Structural Studies of Fertilization-associated Carbohydrate-rich Glycoproteins (Hyosophorin) Isolated from the Fertilized and Unfertilized Eggs of Flounder, Paralichthys olivaceus

Extraction optimization, partial purification, and characterization of sialoglycoproteins from Labeo rohita roes

In India, fish roes are generally considered worthless garbage and disposed of without recovering the valuable molecules, creating environmental and disposal problems. The present investigation aimed to optimize the extraction conditions, partial purification, and characterization of sialoglycoproteins (RRSGP) from Labeo rohita (rohu) roes. RSM generated optimum conditions for maximum RRSGP (70.49 %) extraction, which were 1.25 M NaCl, 1:32.5(w/v) solid-to-liquid ratio, 47.5 °C temperature, and 3 h time. Further, sialoglycoproteins from RRSGPs were partially purified, and result revealed that obtained peak-1 (PRRSGP) using QFF anion exchange chromatography exhibited higher glycoprotein and sialic acid content (p < 0.05). SDS-PAGE pattern of PRRSGP presented dominant bands of 97 kDa and 27 kDa glycoproteins. FTIR spectrum of PRRSGP confirmed the presence of glycated proteins. HPLC analysis revealed that PRRSGP consists of Neu5Ac. Furthermore, β-elimination reaction elucidated that PRRSGP contained N-glycosidic linkage. PRRSGP exhibited tyrosine and glutamate as primary amino acids. Glycan part of PRRSGP presented mannose and N-acetyl galactosamine as dominant neutral and amino sugar, respectively. Furthermore, PRRSGP exhibited antioxidant activity with EC50 value for DPPH (8.79 mg/ml) and ABTS (2.21 mg/ml). Besides, RRSGP displayed better protein solubility, foaming, and emulsion properties. Therefore, rohu roes are potential source of sialoglycoproteins that can be recovered and used as bio-functional ingredients in food and nutraceutical applications.

Diversity of sialic acids and sialoglycoproteins in gametes and at fertilization

Sialic acids are a family of 9-carbon monosaccharides with particular physicochemical properties. They modulate the biological functions of the molecules that carry them and are involved in several steps of the reproductive process. Sialoglycoproteins participate in the balance between species recognition and specificity, and the mechanisms of these aspects remain an issue in gametes formation and binding in metazoan reproduction. Sialoglycoproteins form a specific coat at the gametes surface and specific polysialylated chains are present on marine species oocytes. Spermatozoa are submitted to critical sialic acid changes in the female reproductive tract facilitating their migration, their survival through the modulation of the female innate immune response, and the final oocyte-binding event. To decipher the role of sialic acids in gametes and at fertilization, the dynamical changes of enzymes involved in their synthesis and removal have to be further considered.

Comprehensive Analysis of the Structure and Function of Peptide:N-Glycanase 1 and Relationship with Congenital Disorder of Deglycosylation

The cytosolic PNGase (peptide:N-glycanase), also known as peptide-N4-(N-acetyl-β-glucosaminyl)-asparagine amidase, is a well-conserved deglycosylation enzyme (EC 3.5.1.52) which catalyzes the non-lysosomal hydrolysis of an N(4)-(acetyl-β-d-glucosaminyl) asparagine residue (Asn, N) into a N-acetyl-β-d-glucosaminyl-amine and a peptide containing an aspartate residue (Asp, D). This enzyme (NGLY1) plays an essential role in the clearance of misfolded or unassembled glycoproteins through a process named ER-associated degradation (ERAD). Accumulating evidence also points out that NGLY1 deficiency can cause an autosomal recessive (AR) human genetic disorder associated with abnormal development and congenital disorder of deglycosylation. In addition, the loss of NGLY1 can affect multiple cellular pathways, including but not limited to NFE2L1 pathway, Creb1/Atf1-AQP pathway, BMP pathway, AMPK pathway, and SLC12A2 ion transporter, which might be the underlying reasons for a constellation of clinical phenotypes of NGLY1 deficiency. The current comprehensive review uncovers the NGLY1’ssdetailed structure and its important roles for participation in ERAD, involvement in CDDG and potential treatment for NGLY1 deficiency.

Isolation and Characterization of a Novel Sialoglycopeptide Promoting Osteogenesis from Gadus morhua Eggs

Gadus morhua eggs contain several nutrients, including polyunsaturated fatty acids, lecithin and glycoproteins. A novel sialoglycopeptide from the eggs of G. morhua (Gm-SGPP) was extracted with 90% phenol and purified by Q Sepharose Fast Flow (QFF) ion exchange chromatography, followed by S-300 gel filtration chromatography. Gm-SGPP contained 63.7% carbohydrate, 16.2% protein and 18.6% N-acetylneuraminic acid. High-performance size exclusion chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated that Gm-SGPP is a 7000-Da pure sialoglycopeptide. β-elimination reaction suggested that Gm-SGPP contained N-glycan units. Amino acid N-terminal sequence analysis indicated the presence of Ala-Ser-Asn-Gly-Thr-Gln-Ala-Pro amino acid sequence. Moreover, N-glycan was connected at the third Asn location of the peptide chain through GlcNAc. Gm-SGPP was composed of D-mannose, D-glucuronic acid and D-galactose. Fourier transform-infrared spectroscopy (FT-IR), ¹H-nuclear magnetic resonance spectroscopy (¹H-NMR) and methylation analysis were performed to reveal the structure profile of Gm-SGPP. In vitro results showed that the proliferation activity of MC3T3-E1 cells was significantly promoted by Gm-SGPP. In vivo data revealed that Gm-SGPP increased the calcium and phosphorus content of tibias and promoted longitudinal bone growth in adolescent rats.

Exploring the Arctic Charr Intestinal Glycome: Evidence of Increased N-Glycolylneuraminic Acid Levels and Changed Host-Pathogen Interactions in Response to Inflammation

Disease outbreaks are a limiting factor for the sustainable development of the aquaculture industry. The intestinal tract is covered by a mucus layer mainly comprised by highly glycosylated proteins called mucins. Mucins regulate pathogen adhesion, growth, and virulence, and the glycans are vital for these functions. We analyzed intestinal mucin O-glycans on mucins from control and full-fat extruded soy-bean-fed (known to cause enteritis) Arctic charr using liquid chromatography-tandem mass spectrometry. In total, 56 glycans were identified on Arctic charr intestinal mucins, with a high prevalence of core-5-type and sialylated O-glycans. Disialic-acid-epitope-containing structures including NeuAcα2,8NeuAc, NeuAc(Gc)α2,8NeuGc(Ac), and NeuGcα2,8NeuGc were the hallmark of Arctic charr intestinal mucin glycosylation. Arctic charr fed with soy bean meal diet had lower (i) number of structures detected, (ii) interindividual variation, and (iii) N-glycolylneuraminic-acid-containing glycans compared with control Arctic charr. Furthermore, Aeromonas salmonicida grew less in response to mucins from inflamed Arctic charr than from the control group. The Arctic charr glycan repertoire differed from that of Atlantic salmon. In conclusion, the loss of N-glycolylneuraminic acid may be a biomarker for inflammation in Arctic char, and inflammation-induced glycosylation changes affect host-pathogen interactions.

Structures and developmental alterations of N-glycans of zebrafish embryos

Zebrafish is a model organism suitable for studying vertebrate development. We analyzed the N-glycan structures of zebrafish embryos and their alterations during zebrafish embryogenesis to obtain basic data for studying the roles of N-glycosylation. Multiple modes of high-performance liquid chromatography and multistage mass spectrometry were used for structural analysis of N-glycans. The N-glycans from deyolked embryos at 36 hours postfertilization, a mid-pharyngula stage, contained relatively higher amounts of complex- and hybrid-type glycans with LacNAc (Galβ1-4GlcNAc) and/or sialyl LacNAc without additional β1,4-Gal, which are commonly found in mammalian tissues, as well as abundant oligomannose-type glycans. Some of the complex- and hybrid-type glycans possessed various extended LacNAc structures, such as Galβ1-4LacNAc, LacNAc-repeat or unique (+/- dHex)-GalNAcα1-GlcNAcβ1-LacNAc. In contrast, the yolk of the embryo contains predominant oligomannose-type glycans and complex-type glycans with Galβ1-4(Siaα2-3)Galβ1-4(Fucα1-3)GlcNAc antennae. N-Glycan profiles obtained from deyolked embryos at different stages showed stage-dependent variation of complex- and hybrid-type glycans. At gastrula and early segmentation stages, complex- and hybrid-type glycans were minor components, and their antenna structures were mainly sialyl LacdiNAc (Siaα2-6GalNAcβ1-4GlcNAc). From the mid-segmentation to pharyngula stages, those with LacNAc and/or α2,6-sialyl LacNAc antenna structures increased remarkably, and those with α2,3-sialyl LacNAc antenna, core α1,6-Fuc and bisecting GlcNAc modifications increased gradually. These results suggest the presence of mechanisms for regulating the antenna structures of complex/hybrid N-glycan biosynthesis in the phylotypic stage of vertebrate development.

Atlantic Salmon Carries a Range of Novel O-Glycan Structures Differentially Localized on Skin and Intestinal Mucins

Aquaculture is a growing industry, increasing the need for understanding host-pathogen interactions in fish. The skin and mucosal surfaces, covered by a mucus layer composed of mucins, is the first point of contact between fish and pathogens. Highly O-glycosylated mucins have been shown to be an important part of the defense against pathogens, and pathogens bind to host surfaces using lectin-like adhesins. However, knowledge of piscine O-glycosylation is very limited. We characterized mucin O-glycosylation of five freshwater acclimated Atlantic salmon, using mass spectrometry. Of the 109 O-glycans found, most were sialylated and differed in distribution among skin, pyloric ceca, and proximal and distal intestine. Skin O-glycans were shorter (2-6 residues) and less diverse (33 structures) than intestinal O-glycans (2-13 residues, 93 structures). Skin mucins carried O-glycan cores 1, 2, 3, and 5 and three types of sialic acids (Neu5Ac, Neu5Gc, and Kdn) and had sialyl-Tn as the predominant structure. Intestinal mucins carried only cores 1, 2, and 5, Neu5Ac was the only sialic acid present, and sialylated core 5 was the most dominant structure. This structural characterization can be used for identifying structures of putative importance in host-pathogen interactions for further testing in biological assays and disease intervention therapies.

Glycobiology of reproductive processes in marine animals: the state of the art

Glycobiology is the study of complex carbohydrates in biological systems and represents a developing field of science that has made huge advances in the last half century. In fact, it combines all branches of biomedical research, revealing the vast and diverse forms of carbohydrate structures that exist in nature. Advances in structure determination have enabled scientists to study the function of complex carbohydrates in more depth and to determine the role that they play in a wide range of biological processes. Glycobiology research in marine systems has primarily focused on reproduction, in particular for what concern the chemical communication between the gametes. The current status of marine glycobiology is primarily descriptive, devoted to characterizing marine glycoconjugates with potential biomedical and biotechnological applications. In this review, we describe the current status of the glycobiology in the reproductive processes from gametogenesis to fertilization and embryo development of marine animals.

Free N-linked oligosaccharide chains: formation and degradation

There is growing evidence that N-linked glycans play pivotal roles in protein folding and intra- and/or intercellular trafficking of N-glycosylated proteins. It has been shown that during the N-glycosylation of proteins, significant amounts of free oligosaccharides (free OSs) are generated in the lumen of the endoplasmic reticulum (ER) by a mechanism which remains to be clarified. Free OSs are also formed in the cytosol by enzymatic deglycosylation of misfolded glycoproteins, which are subjected to destruction by a cellular system called "ER-associated degradation (ERAD)." While the precise functions of free OSs remain obscure, biochemical studies have revealed that a novel cellular process enables them to be catabolized in a specialized manner, that involves pumping free OSs in the lumen of the ER into the cytosol where further processing occurs. This process is followed by entry into the lysosomes. In this review we summarize current knowledge about the formation, processing and degradation of free OSs in eukaryotes and also discuss the potential biological significance of this pathway. Other evidence for the occurrence of free OSs in various cellular processes is also presented.

Identification of two discrete peptide: N-glycanases in Oryzias latipes during embryogenesis

Two different types of peptide:N-glycanase (PNGase) were identified in developing embryos of medaka fish ( Oryzias latipes ). Because the optimum pH values for their activities were acidic and neutral, they were designated as acid PNGase M and neutral PNGase M, respectively. The acid PNGase M corresponded to the enzyme that had been partially purified from medaka embryos (Seko,A., Kitajima,K., Inoue,Y. and Inoue,S. (1991) J. Biol. Chem., 266, 22110-22114). The apparent molecular weight of this enzyme was 150 K, and the optimal pH was 3.5-4.0, and the K m for L-hyosophorin was 44 microM. L-Hyosophorin is a cortical alveolus-derived glycononapeptide with a large N-linked glycan chain present in the perivitelline space of the developing embryo. Acid PNGase M was competitively inhibited by a free de-N-glycosylated nonapeptide derived from L-hyosophorin. This enzyme was expressed in ovaries and embryos at all developmental stages after gastrulation, but activity was not detected in embryos at developmental stages between fertilization and gastrula. Several independent lines of evidence suggested that acid PNGase M may be responsible for the unusual accumulation of free N-glycans derived from yolk glycoproteins (Iwasaki,M., Seko,A., Kitajima,K., Inoue,Y. and Inoue,S. (1992) J. Biol. Chem., 267, 24287-24296). In contrast, the neutral PNGase M was expressed in blastoderms from the 4-8 cell stage and in cells up to early gastrula. The general significance of these findings is that they show a developmental stage-dependent expression of the two PNGase activities, and that expression of the neutral PNGase M activity occurs concomitantly with the de-N-glycosylation of L-hyosophorin. These data thus support our conclusion that the neutral PNGase M is responsible for the developmental-stage-related de-N-glycosylation of the L-hyosophorin.

Purification, characterization, and molecular cloning of carp hyosophorin

Carp hyosophorin (HSP) is purified from oocytes. It is a highly glycosylated protein (10% protein and 90% carbohydrate) of high molecular weight (>100 kDa) and is localized in the cortical granules of oocytes. During cortical reaction carp HSP is exocytosed into the perivitelline space and is rapidly cleaved to the low-molecular-weight forms of 20 to 30 kDa. The major part of carp HSP cDNA is composed of tandem repeats, the repetitive domain. A repeat is 36 base pairs (bp) in length, which encodes 12 amino acid residues. The sequences of repeats vary within a given cDNA and among different cDNAs. The predominant sequences of repeats are DDGSGSNATTTQ. In addition, the length of the repetitive domain is highly variable among different genes and cDNAs, and ranges from 170 to 1,010 bp. Transcription of carp HSP is restricted in oocytes and starts very early during oogenesis. Carp HSP is highly species-specific. The RNA of goldfish ovary shows no positive signals when probed by carp HSP cDNA.

Site-specific de-N-glycosylation of diglycosylated ovalbumin in hen oviduct by endogenous peptide: N-glycanase as a quality control system for newly synthesized proteins

Hen ovalbumin (OVA) is known to exist as a singly N-glycosylated form with a glycan chain on Asn-292 in egg white. Previous studies showed that di-N-glycosylated form of OVA [Di-OVA; CHO-Asn-292/CHO-Asn-311 (CHO, N-glycan chain)], which has two N-glycan chains on Asn-292 and Asn-311, was expressed only transiently in hen oviduct. Di-OVA was not found in egg white, suggesting that this form cannot be secreted normally and may possibly be converted to mono-N-glycosylated OVA (CHO-Asn-292/Asp-311) by the action of peptide:N-glycanase (PNGase) during synthesis and secretion. In this study, we have identified the putative PNGase activity in the homogenate of hen oviduct, purified 1,000-fold, and designated as PNGase HO. We examined the reactivity of Di-OVA to PNGase HO and found that this enzyme site-specifically cleaved off the glycan chain at Asn-311 to convert Di-OVA into the mono-N-glycosylated form (CHO-Asn-292/Asp-311). In contrast, this enzyme was found not to act on the mono-N-glycosylated OVA (CHO-Asn-292/Asn-311) found in egg white when it was tested as a substrate. The present findings support our view that de-N-glycosylation catalyzed by PNGase may be involved in quality control of newly synthesized proteins by converting its diglycosylated form into the mono-N-glycosylated form that can be secreted. However, the alternative possibility that de-N-glycosylation may trigger cytosolic degradation of the aberrantly glycosylated glycoprotein cannot be ruled out.

Activity of UDP-GlcNAc:GlcNAc beta 1-->6(GlcNAc beta 1-->2) Man alpha 1-->R[GlcNAc to Man] beta 1-->4N-acetylglucosaminyltransferase VI (GnT VI) from the ovaries of Oryzias latipes (Medaka fish)

UDP-GlcNAc:GlcNAc beta 1-->(GlcNAc beta 1-->2)Man alpha 1-R[GlcNAc to Man] beta 1-->4N-acetylglucosaminyltransferase VI (GnT VI) activity was shown to be present in crude homogenates of Medaka fish (Oryzias latipes) ovaries using UDP-[14C]GlcNAc and synthetic GlcNAc beta 1-->6 (GlcNAc beta 1-->2)Man alpha 1-->6Glc beta 1-->octyl as substrates. Characterization of this activity showed a pH optimum at about pH 7.0 and an absolute requirement for divalent cations. The optimum concentration of Mn2+ was at about 25 mM. This finding is the first report on GnT VI activity in fish; the enzyme has previously been described only in avian tissues.

A molecular builder for carbohydrates: application to polysaccharides and complex carbohydrates

A new procedure (POLYS) for producing three-dimensional structures of polysaccharides and complex carbohydrates is described. This employs a builder concept combining a database of monosaccharide structures with a database containing information on populations of independent neighboring glycosidic linkages in disaccharide fragments. The computer program is written in C, and it can cope with both the complexity and the diversity of carbohydrates and the unique topological features arising from multiple branching. A simple ASCII syntax was developed for describing the primary structures in accordance with IUPAC nomenclature. The translation of the primary structure is made through the combined use of a lexical analyzer and a command interpreter. In this way the program can be considered as a compiler of primary structures of carbohydrates. However, it also generates secondary and tertiary structures in the form of Cartesian coordinates in formats used by most molecular mechanics programs and packages. In our laboratory POLYS was exhaustively tested on standard homopolysaccharide systems such as cellulose and mannan and found to work very well. We now report the ease of use and the efficiency of the molecular builder in applications to more complex carbohydrate systems. These include the structural exploration of a pentaantennary oligosaccharide having 135 residues, the complex family of pectic polysaccharides including the organization and distribution of side chains (arabinan, arabinogalactan, and galactan) on the rhamnogalacturonan backbone.

Occurrence and structural analysis of highly sulfated multiantennary N-linked glycan chains derived from a fertilization-associated carbohydrate-rich glycoprotein in unfertilized eggs of Tribolodon hakonensis

This study represents the first detailed investigation of the nature of highly sulfated (keratan-sulfate-like) complex-type asparagine-linked glycans having a tetraantennary core structure and shows the effectiveness of fast-atom-bombardment mass spectrometric (FAB-MS) methods incorporating derivatization and mild methanolysis for analyzing such complex types of sulfated glycans. The structure of the N-glycan chains was unambiguously established by a combination of compositional analysis, methylation analysis, mild methanolysis for desulfation, hydrazinolysis/nitrous acid deamination, enzymatic (endo-beta-galactosidase and peptide:N-glycosidase F) digestions, and instrumental analyses (1H-NMR spectroscopy and FAB-MS) which revealed the novel repeating sulfated carbohydrate sequences, +/- Gal beta 1-->4Gal beta 1[-->(HSO3-->6)GlcNAc beta 1-->3(+/- Gal beta 1-->4)Gal beta 1]n--> (see Structure I; p + q + r + s approximately 14). This sequence is unique in: (a) the skeletal structure is similar to that of keratan sulfate but is completely devoid of 6-O-sulfated Gal residues and (b) the presence of branched Gal residues in the sequence -->4GlcNAc beta 1-->3(Gal beta 1-->4)Gal beta 1-->. [formula: see text]

A unique multifucosylated -3GalNAc beta 1-->4GlcNAc beta 1-->3Gal alpha 1- motif constitutes the repeating unit of the complex O-glycans derived from the cercarial glycocalyx of Schistosoma mansoni

The entire surface of the cercarial stage of the human blood fluke Schistosoma mansoni is covered by a 1-microns thick, highly immunogenic, fucose-rich glycocalyx (GCX). Using strategies based on enzymatic, chemical, and mass spectrometric analysis, we have defined the structures of the major glycans released by reductive elimination from GCX. They comprise a heterogeneous population of multifocosylated complex oligosaccharides with the following nonreducing terminal sequences: [formula: see text] Our structural data suggest that these tri- to pentafucosylated epitopes are carried on type 1, R-->Gal beta-1-->3GalNAc, and type 2, R-->Gal beta 1-->3(R-->GlcNAc beta-1-->6)GalNAc, core structures via repeat units of (3GalNAc beta 1-->4(Fuc alpha 1-->2Fuc alpha 1-->2Fuc alpha 1-->3)GlcNAc beta-1-->3Gal alpha-->)n, where n is mainly 0 and 1, and all sugars are in the pyranose form. The proposed structure represents the first instance where an alpha-galactosylated beta-GalNAc(1-->4)-beta-GlcNAc sequence occurs as a repeating unit in a glycoprotein. It is also unique in being substituted with oligofucosyl appendages. The unusual oligosaccharide structures described here, particularly the potentially immunodominant oligofucosyl moieties, are most likely responsible for the known potency of GCX in modulating various immune responses including complement activation, B cell mitogenesis, and delayed type hypersensitivity in schistosomiasis.

Carbohydrate-binding property of peptide: N-glycanase from mouse fibroblast L-929 cells as evaluated by inhibition and binding experiments using various oligosaccharides

Carbohydrate binding to peptide: N-glycanase from mouse fibroblast L-929 cells (L-929 PNGase) and inhibition by oligosaccharides of its catalytic activity were studied. L-929 PNGase was found to bind strongly with oligosaccharides having triomannosido-N,N'-diacetyl-chitobiosyl (Man3GlcNAc2) structure (Kd = approximately 10 microM). This binding was inhibited by mannotriose (Man3; Man alpha 1-->3[Man alpha 1-->6]Man) but not by N,N'-diacetylchitobiose (GlcNAc2; GlcNAc beta 1-->4GlcNAc). Scatchard analysis indicated that there exist two binding sites for Man3 on a homodimeric form of a 105-kDa subunit. Oligosaccharides having Man3GlcNAc2 structure were also shown to be strong inhibitors for the PNGase-catalyzed reaction (Ki = approximately 10 microM). The minimum structural requirements for inhibition of the PNGase activity were Man3 and GlcNAc2. Enzyme kinetic studies showed that the mechanism of inhibition by the oligosaccharides and Man3 fits well with a model wherein two inhibitor binding sites reside on L-929 PNGase. The conformity of Kd with IC50 values may be taken as an evidence for inhibition of the catalytic activity by the oligosaccharides and Man3 through the occupation of the binding sites with these molecules. On the other hand, inhibition by GlcNAc2 followed the simple competitive mode. Since the minimum substrate for the L-929 PNGase was shown to be Man beta 1-->4GlcNAc beta 1-->4GlcNAc beta 1-->peptide, GlcNAc2 may be directly accessible to the catalytic site in competition with substrate. Interestingly, alkylation of -SH group in L-929 PNGase caused complete loss of the catalytic activity, but the carbohydrate binding activity was completely retained, indicating that the catalytic site(s) is discriminated from the carbohydrate-binding sites in the active site of this enzyme. The carbohydrate-binding property seems to be unique to soluble PNGases from mammals and may be associated not only with regulation of the enzyme activity, but also with receptor and carrier functions for glycoconjugates in certain intracellular processes.

Proton NMR study of the trimannosyl unit in a pentaantennary N-linked decasaccharide structure. Complete assignment of the proton resonances and conformational characterization

The chemical shifts of all the ring protons of the three Man residues in a pentaantennary glycan chain have been unambiguously assigned by two-dimensional proton nuclear magnetic resonance (1H-NMR) spectroscopic methods. The study, using chemical shift and J values on the conformation of the trimannosyl unit, revealed that the rotamer about the C5-C6 bond of the alpha 1-->6 linkage in the sequence of Man alpha 1-->6Man beta 1--> is predominantly confined to a gauche-gauche rotamer (omega = 180 degrees, omega = O6-C6-C5-H5) and not to a gauche-trans rotamer (omega = -60 degrees). We do not know of any previous demonstration that the dihedral angle omega (O6-C6-C5-H5) in Man alpha 1-->6Man beta 1--> is preferentially 180 degrees in complex-type N-linked glycans having no bisecting GlcNAc residue.

Do de-N-glycosylation enzymes have an important role in plant cells?

In this review de-N-glycosylation was defined as the removal of the glycan(s) from a N-glycosylprotein, by means of enzymes acting on the di-N-acetylchitobiosyl part of the invariant pentasaccharide inner-core of N-glycosylproteins. Peptide-N4-(N-acetyl-beta-D-glucosaminyl) asparagine amidases (PNGase) and endo-N-acetyl-beta-D-glucosaminidases (ENGase) were both considered as de-N-glycosylation enzymes. A detailed description of the characterization and the function of plant PNGases and ENGases is presented, together with a brief presentation on the occurrence and the current knowledge on the function of microbial and animal enzymes. De-N-glycosylation of plant glycoproteins was proposed as a possible mechanism for the release of oligosaccharides displaying biological activities and the removal of N-glycans could also explain the regulation of protein activity. Each enzyme seems to have a specific function during germination and post-germinative development. All the arguments concur that de-N-glycosylation enzymes have an important role in plant cells and confirm that the N-glycosylation/de-N-glycosylation system should occur more commonly than presently recognized in living organisms.

Does an animal peptide: N-glycanase have the dual role as an enzyme and a carbohydrate-binding protein?

Recently, we have reported purification and characterization of a de-N-glycosylating enzyme, peptide: N-glycanase (PNGase) found in C3H mouse fibroblast L-929 cells, and designated L-929 PNGase [Suzuki T, Seko A, Kitajima K, Inoue Y, Inoue S (1994) J Biol Chem 269, 17611-18]. The unique properties of L-929 PNGase are that the enzyme had a high affinity to the substrate glycopeptide (e.g. Km = 114 microM for fetuin derived glycopentapeptide) and that the PNGase-catalysed reaction is strongly inhibited by the released free oligosaccharides but not by the free peptides formed, suggesting that L-929 PNGase is able to bind to a certain type of carbohydrate chain. In this study, we report the new findings of the mannan-binding property of L-929 PNGase: the de-N-glycosylating enzyme activity of L-929 PNGase was inhibited by yeast mannan and triomannose, Man alpha 1-->3(Man alpha 1-->6)Man, but not by mannose and alpha-methyl-D-mannoside. Furthermore, L-929 PNGase was revealed to bind to the glycan moiety of yeast mannan by using mannan-conjugated Sepharose 4B gel as a ligand, suggesting that L-929 PNGase could serve not only as an enzyme but also as a carbohydrate recognition protein in vivo. Such 'dual' properties found for animal-derived L-929 PNGase are unique and are not shared with other previously characterized plant- and bacterial-origin PNGases--PNGase A and PNGase F, respectively.

Identification of free glycan chain liberated by de-N-glycosylation of the cortical alveolar glycopolyprotein (hyosophorin) during early embryogenesis of the Medaka fish, Oryzias latipes

In Medaka embryos (at the stages of blastulation to organogenesis), we found the presence of free glycan of which structure is identical with the multiantennary N-linked sugar chain of L-hyosophorin molecules which were originally present in the cortical alveoli of the unfertilized eggs in their precursor high molecular form. The free glycan-enriched fraction was separated from L-hyosophorin by chromatography on DEAE-Sephadex A-25 and Sephadex G-50 after removal of the sialic acid residues with exo-sialidase. Composition analysis, 400-MHz 1H NMR spectroscopy, and pyridylamination-hydrazinolysis-nitrous acid deamination of the free glycan showed the presence of di-N-acetylchitobiosyl structure at the reducing end, suggesting that the free glycan chain was derived from L-hyosophorin by the action of a specific peptide:N-glycosidase (PNGase). When we combine the previous finding of the hyosophorin-derived unique pentaantennary free glycan chain in the flounder embryos [A. Seko et al. (1989) J. Biol. Chem. 264, 15922-15929], it is anticipated that PNGase-catalyzed de-N-glycosylation of L-hyosophorin would be required at a certain stage of embryogenesis for L-hyosophorin to play a yet undefined functional role during early development.

New tandem-repeating peptide structures in polysialoglycoproteins from the unfertilized eggs of kokanee salmon

New polysialoglycoproteins, designated PSGP(On), were isolated from the fertilized and unfertilized eggs of the kokanee salmon, Oncorhynchus nerka adonis. The polysialylglycan chains consisting of alpha-2,8-linked O-acetylated poly(N-glycolylneuraminyl) chains have recently been characterized. We have now determined the complete amino acid sequence of the tandem-repeating units of PSGP(On) from the unfertilized eggs of kokanee salmon and found that the following two distinct forms are present in PSGP(On) in almost identical amounts: [formula: see text] and [formula: see text] where * denotes the O-glycosylation site and mean value of m, n = about 20. Upon fertilization these high-molecular-weight forms of PSGP(On) were proteolytically cleaved to the corresponding repeating units, low-molecular-weight PSGP(On), by the action of a specific protease (PSGPase) at the position two residues set C-terminally to the Pro residue and N-terminally to the Asp residue, i.e. -Pro-Ser-Xaa-Asp-: [formula: see text] and [formula: see text].