Conformational flexibility of the group B meningococcal polysaccharide in solution

Molecular Recognition Insights of Sialic Acid Glycans by Distinct Receptors Unveiled by NMR and Molecular Modeling

All cells are decorated with a highly dense and complex structure of glycan chains, which are mostly attached to proteins and lipids. In this context, sialic acids are a family of nine-carbon acidic monosaccharides typically found at the terminal position of glycan chains, modulating several physiological and pathological processes. Sialic acids have many structural and modulatory roles due to their negative charge and hydrophilicity. In addition, the recognition of sialic acid glycans by mammalian cell lectins, such as siglecs, has been described as an important immunological checkpoint. Furthermore, sialic acid glycans also play a pivotal role in host-pathogen interactions. Various pathogen receptors exposed on the surface of viruses and bacteria are responsible for the binding to sialic acid sugars located on the surface of host cells, becoming a critical point of contact in the infection process. Understanding the molecular mechanism of sialic acid glycans recognition by sialic acid-binding proteins, present on the surface of pathogens or human cells, is essential to realize the biological mechanism of these events and paves the way for the rational development of strategies to modulate sialic acid-protein interactions in diseases. In this perspective, nuclear magnetic resonance (NMR) spectroscopy, assisted with molecular modeling protocols, is a versatile and powerful technique to investigate the structural and dynamic aspects of glycoconjugates and their interactions in solution at the atomic level. NMR provides the corresponding ligand and protein epitopes, essential for designing and developing potential glycan-based therapies. In this review, we critically discuss the current state of knowledge about the structural features behind the molecular recognition of sialic acid glycans by different receptors, naturally present on human cells or pathogens, disclosed by NMR spectroscopy and molecular modeling protocols.

The polyfunctional polysialic acid: A structural view

Polysialic acid (polySia), a homopolymer of α2,8-linked sialic acid residues, modifies a small number of proteins and has central functions in vertebrate signalling. Here, we review the regulatory functions of polySia in signalling processes and the immune system of adult humans, as well as functions based on their chemical properties. The main focus will be on the structure-function relationship of polySia with its interaction partners in humans. Recent studies have indicated that the degree of polymerisation is an important parameter that can guide the regulatory effect of polySia in addition to its binding to target proteins. Therefore, the structures of polySia in solution and bound to interaction partners are compared in order to identify the key factors that define binding specificity.

Surface architecture of Neisseria meningitidis capsule and outer membrane as revealed by Atomic Force Microscopy

An extensive morphological analysis of the Neisseria meningitidis cell envelope, including serogroup B capsule and outer membrane, based on atomic force microscopy (AFM) together with mechanical characterization by force spectroscopic measurements, has been carried out. Three meningococcal strains were used: the encapsulated serogroup B strain B1940, and the isogenic mutants B1940 siaD(+C) (lacking capsule), and B1940 cps (lacking both capsule and lipooligosaccharide outer core). regularly structured AFM experiments with the encapsulated strain B1940 provided unprecedented images of the meningococcal capsule, which seems to be characterized by protrusions ("bumps") with the lateral dimensions of about 30 nm. Measurement of the Young's modulus provided quantitative assessment of the property of the capsule to confer resistance to mechanical stress. Moreover, Raman spectroscopy gave a fingerprint by which it was possible to identify the specific molecular species of the three strains analyzed, and to highlight major differences between them.

The Incorporation of Labile Protons into Multidimensional NMR Analyses: Glycan Structures Revisited

Glycan structures are often stabilized by a repertoire of hydrogen-bonded donor/acceptor groups, revealing longer-lived structures that could represent biologically relevant conformations. NMR provides unique data on these hydrogen-bonded networks from multidimensional experiments detecting cross-peaks resulting from through-bond (TOCSY) or through-space (NOESY) interactions. However, fast OH/H₂O exchange, and the spectral proximity among these NMR resonances, hamper the use of glycans’ labile protons in such analyses; consequently, studies are often restricted to aprotic solvents or supercooled aqueous solutions. These nonphysiological conditions may lead to unrepresentative structures or to probing a small subset of accessible conformations that may miss “active” glycan conformations. Looped, projected spectroscopy (L-PROSY) has been recently shown to substantially enhance protein NOESY and TOCSY cross-peaks, for ¹Hs that undergo fast exchange with water. This study shows that even larger enhancements can be obtained for rapidly exchanging OHs in saccharides, leading to the retrieval of previously undetectable 2D TOCSY/NOESY cross-peaks with nonlabile protons. After demonstrating ≥300% signal enhancements on model monosaccharides, these experiments were applied at 1 GHz to elucidate the structural network adopted by a sialic acid homotetramer, used as a model for α,2–8 linked polysaccharides. High-field L-PROSY NMR enabled these studies at higher temperatures and provided insight previously unavailable from lower-field NMR investigations on supercooled samples, involving mostly nonlabile nuclei. Using L-PROSY’s NOEs and other restraints, a revised structural model for the homotetramer was obtained combining rigid motifs and flexible segments, that is well represented by conformations derived from 40 μs molecular dynamics simulations.

Bacterial polysaccharides: conformation, dynamics and molecular recognition by antibodies

Bacterial infections are the cause of different severe health conditions and new therapies to combat these pathogens have been widely investigated. Carbohydrates, being complex structures covering the surface of bacteria, are considered relevant targets for antibody and vaccine development. The biological activities in pathogenesis of bacterial capsular polysaccharides and lipopolisaccharides and their unique structures have boosted the study of the minimal antigenic binding epitopes and the structural details of antibody-carbohydrate recognition. This review describes the most recent advances on the field, examining the structure, conformation and dynamics of relevant bacterial carbohydrates and their complexes with antibodies. The understanding of key factors governing the recognition process is fundamental for the progress toward the development of specific and efficient bacterial therapeutics.

Molecular Conformations of Di-, Tri-, and Tetra-α-(2→8)-Linked Sialic Acid from NMR Spectroscopy and MD Simulations

By using molecular dynamics simulations with an efficient enhanced sampling technique and in combination with nuclear magnetic resonance (NMR) spectroscopy quantitative structural information on α-2,8-linked sialic acids is presented. We used a bottom-up approach to obtain a set of larger ensembles for tetra- and deca-sialic acid from model dimer and trimer systems that are in agreement with the available J-coupling constants and nuclear Overhauser effects. The molecular dynamic (MD) simulations with enhanced sampling are used to validate the force field used in this study for its further use. This empowered us to couple NMR observables in the MD framework via J-coupling and distance restraining simulations to obtain conformations that are supported by experimental data. We used these conformations in thermodynamic integration and one-step perturbation simulations to calculate the free-energy of suggested helical conformations. This study brings most of the available NMR experiments together and supplies information to resolve the conflict on the structures of poly-α-2,8-linked sialic acid.

Physical biology of the cancer cell glycocalyx

The glycocalyx coating the outside of most cells is a polymer meshwork comprising proteins and complex sugar chains called glycans. From a physical perspective, the glycocalyx has long been considered a simple 'slime' that protects cells from mechanical disruption or against pathogen interactions, but the great complexity of the structure argues for the evolution of more advanced functionality: the glycocalyx serves as the complex physical environment within which cell-surface receptors reside and operate. Recent studies have demonstrated that the glycocalyx can exert thermodynamic and kinetic control over cell signalling by serving as the local medium within which receptors diffuse, assemble and function. The composition and structure of the glycocalyx change markedly with changes in cell state, including transformation. Notably, cancer-specific changes fuel the synthesis of monomeric building blocks and machinery for production of long-chain polymers that alter the physical and chemical structure of the glycocalyx. In this Review, we discuss these changes and their physical consequences on receptor function and emergent cell behaviours.

The β-reducing end in α(2-8)-polysialic acid constitutes a unique structural motif

Over the years, structural characterizations of α(2-8)-polysialic acid (polySia) in solution have produced inconclusive results. Efforts for obtaining detailed information in this important antigen have focused primarily on the α-linked residues and not on the distinctive characteristics of the terminal ones. The thermodynamically preferred anomeric configuration for the reducing end of sialic acids is β, which has the [I]CO2- group equatorial and the OH ([I]OH2) axial, while for all other residues the CO2- group is axial. We show that this purportedly minor difference has distinct consequences for the structure of α(2-8)-polySia near the reducing end, as the β configuration places the [I]OH2 in a favorable position for the formation of a hydrogen bond with the carboxylate group of the following residue ([II]CO2-). Molecular dynamics (MD) simulations predicted the hydrogen bond, which we subsequently directly detected by NMR. The combination of MD and residual dipolar couplings shows that the net result for the structure of Sia2-βOH is a stable conformation with well-defined hydration and charge patterns, and consistent with experimental NOE-based hydroxyl and aliphatic inter-proton distances. Moreover, we provide evidence that this distinct conformation is preserved on Sia oligosaccharides, thus constituting a motif that determines the structure and dynamics of α(2-8)-polySia for at least the first two residues of the polymer. We suggest the hypothesis that this structural motif sheds light on a longtime puzzling observation for the requirement of 10 residues of α(2-8)-polySia in order to bind effectively to specific antibodies, about four units more than for analogous cases.

Toll-like receptor 4-related immunostimulatory polysaccharides: Primary structure, activity relationships, and possible interaction models

Toll-like receptor (TLR) 4 is an important polysaccharide receptor; however, the relationships between the structures and biological activities of TLR4 and polysaccharides remain unknown. Many recent findings have revealed the primary structure of TLR4/MD-2-related polysaccharides, and several three-dimensional structure models of polysaccharide-binding proteins have been reported; and these models provide insights into the mechanisms through which polysaccharides interact with TLR4. In this review, we first discuss the origins of polysaccharides related to TLR4, including polysaccharides from higher plants, fungi, bacteria, algae, and animals. We then briefly describe the glucosidic bond types of TLR4-related heteroglycans and homoglycans and describe the typical molecular weights of TLR4-related polysaccharides. The primary structures and activity relationships of polysaccharides with TLR4/MD-2 are also discussed. Finally, based on the existing interaction models of LPS with TLR4/MD-2 and linear polysaccharides with proteins, we provide insights into the possible interaction models of polysaccharide ligands with TLR4/MD-2. To our knowledge, this review is the first to summarize the primary structures and activity relationships of TLR4-related polysaccharides and the possible mechanisms of interaction for TLR4 and TLR4-related polysaccharides.

Three-dimensional structural aspects of protein-polysaccharide interactions

Linear polysaccharides are typically composed of repeating mono- or disaccharide units and are ubiquitous among living organisms. Polysaccharide diversity arises from chain-length variation, branching, and additional modifications. Structural diversity is associated with various physiological functions, which are often regulated by cognate polysaccharide-binding proteins. Proteins that interact with linear polysaccharides have been identified or developed, such as galectins and polysaccharide-specific antibodies, respectively. Currently, data is accumulating on the three-dimensional structure of polysaccharide-binding proteins. These proteins are classified into two types: exo-type and endo-type. The former group specifically interacts with the terminal units of polysaccharides, whereas the latter with internal units. In this review, we describe the structural aspects of exo-type and endo-type protein-polysaccharide interactions. Further, we discuss the structural basis for affinity and specificity enhancement in the face of inherently weak binding interactions.

Elucidating Structural Features of an Entirely Carbohydrate Cancer Vaccine Construct Employing Circular Dichroism and Fluorescent Labeling

The zwitterionic polysaccharide PS A1 from anaerobe Bacteroides fragilis ATCC 25285/NCTC 9343 is known to elicit a T-cell-dependent, major histocompatibility complex class II (MHCII) immune response through a correspondingly similar protein-antigen-based mechanism/pathway. The biological activity of PS A1 is known to arise from alternating charged motifs on adjacent monosaccharides comprising a tetrameric repeating oligomeric unit creating an alpha-helical secondary structure. However, we have learned that this alpha-helical structural characteristic may not play a role in immune activation. Paradoxically, our current knowledge of structure - activity relationships (SARs) with electrostatically charged polysaccharides has become more clearly defined, yet a lack of tools/probes for measuring dynamic structural changes hinders progress in carbohydrate-based vaccine development. Site- and region-specific structural modifications of PS A1, followed by conjugation with a known carbohydrate cancer antigen, the Thomsen-nouveau (Tn = alpha-D-GalNAc-OSer/Thr) antigen, does not alter antibody isotype switching ability and leads to specific IgG3 antibodies in C57BL/6 mice. Circular dichroism (CD) and studies using fluorescently labeled PS A1, described herein, reveal information pertaining to structure - activity relationships and the nature of Tn conjugation to chemically modified PS A1. The CD spectra of a Tn-PS A1 construct at 8.5 ≥ pH ≤ 3.5 illustrates complete loss of alpha-helical character while spectra obtained in the 3.6 ≤ pH ≥ 8.4 range denotes minimal alpha-helicity in comparison to naturally occurring PS A1. Temperatures exceeding 60 °C reveal complete loss of helical character. Two methods for Alexa Fluor488^® fluorescent labeling studies of chemically oxidized PS A1 have given rise to percent conjugation values (% loading) calculated to be on average 35 Tn molecules bound. Combined, our results argue that altering the structure of PS A1, without chemically modifying the electrostatic charge character, does not alter immune response/recognition in mice. These findings have important implications for the design of entirely carbohydrate-based vaccine constructs.

Crystal structure of anti-polysialic acid antibody single chain Fv fragment complexed with octasialic acid: insight into the binding preference for polysialic acid

Polysialic acid is a linear homopolymer of α2-8-linked sialic acids attached mainly onto glycoproteins. Cell surface polysialic acid plays roles in cell adhesion and differentiation events in a manner that is often dependent on the degree of polymerization (DP). Anti-oligo/polysialic acid antibodies have DP-dependent antigenic specificity, and such antibodies are widely utilized in biological studies for detecting and distinguishing between different oligo/polysialic acids. A murine monoclonal antibody mAb735 has a unique preference for longer polymers of polysialic acid (DP >10), yet the mechanism of recognition at the atomic level remains unclear. Here, we report the crystal structure of mAb735 single chain variable fragment (scFv735) in complex with octasialic acid at 1.8 Å resolution. In the asymmetric unit, two scFv735 molecules associate with one octasialic acid. In both complexes of the unit, all the complementarity-determining regions except for L3 interact with three consecutive sialic acid residues out of the eight. A striking feature of the complex is that 11 ordered water molecules bridge the gap between antibody and ligand, whereas the direct antibody-ligand interaction is less extensive. The dihedral angles of the trisialic acid unit directly interacting with scFv735 are not uniform, indicating that mAb735 does not strictly favor the previously proposed helical conformation. Importantly, both reducing and nonreducing ends of the bound ligand are completely exposed to solvent. We suggest that mAb735 gains its apparent high affinity for a longer polysialic acid chain by recognizing every three sialic acid units in a paired manner.

Dynamics of exocyclic groups in the Escherichia coli O91 O-antigen polysaccharide in solution studied by carbon-13 NMR relaxation

Carbon-13 relaxation data are reported for exocyclic groups of hexopyranosyl sugar residues in the repeating unit within the Escherichia coli O91 O-antigen polysaccharide in a dilute D2O solution. The measurements of T 1, T 2 and heteronuclear nuclear Overhauser enhancements were carried out at 310 K at two magnetic fields (16.4 T, 21.1 T). The data were analyzed using the standard and extended Lipari-Szabo models, as well as a conformational jump model. The extended version of the Lipari-Szabo and the two-site jump models were most successful for the hydroxymethyl groups of Gal and GlcNAc sugar residues. Different dynamics was found for the hydroxymethyl groups associated with different configurations (D-gluco, D-galacto) of the sugar residues, the latter being faster than the former.

Evidence for helical structure in a tetramer of α2-8 sialic acid: unveiling a structural antigen

Characteristic H-bonding patterns define secondary structure in proteins and nucleic acids. We show that similar patterns apply for α2-8 sialic acid (SiA) in H(2)O and that H-bonds define its structure. A (15)N,(13)C α2-8 SiA tetramer, (SiA)(4), was used as a model system for the polymer. At 263 K, we detected intra-residue through-H-bond J couplings between (15)N and C8 for residues R-I-R-III of the tetramer, indicating H-bonds between the (15)N's and the O8's of these residues. Additional J couplings between the (15)N's and C2's of the adjacent residues confirm the putative H-bonds. NH groups showing this long-range correlation also experience slower (1)H/(2)H exchange. Additionally, detection of couplings between H7 and C2 for R-II and R-III implies that the conformations of the linkers between these residues are different than in the monomers. These structural elements are consistent with two left-handed helical models: 2 residues/turn (2(4) helix) and 4 residues/turn (1(4) helix). To discriminate between models, we resorted to (1)H,(1)H NOEs. The 2(4) helical model is in better agreement with the experimental data. We provide direct evidence of H-bonding for (SiA)(4) and show how H-bonds can be a determining factor for shaping its 3D structure.

Sucrose in aqueous solution revisited, Part 2: adaptively biased molecular dynamics simulations and computational analysis of NMR relaxation

We report 100 ns molecular dynamics simulations, at various temperatures, of sucrose in water (with concentrations of sucrose ranging from 0.02 to 4M), and in a 7:3 water-DMSO mixture. Convergence of the resulting conformational ensembles was checked using adaptive-biased simulations along the glycosidic Φ and ψ torsion angles. NMR relaxation parameters, including longitudinal (R₁) and transverse (R₂) relaxation rates, nuclear Overhauser enhancements (NOE), and generalized order parameter (S²) were computed from the resulting time-correlation functions. The amplitude and time scales of molecular motions change with temperature and concentration in ways that track closely with experimental results, and are consistent with a model in which sucrose conformational fluctuations are limited (with 80-90% of the conformations having ϕ-ψ values within 20° of an average conformation), but with some important differences in conformation between pure water and DMSO-water mixtures.

Capsular polysaccharide vaccine for Group B Neisseria meningitidis, Escherichia coli K1, and Pasteurella haemolytica A2

We reviewed the literature that is the basis for our proposal that (2→8)-α-Neu5Ac conjugates will be safe and effective vaccines for Group B meningococci (GBMs), Escherichia coli K1, and Pasteurella haemolytica A2. Although (2→8)-α-Neu5Ac is a virulence factor and a protective antigen of these three pathogens, it is also a component of normal tissues (neural cell adhesion molecule). Natural, anti-(2→8)-α-Neu5Ac present in most adults, vaccine-induced antibodies, and even high levels of spontaneously appearing monoclonal anti-(2→8)-α-Neu5Ac did not cause autoimmunity. Although it is not possible to prove a null hypothesis, there are no epidemiologic, serologic, immunologic, or clinical data to indicate that (2→8)-α-Neu5Ac antibodies will induce pathology or an autoimmune disease. No increased pathology caused by these antibodies was found, even in neonates and infants of mothers recovered from GBM meningitis. The lack of pathology mediated by anti-(2→8)-α-Neu5Ac may be explained by different presentations of (2→8)-α-Neu5Ac on bacterial and mammalian cells and by the unusual physicochemical properties of anti-(2→8)-α-Neu5Ac. Based on clinical and experimental data collected over 30 y and because (2→8)-α-Neu5Ac is an essential virulence factor and a protective antigen for GBM, E. coli K1, and P. haemolytica A2, protein conjugates of it are easy to prepare using inexpensive and plentiful ingredients, and they would be compatible with routinely administered infant vaccines, clinical studies of these conjugates should proceed.

Searching and optimizing structure ensembles for complex flexible sugars

NMR restrictions are suitable to specify the geometry of a molecule when a single well-defined global free energy minimum exists that is significantly lower than other local minima. Carbohydrates are quite flexible, and therefore, NMR observables do not always correlate with a single conformer but instead with an ensemble of low free energy conformers that can be accessed by thermal fluctuations. In this communication, we describe a novel procedure to identify and weight the contribution to the ensemble of local minima conformers based on comparison to residual dipolar couplings (RDCs) or other NMR observables, such as scalar couplings. A genetic algorithm is implemented to globally minimize the R factor comparing calculated RDCs to experiment. This is done by optimizing the weights of different conformers derived from the exhaustive local minima conformational search program, fast sugar structure prediction software (FSPS). We apply this framework to six human milk sugars, LND-1, LNF-1, LNF-2, LNF-3, LNnT, and LNT, and are able to determine corresponding population weights for the ensemble of conformers. Interestingly, our results indicate that in all cases the RDCs can be well represented by only a few most important conformers. This confirms that several, but not all of the glycosidic linkages in histo-blood group "epitopes" are quite rigid.

Dynamics of chitosan by (1)h NMR relaxation

The dynamics of chitosan (CS) in solution have been studied by (1)H NMR relaxation [longitudinal (T(1)) and transverse (T(2)) relaxation times and NOE] as a function of the degrees of acetylation (DA, 1-70) and polymerization (DP, 10-1200), temperature (278-343 K), concentration (0.1-30 g/L), and ionic strength (50-400 mM). This analysis points to CS as a semirigid polymer with increased flexibility at higher DA in agreement with reduced electrostatic repulsions between protonated amino groups.

Biosynthesis and production of polysialic acids in bacteria

Polysialic acids (PA) are protective capsular sialohomopolymers present in some bacteria which can invade the mammalian host and cause lethal bacteremia and meningitis. Biosynthesis and translocation of PA to the cell surface are equivalent in different species and bacterial strains which are produced. The diversity in PA structure is derived from the PA linkages and is a consequence of the specific sialyltransferase activities. The monomer acetylation and the polymer length could be important factors in the potential virulence. In vivo PA production is affected by different physical and chemical factors. The temperature of cellular growth strictly regulates PA genesis through a molecular complex and multifactorial mechanism that operate to transcription level.

The conformational properties of methyl alpha-(2,8)-di/trisialosides and their N-acyl analogues: implications for anti-Neisseria meningitidis B vaccine design

The conformational properties of di- and trisaccharide fragments of the polysialic acid O-antigen capsular polysaccharide (CPS) of Neisseria meningitidis B (NmB) have been investigated by a combination of solution phase NMR spectroscopy and explicit-solvent molecular dynamics (MD) simulations. Simulations employing 100 ns of conventional MD, as well as 160 ns of replica exchange MD (REMD), with the GLYCAM06 force field were shown to be in agreement with experimental NMR scalar J-coupling and NOE values. The presence of conformational families has been determined by monitoring interglycosidic torsion angles, by comparing structural superimpositions, as well as via a Bayesian statistical analysis of the torsional data. Attempts to augment the immunogenicity of NmB CPS often involve chemical modifications of the N-acetyl moiety. Here the effects of these chemical group modifications on the conformational properties of the trisialoside have been probed via REMD simulations of the N-glycolyl, N-propionyl, N-propyl and N-butanoyl analogues. Although there were conformational families unique to each non-native analogue, the chemical modifications resulted in largely equivalent overall conformational phase-spaces compared to the native trisialoside. On the basis of the conformational distributions, these shared conformational properties suggest that a recurrent global conformational epitope may be present in both the native and chemically modified CPS fragments. Explanations are therefore provided for monoclonal antibody cross-reactivity, in terms of recognition of a shared global CPS conformation, as well as for lack of cross-reactivity, in terms of fine structural differences associated with the N-acyl groups, which may be dominant in highly matured antibody responses.

Extracellular structure of polysialic acid explored by on cell solution NMR

The capsular polysaccharide of the pathogens Neisseria meningitidis serogroup B and of Escherichia coli K1, alpha(2 --> 8) polysialic acid (PSA), is unusual, because when injected into adult humans, it generates little or no antibody. In contrast, people infected with these pathogens generate specific serum antibodies. A structural study on cells is used to address this anomaly by characterizing antigen structures in vivo. We introduce on cell multidimensional solution NMR spectroscopy for direct observation of PSA on E. coli bacteria. Using 13C,15N-labeled PSA, we applied a combination of heteronuclear NMR methods, such as heteronuclear single quantum coherence, HNCA, and HNCO, in vivo. Analysis reveals that free and cell-bound PSA are structurally similar, indicating that the poor immunogenicity of PSA is not due to major structural differences between cells and purified PSA. The 13C linewidths of PSA on cells are 2 to 3 times larger than the corresponding ones in free PSA. The possible implications of the differences between free and on cell PSA are discussed. In addition, we demonstrate the suitability of the method for in vivo kinetic studies.

Going for baroque at the Escherichia coli K1 cell surface

Phase variation is usually thought of as the stochastic switching between alternatively expressed ('on') and unexpressed ('off') phenotypic states. However, coupling synthesis of a monotonous homopolysaccharide to a mechanism of random but incomplete chemical modification produces almost infinite structural variation. Potentially limitless variability implies that evolution can produce highly ornate or extravagant flourishes reminiscent of the baroque style. Here, we describe an analysis of capsular polysialic acid form variation in Escherichia coli K1, demonstrating that the large number of variant structures is controlled by a single contingency locus. The mechanism for generating maximum structural diversity from maximal genetic parsimony is conferred by a simple translational switch carried on a K1-specific prophage.

Structure and function relations with a T-cell-activating polysaccharide antigen using circular dichroism

Studies centered on understanding how molecular structure affects biological function have historically focused on proteins. Circular dichroism (CD) is commonly used to analyze protein secondary structure, yet its application to other molecules is far less explored. In fact, little is known about how glycan conformation might affect function, likely because of a lack of tools for measuring dynamic structural changes of carbohydrates. In the present study, we developed a method based on CD to monitor conformational changes in the zwitterionic T-cell-activating glycoantigen polysaccharide A1 (PSA). We found that PSA helical structure produces a CD spectrum that is strikingly similar to proteins rich in alpha-helical content and is equally sensitive to nonpolar solvents. Like conventional T-cell-dependent proteins, PSA requires processing before major histocompatibility complex class II (MHCII) binding. CD spectra of PSA fragments of varying sizes indicated that fragments smaller than three repeating units lack helical content and are incapable of MHCII binding. Likewise, neutralization of charged groups in the repeating unit resulted in major conformational changes as measured by CD, which correlated with a lack of MHCII presentation. These data represent two significant findings: CD can be used to measure conformational changes in carbohydrates and the functional epitope from PSA is dependent on a specific conformation that is stabilized by adjacent repeating units and a zwitterionic charge motif. As a result, this work demonstrates that CD is a valuable tool for use in functional glycomics efforts that seek to align chemical and conformational structure with biological activity.

Coaggregation of Porphyromonas gingivalis and Fusobacterium nucleatum PK 1594 is mediated by capsular polysaccharide and lipopolysaccharide

Previous reports have shown that coaggregation between Porphyromonas gingivalis and Fusobacterium nucleatum, two important periodontopathogens, is mediated by a galactoside on the surface of P. gingivalis and a lectin on F. nucleatum. In the present study, purified capsular polysaccharide (CPS) and lipopolysaccharide (LPS) of P. gingivalis PK 1924 (serotype K5) were found to be able to bind to F. nucleatum cells and to inhibit binding of F. nucleatum to P. gingivalis serotype K5. Sugar binding studies showed that the requirements for binding of P. gingivalis serotype K5 CPS and LPS to the F. nucleatum lectin are: the presence of a metal divalent ion, an axial free hydroxyl group at position 4 and free equatorial hydroxyl groups at position 3 and 6 of d-galactose. These data suggest that P. gingivalis serotype K5- CPS and LPS act as receptors mediating coaggregation between P. gingivalis and fusobacteria.

Epitopes recognized by a nonautoreactive murine anti-N-propionyl meningococcal group B polysaccharide monoclonal antibody

The capsular polysaccharide of Neisseria meningitidis group B (MBPS) is a polymer of alpha (2-->8) N-acetyl neuraminic acid. The polysaccharide is chemically identical to an autoantigen, polysialic acid (PSA), and is a poor immunogen, even when conjugated to protein carriers. Immunization of mice with MBPS-protein conjugate vaccines, in which N-acetyl groups have been replaced by propionyl groups (N-Pr MBPS), elicits serum bactericidal antibodies. A subpopulation of these antibodies do not cross-react with human PSA. The reasons for the increased immunogenicity of N-Pr MBPS and the antigenic targets of the bactericidal nonautoreactive antibodies are unknown. In this study, we investigated the antigenic targets of a protective murine monoclonal antibody (MAb) prepared against a N-Pr MBPS-tetanus toxoid conjugate vaccine. Binding of the MAb to N-Pr MBPS (as demonstrated by an enzyme-linked immunosorbent assay) and bactericidal activity were inhibited by de-N-acetylated MBPS and re-N-acetylated MBPS, which indicate that N-propionyl groups are not obligatory determinants for binding. The results of affinity selection from a preparation of N-Pr MBPS and matrix-assisted laser desorption ionization-time of flight mass spectroscopic analysis indicated that the minimal epitope recognized by the MAb is a MBPS disaccharide containing one de-N-acetylated residue. Thus, the bacterial capsular epitope recognized by this bactericidal, nonautoreactive, anti-group-B MAb likely contains de-N-acetyl residues.

Dynamics of the Escherichia coli O91 O-Antigen Polysaccharide in Solution as Studied by Carbon-13 NMR Relaxation

The dynamics of the O-antigen part of the lipopolysaccharide from the enterohemorrhagic Escherichia coli O91 has been determined in solution using (13)C NMR relaxation measurements at two magnetic field strengths, 9.4 and 14.1 T, thereby facilitating the testing of several dynamical models. The biological repeating unit, consisting of five sugar residues and substituents, could be determined by spectral analysis of different (1)H,(13)C correlations and corroborated by the relaxation data. The site specifically (13)C-labeled material was shown to have approximately 10 repeating units with a narrow distribution. A model-free analysis of the relaxation data revealed a complex dynamical behavior where the sugar residues could be described by a global correlation time (tau(m) = 5.4 ns), generalized order parameters (S(2) approximately 0.63), and different correlation times for internal motions related to their position in the repeating unit along the polymer (tau(e) approximately 360-520 ps). One of the sugar residues showed, in addition, a chemical exchange contribution. Furthermore, a substituent on another sugar residue was described by two order parameters (S(f)(2) = 0.51 and S(s)(2) = 0.21). The solution dynamics of the polysaccharide are thus described by highly intricate motions, both in amplitude and time scales. These results are of significance in the general description of polysaccharides surrounding bacterial cell surfaces and in the presentation of antigenic epitopes to the immune system of an invaded host.