Monosaccharide composition and properties of a deglycosylated turnip peroxidase isozyme

A neutral peroxidase isozyme (TP) purified from turnip (Brassica napus L. var. purple top white globe) was partially deglycosylated, using chemical and enzymatic treatment. A 32% carbohydrate removal was achieved by exposing TP to a mixture of PNGase F, O-glycosidase, NANase, GALase III and HEXase I, while m-periodate treatment removed about 88% of TP carbohydrate moiety. The glycoprotein fraction of the TP contained a relatively high mannose and fucose content (37 and 31%, w/w, respectively), 16% (w/w) galactose, and 15% (w/w) GlcNAc. Thus, the carbohydrate moiety was classified as a hybrid type. Partially deglycosylated TP had reduced activity (by 50-85%), was more susceptible to proteolysis, and showed a slight decrease in thermostability compared to the native enzyme. Circular dichroism studies strongly suggested that although the carbohydrate moiety of TP did not influence the conformation of the polypeptide backbone, its presence considerably enhanced protein conformational stability toward heat. Removal of oligosaccharide chains from TP caused a decrease in K(m) and V(max) for hydrogen peroxide. Native and chemically deglycosylated TP were similarly immunodetected by rabbit polyclonal antibodies raised against TP. The results suggest that the carbohydrate moiety of TP is important for peroxidase activity and stability.     

Alkaline treatment has contrasting effects on the structure of deglycosylated and glycosylated forms of glucose oxidase

X-ray crystallographic studies on glucose oxidase showed a strong interaction between carbohydrate and protein moieties of the glycoprotein. However, experimental studies under physiological conditions reported no influence of carbohydrate moiety on the structural and functional properties of glucose oxidase. In order to demonstrate the role of carbohydrate moiety on the structure and stability, we carried out a detailed comparative study on the pH-induced structural changes in the native and deglycosylated forms of glucose oxidase. Our studies demonstrate that at physiological pH both forms of enzyme have very similar structural and stability properties. Acid denaturation also showed similar structural changes in both forms of the enzyme. However, on alkaline treatment contrasting effects on the structure and stability of the two forms of enzyme were observed. The glycosylated enzyme undergoes partial unfolding with decreased stability at alkaline pH; however, a compaction of native conformation and enhanced stability of enzyme was observed for the deglycosylated enzyme under similar conditions. This is the first experimental demonstration of the influence of carbohydrate moiety on structure and stability of glucose oxidase. The studies also indicate the importance of pH studies in evaluating the effect of carbohydrate moiety on the structural and stability properties of glycoprotein.     

Ovalbumin-related gene Y protein bears carbohydrate chains of the ovomucoid type

We have recently established the monoclonal antibodies (mAbs) specific to the major food allergen, ovomucoid, as mAb 7D, recognizing the carbohydrate moiety of ovomucoid, and mAb 6H, the peptide moiety (Biosci. Biothechnol. Biochem., 68, 2490-2497, (2004)). Using these mAbs, we found commercially available ovalbumin preparations contaminated with a considerable amount of ovomucoid together with other glycoproteins. To examine the contaminants, egg white was subjected to cation-exchange chromatography. An unidentified protein was found in egg white that reacted with mAb 7D but not with mAb 6H, having a molecular size of about 52 kDa and a blocked N-terminus. Two internal amino acid sequences of the fragments obtained after a lysyl endopeptidase and a hydroxylamine treatment revealed the protein to be ovalbumin Y (ovalbumin-related gene Y protein). We conclude that ovalbumin Y is a unique chimeric glycoprotein having an amino acid sequence similar to that of ovalbumin, but having a carbohydrate moiety similar to that of ovomucoid.     

Biochemical and immunologic studies on the native and deglycosylated forms of gamma-glutamyltranspeptidase of rat kidney

We have deglycosylated the enzyme gamma-glutamyl transpeptidase by treatment of the protein with anhydrous hydrofluoric acid at 0 degree C. After deglycosylation, the heavy and light subunits showed a molecular weight of 43 and 23 Kd respectively. Whereas the antiserum against the native enzyme recognized both proteins, the antiserum against the deglycosylated enzyme failed to recognize the native enzyme, indicating that some of the determinants of the native enzyme are masked by the carbohydrate moiety.     

Ovalbumin-Related Gene Y Protein Bears Carbohydrate Chains of the Ovomucoid Type

We have recently established the monoclonal antibodies (mAbs) specific to the major food allergen, ovomucoid, as mAb 7D, recognizing the carbohydrate moiety of ovomucoid, and mAb 6H, the peptide moiety (Biosci. Biothechnol. Biochem., 68, 2490–2497, (2004)). Using these mAbs, we found commercially available ovalbumin preparations contaminated with a considerable amount of ovomucoid together with other glycoproteins. To examine the contaminants, egg white was subjected to cation-exchange chromatography. An unidentified protein was found in egg white that reacted with mAb 7D but not with mAb 6H, having a molecular size of about 52 kDa and a blocked N-terminus. Two internal amino acid sequences of the fragments obtained after a lysyl endopeptidase and a hydroxylamine treatment revealed the protein to be ovalbumin Y (ovalbumin-related gene Y protein). We conclude that ovalbumin Y is a unique chimeric glycoprotein having an amino acid sequence similar to that of ovalbumin, but having a carbohydrate moiety similar to that of ovomucoid.     

Turtle-dove ovomucoid, a glycoprotein proteinase inhibitor with P1-blood-group antigen activity.

Ovomucoid from the egg white of turtle-dove (Streptopelia risoria) was purified and shown to be a glycoprotein of mol. wt. 29 400, with valine as N-terminal residue. It is an inhibitor of both trypsin and chymotrypsin, but has a lower affinity for trypsin than has hen ovomucoid. Turtle-dove ovomucoid contains antigenic activity cross-reacting with the blood-group-P1 antigen of human erythrocytes. Hen ovomucoid has no detectable blood group-P1 activity. The carbohydrate composition of turtle-dove ovomucoid differs from hen ovomucoid in having substantially higher galactose content. The possible relationship between carbohydrate composition and antigenic activity is discussed.     

Role of the carbohydrate part of yeast acid phosphatase

Acid phosphatase, purified from the yeast Saccharomyces cerevisiae, was completely deglycosylated by endo-beta-N-acetylglucosaminidase H or by HF treatment. Three protein bands were obtained on sodium dodecyl sulfate (SDS)-electrophoresis, with molecular weights of 73,000, 71,000 and 61,500. The released carbohydrate chains varied in size from 12 to 142 mannose units. To study the role of carbohydrate chains in the structure and function of acid phosphatase, a comparison of the properties of the partially deglycosylated enzyme with the native one was performed. The 60% deglycosylated enzyme retained the original activity, and CD and fluorescence spectra showed that the native conformation of the enzyme was preserved. The 90% deglycosylated enzyme showed a pronounced loss of enzyme activity, accompanied by the disruption of the three-dimensional structure. The partially deglycosylated enzyme was less soluble and more susceptible to denaturing effects of heat, pH, urea, and guanidine hydrochloride. Under conditions of electrophoresis, the partially deglycosylated enzyme dissociated, indicating a possible role of carbohydrate chains in maintaining the dimeric structure of the enzyme. Susceptibility of acid phosphatase toward proteolysis was drastically increased by deglycosylation.     

Function of carbohydrate moiety in acid phosphatase of Rhodotorula glutinis

The properties of native and partially deglycosylated forms of acid phosphatase from Rhodotorula glutinis were compared. The removal of carbohydrate moiety resulted in higher thermostability and resistance to proteolysis whereas specific activity, pH optimum and Km value with p-nitrophenyl phosphate remained unchanged. The role of carbohydrate moiety in stabilization of the enzyme structure and protection against proteolysis is suggested.     

The absence of a role for the carbohydrate moiety in the binding of apolipoprotein B to the low density lipoprotein receptor.

The binding of low density lipoprotein (LDL) to fibroblasts occurs through apolipoprotein B, a glycoprotein. The role of the carbohydrate in binding was assessed in two ways: (1) LDL, freed of sialic acid and most of the glucosamine and hexoses by digestion with a mixture of glycosidases, bound to fibroblasts as does native LDL. (2) The glycopeptides liberated from apoprotein B by trypsin and pronase failed to inhibit LDL binding to fibroblasts. Apparently the carbohydrate moiety of LDL does not interact with the plasma membrane receptor.     

N-linked glycosylation influences on the catalytic and biochemical properties of Penicillium purpurogenum β-d-glucuronidase

To study the influence of N-linked carbohydrate moiety on the catalytic and biochemical properties of glycosylated enzyme, a recombinant β-d-glucuronidase (PGUS-P) from Penicillium purpurogenum as a model glycoprotein, was deglycosylated with peptide-N-glycosidase F (PNGase-F) under native conditions. The enzymatic deglycosylation procedure resulted in the complete removal of carbohydrate moiety. Compared with the glycosylated PGUS-P, the deglycosylated PGUS-P exhibited 20-70% higher activity (p<0.05) within pH 6-9, but 15-45% lower activity (p<0.05) at 45-70°C. The apparent decrease in the thermal stability of the deglycosylated enzyme was reflected by a decrease in the denaturation temperature (T(d)) values determined by differential scanning calorimetry (DSC). The removal of N-linked glycans also reduced enzyme's sensitivity to certain metal ions. The deglycosylated PGUS-P displayed lower K(m) vaules, but higher k(cat)/K(m) ratios than the glycosylated isoform towards glycyrrhizin. The consequent conformational changes were also determined by circular dichroism (CD) and fluorescence spectroscopy which revealed no significant difference in the secondary but a slight dissimilarity between the tertiary structures of both isoforms of PGUS-P.     

Deglycosylation of ovalbumin prohibits formation of a heat-stable conformer

To study the influence of the carbohydrate-moiety of ovalbumin on the formation of the heat-stable conformer S-ovalbumin, ovalbumin is deglycosylated with PNGase-F under native conditions. Although the enzymatic deglycosylation procedure resulted in a complete loss of the ability to bind to Concavalin A column-material, only in about 50% the proteins lost their complete carbohydrate moiety, as demonstrated by mass spectrometry and size exclusion chromatography. Thermal stability and conformational changes were determined using circular dichroism and differential scanning calorimetry and demonstrated at ambient temperature no conformational changes due to the deglycosylation. Also the denaturation temperature of the processed proteins remained the same (77.4 +/- 0.4 degrees C). After heat treatment of the processed protein at 55 degrees C and pH 9.9 for 72 h, the condition that converts native ovalbumin into the heat-stable conformer (S-ovalbumin), only the material with the intact carbohydrate moiety forms this heat-stable conformer. The material that effectively lost its carbohydrate moiety appeared fully denatured and aggregated due to these processing conditions. These results indicate that the PNGase-F treatment of ovalbumin prohibits the formation and stabilization of the heat-stable conformer S-ovalbumin. Since S-ovalbumin in egg protein samples is known to affect functional properties, this work illustrates a potential route to control the quality of egg protein ingredients.     

Monoclonal antibodies against hen's egg ovomucoid

Two monoclonal antibodies (mAb 23E5 and 32A8) to hen's egg ovomucoid (OM), which causes hen's egg allergy and has trypsin inhibitory activity, were prepared and purified. Their affinity to the three separate domains of the ovomucoid, which are homologous in primary structure and are designated as DI, DII, and DIII, was studied by a competitive radioimmunoassay. MAb 23E5 bound to OM more efficiently than to DI, DII, or DIII-2 (with carbohydrate), but reacted with DIII-1 (free from carbohydrate) more efficiently than with OM. Except for the binding to OM, mAb 32A8 bound to DIII-2 most efficiently and to DIII-1 least efficiently, suggesting that this antibody recognized the carbohydrate moiety of DIII. MAb 32A8 inhibited the trypsin inhibitory activity of OM, whereas mAb 23E5 had no effect on it. These monoclonal antibodies should be useful for analyzing the antigenic determinants and trypsin inhibitory activity of ovomucoid.     

Determination of a sugar chain and its linkage site on a glycoprotein TIME-EA4 from silkworm diapause eggs by means of LC-ESI-Q-TOF-MS and MS/MS

The electrospray ionization (ESI)-tandem quadrupole/orthogonal-acceleration time-of-flight (Q-TOF) mass spectrometer combined with the nano-HPLC system was utilized to determine the glycosylation site and the glycan structure in glycoprotein TIME-EA4 (EA4) from Bombyx diapause eggs. LC-MS analysis of EA4 and deglycosylated EA4 indicated that the carbohydrate moiety of EA4 has the mass of 730.58 Da. Then, EA4 was digested with trypsin and chymotrypsin to identify the glycosylated peptide. The peptide fragment from G1y21 to Phe25 was found to carry the carbohydrate moiety. LC-MS/MS analysis of this peptide fragment revealed the sequence of the attached oligosaccharide and the glycosylation site at the same time. The present methodology utilizing the combination of the nano-HPLC system and a highly sensitive Q-TOF mass spectrometer is demonstrated to be quite effective for analyses of glycoproteins of relatively low purity and limited availability from natural sources.     

Studies on bonnet monkey cervical mucus the effect of proteases on mucus glycoproteins of macaca radiata

The influence of proteinases on monkey cervical glycoproteins was investigated to assess their effect on cervical mucus and, thereby, on sperm penetration. The major component of periovulatory cervical mucus, a high molecular weight glycoprotein, was treated with Pronase, trypsin, chymotrypsin, papain, and bovine seminal peptidase, and the enzyme-resistant glycoprotein was purified by gel filtration on Sepharose 2B. A macromolecular component in high yield was recovered containing carbohydrate and protein moieties. Asialoglycoprotein, on treatment with Pronase, trypsin, and bovine seminal peptidase released more than one glycoprotein fragment. The carbohydrate and amino acid components of the native and degraded glycoproteins were similar in composition with variations in proportions. The structure of the carbohydrate-rich, pronase-resistant glycoprotein, further purified on Sepharose 2B, was examined. Sequential Smith degradation and methylation of the degraded glycoprotein fragment established a structure that shows some differences to that of the native glycoprotein. The influence of proteinases on cervical-mucus glycoproteins and a possible mechanism of sperm penetration through Pronase-treated glycoproteins is discussed.     

Trypsin inhibition activity of heat-denatured ovomucoid: a kinetic study

A kinetic study was conducted on the effect of heating in the temperature range of 75-110 degrees C on the trypsin inhibition activity of ovomucoid. Heat treatment of isolated ovomucoid resulted in a time-dependent decrease in trypsin inhibition activity that could accurately be described by a first-order kinetic model. The magnitude and the temperature dependence of the rate constants was affected by the pH during heat treatment. The heat stability of ovomucoid was the lowest at pH 7.6. Heat treatments intended to decrease the trypsin inhibition activity should therefore be carried out as soon as possible after laying, because the ovomucoid was inactivated faster at the pH of fresh egg white (pH 7.6). The presence of the other egg white constituents decreased the heat stability of ovomucoid compared to that of the model system of ovomucoid in buffer, presumably by the formation of ovomucoid-lysozyme complexes in the former.     

Immunochemical studies on thermal denaturation of ovomucoid

The thermal denaturation of ovomucoid was investigated by immunochemical methods, namely immunoprecipitation analyses and antibody-Sepharose 4B column chromatography. In the immunoprecipitation analyses, heated ovomucoid (90 degrees C, 90 min, pH 7.2) required about twice the antigen addition of the native protein to approach maximal precipitation with specific antibody, and the maximal immunoprecipitation was decreased to 80% of that by native ovomucoid. However, heated protein inhibited the binding of antibody with native ovomucoid, and 100% inhibition was attained at about 4-times the antigen addition necessary for the native protein. Heated ovomucoid (100 degrees C, 120 min) showed little immunoprecipitation and inhibition. To ovomucoid antigenicity was diminished more slowly than the trypsin inhibitory activity by heating, e.g., heated ovomucoid (90 degrees C, 120 min) retained more than 30% of the antigenicity but little trypsin inhibitory activity. By passing through the immunoaffinity column, heated ovomucoid (90 degrees C, 90 min) was separated into two fractions, either with (fraction II) or without (fraction I) antigenicity. Fraction II contained smaller fractions of ordered secondary structure than native ovomucoid, and trypsin inhibitory activity of fraction II was only 24% of the native one. These results indicated that thermally denatured ovomucoid was heterogeneous regarding the conformational damage caused by heating, and the structure around some antigenic sites in an ovomucoid molecule was retained even after the backbone conformation was partially destroyed and trypsin inhibitory activity was lost.     

The absence of a role for the carbohydrate moiety in the binding of apolipoprotein B to the low density lipoprotein receptor

The binding of low density lipoprotein (LDL) to fibroblasts occurs through apolipoprotein B, a glycoprotein. The role of the carbohydrate in binding was assessed in two ways:

• 1.(1) LDL, freed of sialic acid and most of the glucosamine and hexoses by digestion with a mixture of glycosidases, bound to fibroblasts as does native LDL.
• 2.(2) The glycopeptides liberated from apoprotein B by trypsin and pronase failed to inhibit LDL binding to fibroblasts. Apparently the carbohydrate moiety of LDL does not interact with the plasma membrane receptor.

     

Partial deglycosylation of an anionic isoperoxidase from peach seeds – effect on enzyme activity, stability and antigenicity

An anionic isoperoxidase (EC 1.11.1.7) purified from peach seeds ( Prunus persica L. Batsch cv. Merry) was partially deglycosylated by glycopeptidase F (EC 3.2.2.18) treatment. A 40% deglycosylation resulted in an activity loss of 50% when assayed with o -dianisidine. 60% with guaiacol and 78% with 2,2'-azino-bis(3-ethyl)benzethiozoline-6-sulfonic acid (ABTS) as substrate. The indole-3-acetic acid oxidase activity loss was close to 55%. The partially deglycosylated isoperoxidase also showed a higher K_m value for H₂O₂ and higher values for Arrhenius activation energy and enthalpy of activation. There was a decrease in enzyme stability at 4°C after deglycosylation. Native and partially deglycosylated isoperoxidase reacted equally well in an enzyme-linked immunosorbent assay (ELISA) with rabbit polyclonal antibodies raised against the native enzyme. The carbohydrate moiety of this peach seed isoperoxidase appears to be important for enzyme activity and stability.     

Biochemical, biological, and immunological properties of chemically deglycosylated human choriogonadotropin

A chemical method of deglycosylation of human choriogonadotropin (hCG) was used to assess the role of carbohydrate moiety in the maintenance of quaternary structure and functional parameters such as receptor binding, immunological activity, and in vitro biological response. Treatment of purified hCG with anhydrous HF at 0 degrees C for 60 min was effective in removing more than 75% of the carbohydrate moiety. This extent of deglycosylation altered its chromatographic characteristics as revealed by retarded behavior on Sephadex G-100 and failure to be retained on concanavalin A-Sepharose. The electrophoretic heterogeneity present in native hCG was markedly reduced by deglycosylation. The deglycosylated hCG was stable in the lyophilized form and retained its quaternary structure as revealed by the fluorescence probe 8-anilino 1-naphthalene sulfonic acid, receptor binding, and immunological activities. Unlike receptor binding and immunological activities, which were fully retained, the ability of the hormone to stimulate cyclic AMP accumulation in vitro in rat interstitial cells was completely abolished.     

Active fragments obtained by cyanogen bromide cleavage of ovomucoid.

Cleavage of the two methionine residues in the glycoprotein trypsin inhibitor ovomucoid, variant O1, with CNBr resulted in two fragments whose mol.wts. were approx. 16 600 (fragment LS) and 11 000 (fragment M). Both fragments formed precipitates with antisera to ovomucoid. Fragment LS retained 56% of the trypsin-inhibitory activity of ovomucoid, but fragment M did not inhibit. After reduction and alkylation, the molecular weight of fragment M was unchanged, but fragment LS could be resolved into two segments of peptide chain with mol.wts. of approx. 12000 (fragment L) and 4700 (fragment S). Each of these peptides contained carbohydrate. Marked heterogeneity was observed in the hexose and hexosamine contents of fragment L. This may account for much of the heterogeneity in neutral carbohydrate occurring in ovomucoid preparations. It was found that fragment M was located at the N-terminal end, fragment S was in the centre and fragment L made up the C-terminal portion of the molecule.