for a sugar compound must be localized. Carrier-immobilized carbohydrates (neoglycoconjugates) are adequate for this purpose. Chemical synthesis gains access to such probes. In the first stage, the presence of binding sites such as lectins in the tissue is ascertained. The next step toward proving the outlined hypothesis is the application of the first localized then purified endogenous receptors as glycohistochemical markers. It is essential to point out that the fine specificities of plant and animal lectins can differ, although they share an identical monosaccharide specificity. Thus, neoglycoconjugates for localizing sugar ligand-binding proteins and endogenous lectins to detect suitable binding partners are promising probes to enhance our knowledge about the capacities of cells to be engaged in protein-carbohydrate recognition in situ.