Circular dichroism of oligosaccharides containing N-acetyl amino sugars

Distinct Fcα receptor N-glycans modulate the binding affinity to immunoglobulin A (IgA) antibodies

Human immunoglobulin A (IgA) is the most prevalent antibody class at mucosal sites with an important role in mucosal defense. Little is known about the impact of N-glycan modifications of IgA1 and IgA2 on binding to the Fcα receptor (FcαRI), which is also heavily glycosylated at its extracellular domain. Here, we transiently expressed human epidermal growth factor receptor 2 (HER2)-binding monomeric IgA1, IgA2m(1), and IgA2m(2) variants in Nicotiana benthamiana ΔXT/FT plants lacking the enzymes responsible for generating nonhuman N-glycan structures. By coinfiltrating IgA with the respective glycan-modifying enzymes, we generated IgA carrying distinct homogenous N-glycans. We demonstrate that distinctly different N-glycan profiles did not influence antigen binding or the overall structure and integrity of the IgA antibodies but did affect their thermal stability. Using size-exclusion chromatography, differential scanning and isothermal titration calorimetry, surface plasmon resonance spectroscopy, and molecular modeling, we probed distinct IgA1 and IgA2 glycoforms for binding to four different FcαRI glycoforms and investigated the thermodynamics and kinetics of complex formation. Our results suggest that different N-glycans on the receptor significantly contribute to binding affinities for its cognate ligand. We also noted that full-length IgA and FcαRI form a mixture of 1:1 and 1:2 complexes tending toward a 1:1 stoichiometry due to different IgA tailpiece conformations that make it less likely that both binding sites are simultaneously occupied. In conclusion, N-glycans of human IgA do not affect its structure and integrity but its thermal stability, and FcαRI N-glycans significantly modulate binding affinity to IgA.

Synchrotron-Radiation Vacuum-Ultraviolet Circular-Dichroism Spectroscopy for Characterizing the Structure of Saccharides

Circular-dichroism (CD) spectroscopy is a powerful tool for analyzing the structures of chiral molecules and biomolecules. The development of CD instruments using synchrotron radiation has greatly expanded the utility of this method by extending the spectra to the vacuum-ultraviolet (VUV) region below 190 nm and thereby yielding information that is unobtainable by conventional CD instruments. This technique is especially advantageous for monitoring the structure of saccharides that contain hydroxy and acetal groups with high-energy transitions in the VUV region. Combining VUVCD spectra with theoretical calculations provides new insight into the contributions of anomeric hydroxy groups and rotational isomers of hydroxymethyl groups to the dynamics, intramolecular hydrogen bonds, and hydration of saccharides in aqueous solution.

Synchrotron-radiation vacuum-ultraviolet circular dichroism spectroscopy in structural biology: an overview

Circular dichroism spectroscopy is widely used for analyzing the structures of chiral molecules, including biomolecules. Vacuum-ultraviolet circular dichroism (VUVCD) spectroscopy using synchrotron radiation can extend the short-wavelength limit into the vacuum-ultraviolet region (down to ~160 nm) to provide detailed and new information about the structures of biomolecules in combination with theoretical analysis and bioinformatics. The VUVCD spectra of saccharides can detect the high-energy transitions of chromophores such as hydroxy and acetal groups, disclosing the contributions of inter- or intramolecular hydrogen bonds to the equilibrium configuration of monosaccharides in aqueous solution. The roles of hydration in the fluctuation of the dihedral angles of carboxyl and amino groups of amino acids can be clarified by comparing the observed VUVCD spectra with those calculated theoretically. The VUVCD spectra of proteins markedly improves the accuracy of predicting the contents and number of segments of the secondary structures, and their amino acid sequences when combined with bioinformatics, for not only native but also nonnative and membrane-bound proteins. The VUVCD spectra of nucleic acids confirm the contributions of the base composition and sequence to the conformation in comparative analyses of synthetic poly-nucleotides composed of selected bases. This review surveys these recent applications of synchrotron-radiation VUVCD spectroscopy in structural biology, covering saccharides, amino acids, proteins, and nucleic acids.

Structures of intact glycoproteins from vibrational circular dichroism

Vibrational circular dichroism (VCD) spectra for the glycoproteins alpha1-acid glycoprotein (AGP) and bovine submaxillary mucin (BSM), have been measured in D2O solutions and for the films prepared from aqueous (H2O) buffer solutions in the 1800 to 900 cm(-1) region. The solution VCD results revealed that AGP has beta-sheet structure, along with a significant amount of alpha-helix as evidenced from a W pattern in the amide I region. The VCD of BSM solution suggested a polyproline II type structure, characterized by the appearance of strong negative couplet in the amide I region. The film VCD results on AGP and BSM suggested that the secondary structures of polypeptide fold in the film state are similar to those in the solution. The absence of any significant film VCD in the low frequency region (1200-900 cm(-1)), suggested that the dominant linkage for carbohydrate residues is likely to be a beta linkage. VCD spectroscopy gains importance in the secondary structural analysis of polypeptide fold in glycoproteins due to the absence of interfering VCD from the carbohydrate residues in the conformationally sensitive amide I region. Also, film VCD studies permit measurements in the low wavenumber region (1200-900 cm(-1)) that reveal the dominant type of linkage for carbohydrate residues. Such clear structural information is unlike that from ECD, where ECD bands of acylated amino sugar residues interfere with those of polypeptide backbone in the conformationally sensitive far-UV region.

Branched-chain analogues of linear polysaccharides: a spectroscopic and conformational investigation of chitosan derivatives

The solution properties and conformational features of 2-substituted propanoic acid (I) and 2-substituted pentanedioic acid (II) derivatives of chitosan were investigated over a wide range of pH by potentiometric, optical and chiroptical measurements, and by theoretical conformational analysis. No significant change is observed in the solution properties of I upon pH variations, in agreement with computational results showing that the conformational features of the polymer do not vary with respect to the charge state of the ionizable groups. In contrast, spectroscopic titration and preliminary 1H-n.m.r. data indicate that conformational equilibria in II are pH-dependent. Consistently, computed models show that both the charge state of the ionizable groups and the chirality of the carbon atom in the side chain control the structural features of the polymer.

Determination of the absolute configuration of the sugar residues of complex polysaccharides by circular dichroism spectroscopy

Circular dichroism spectroscopy is used to determine the absolute configuration of the constituent sugar residues of bacterial polysaccharides from two strains of Streptococcus sanguis which are important receptors in bacterial coaggregation in the formation of dental plaque. A high-pressure liquid chromatographic method for carbohydrate analysis of glycoproteins, glycolipids, and polysaccharides has been extended to sugar chirality determination by collection of the eluted HPLC fractions and subsequent measurement of their circular dichroism spectra. The method involves methanolysis of the polysaccharide followed by formation of O-benzoyl derivatives and HPLC on reverse-phase columns. Circular dichroism spectra of the collected derivatives in acetonitrile solution in the region 230 nm show large ellipticity bands resulting from "chiral exciton" interaction among the O-benzoyl chromophores which are sensitive to the orientation of hydroxyl groups in the parent sugars. The sensitivity of the method is in the submicrogram range for the absolute configuration of single sugar residues. The circular dichroism of the intact polysaccharide in aqueous solution shows CD bands from the amide chromophore in the region 180 to 220 nm which are sensitive to the chirality of 2-acetamido sugar residues.

Epitope expression of gonococcal lipooligosaccharide (LOS). Importance of the lipoidal moiety for expression of an epitope that exists in the oligosaccharide moiety of LOS

Antigenic expression of lipooligosaccharide (LOS) of strain F62 of Neisseria gonorrhoeae, was investigated with mouse monoclonal IgM antibody 3F11. F62 LOS was modified in various ways in order to understand structural requirements for expression of the 3F11-defined epitope. When the LOS was partially deacylated by treating it with 50 mM NaOH at 80 degrees C for 20 min or with anhydrous hydrazine at 80 degrees C for 20 min, the binding of 3F11 to those deacylated LOS samples decreased significantly. Removal of phosphate groups by treatment of the LOS with HF (4 days at 4 degrees C) did not affect the antigenicity at all. Neither did reduction of carboxyl groups in the LOS molecule (by activation of carboxyl groups with a carbodiimide followed by treatment with NaBH4) alter epitope expression. On oxidation with NaIO4, the LOS lost its antigenicity completely. The presence of Mg2+ did not change the circular dichroism (CD) behavior of F62 LOS. However, the partially deacylated LOS samples showed significantly different CD patterns in the 190-200 nm region compared with F62 LOS, which suggests conformational changes of F62 LOS due to the loss of fatty acids in the lipoidal moiety. Oligosaccharide (OS) and lipoidal components obtained after hydrolysis of F62 LOS with 1% acetic acid, were not recognized by the antibody. The antigenicity of OS was not retained by non-stereospecific acylation of OS with decanoyl chloride. We conclude the following: (1) 3F11-defined epitope exists in the OS moiety of F62 LOS; however, for it to be expressed, the carbohydrate moiety must be in a certain conformation that is defined by an overall structure of the LOS molecule. This structure is significantly influenced by some of the fatty acids in the lipoidal moiety of the LOS molecule; (2) the presence of phosphate or 3-deoxy-manno-2-octulosonic acid (dOclA) is not essential for expression of the 3F11-defined epitope; (3) the presence of divalent cations does not affect epitope expression.

Biochemical and biophysical comparison of two mucins from human submandibular-sublingual saliva

A high-molecular-weight mucin-glycoprotein (MG1) was isolated from human submandibular-sublingual saliva and was comprised of 14.9% protein, 29.0% N-acetylglucosamine, 9.4% N-acetylgalactosamine, 10.5% fucose, 24.2% galactose, 0.9% mannose, 4.0% N-acetylneuraminic acid, and 7.0% sulfate. Carbohydrate units were O-glycosidically linked and ranged in size from 4 to 16 residues. The biophysical properties of MG1 were compared to those of a smaller mucin (MG2) also isolated from submandibular-sublingual saliva. Fluorescence spectroscopy demonstrated that MG1 bound both 1-anilino-8-naphthalenesulfonate (ANS) and N-phenyl-1-naphthylamine (NPNA) in stable hydrophobic binding sites (melting temperature, 47 +/- 2 degrees C), whereas MG2 did not bind these hydrophobic probes. These hydrophobic domains occurred on nonglycosylated or naked portions of MG1 since Pronase treatment eliminated ANS binding. Reduction of disulfide bridges in MG1 increased the number of available hydrophobic binding sites. High ionic strength (0 to 2 M NaCl) had no effect on ligand binding, whereas lowering pH (9 to 2) increased ANS binding without affecting NPNA complexation. Circular dichroism (CD) data suggested that MG1's carbohydrate chains dominated its spectrum. In contrast, the peptide backbone dominated the CD spectrum of MG2. Collectively, the results of this study indicate that human submandibular-sublingual saliva contains two structurally distinct mucins.

Conformational study of O-glycosylated threonine containing peptide models

1H n.m.r. studies of Z-Thr-OMe, Z-Thr-Ala-OMe, Z-Ala-Thr-OMe and their glycosylated derivatives indicate the possibility of an intramolecular hydrogen bond between Thr N alpha H and the N-acetyl carbonyl of the carbohydrate moiety, 3,4,6-tri-O-acetyl-2-acetamido-2-deoxy-alpha-D-galactopyranose (AcGalNAc). This is especially true in the case of Z-Thr(AcGalNAc)-Ala-OMe, suggesting that the strength of this hydrogen bond is dependent on the neighboring amino acids on the carbonyl terminal side of Thr. The existence of such a hydrogen bond implies a conformation in which the carbohydrate moiety is restricted to an orientation with its plane roughly perpendicular to the peptide backbone. In such an orientation, steric problems will be minimized in the case of clustered O-glycosidically linked Thr(Ser) residues as found in human erythrocyte glycophorin. A locked orientation of the carbohydrate moiety with respect to the peptide backbone may also play a conformational role in antifreeze glycoproteins.

Vacuum ultraviolet circular dichroism of keratan sulfate

The vacuum ultraviolet CD of keratan sulfate reveals an intense negative CD band at 171 nm. Its intensity can be rationalized with a recently proposed quadrant rule in terms of the acetamido group being slightly tilted toward the hexosaminidic linkage oxygen. The same structural feature accounts for the particularly intense negative n-pi CD band near 210 nm.

Circular dichroism studies of M and N blood group-specific glycoproteins and glycopeptides

CD spectra of M and N blood group glycoproteins and their NH2-terminal tryptic glycopeptides were compared. The CD spectra of glycoproteins obtained suggest the presence of a low content of alpha-helix. The M glycoprotein contained a higher percentage of a periodical structure. CD spectra of M and N tryptic glycopeptides, both sialoglycopeptides and desialylated preparates, were similar and showed positive ellipticities in the range of 210--240 m. However, the spectra of desialylated glycopeptides were red-shifted in respect to spectra of sialoglycopeptides. The CD spectra of glycopeptides suggest the presence of beta-turns.

Glycosaminoglycans: structure and interaction

In the last few years, there has been considerable progress in the studies on glycosaminoglycans, a group of acidic polysaccharides present in the intercellular matrix of connective tissue. X-ray diffraction studies have indicated that these polymers can exist in the condensed phase in some helical form. Chiroptical and hydrodynamic measurements have provided significant information regarding the molecular conformation in solution and other physicochemical properties of the polymers. Studies related to the interaction properties of glycosaminoglycans with polypeptides, metal ions, and other molecules are numerous. This review covers mainly the results and their interpretations of both published and as yet unpublished material of the 1970s, but certain previous data are also included. A present-day concept regarding the structure and interaction properties of these molecules on the basis of various physicochemical measurements is presented. The biosynthesis and metabolism of glycosaminoglycans, and the structure of proteoglycans and glycoproteins, are not discussed.

Determination of the linkages of disaccharides containing a 2-acetamido-2-deoxy sugar unit by solvent effects in circular dichroism

The circular dichroism spectra of 2-acetamido-2-deoxy sugars in 1,1,1,3,3,3-hexafluoro-2-propanol (F6Pr-2-ol) solutions show a positive band in the n-pi region (209 nm) in contrast to a negative c.d. band in water solution. This difference is interpreted as an indication of a change in the average orientation of the hydroxyl groups adjacent to the amide group. C.d. spectra of 2-acetamido-2-deoxy sugars having a methyl group at O-1 and O-3 confirm this interpretation and suggest that the c.d. spectrum of a disaccharide in F6Pr-2-ol reflects strongly the disaccharide linkage. Large differences in the c.d. spectra of (1 leads to 4) and (1 leads to 6)-linked disaccharides in this solvent lead to rules for distinguishing the linkages of the disaccharides.