Paramagnetic NMR probes for characterization of the dynamic conformations and interactions of oligosaccharides

Exploring multivalent carbohydrate-protein interactions by NMR

Nuclear Magnetic Resonance (NMR) has been widely employed to assess diverse features of glycan-protein molecular recognition events. Different types of qualitative and quantitative information at different degrees of resolution and complexity can be extracted from the proper application of the available NMR-techniques. In fact, affinity, structural, kinetic, conformational, and dynamic characteristics of the binding process are available. Nevertheless, except in particular cases, the affinity of lectin-sugar interactions is weak, mostly at the low mM range. This feature is overcome in biological processes by using multivalency, thus augmenting the strength of the binding. However, the application of NMR methods to monitor multivalent lectin-glycan interactions is intrinsically challenging. It is well known that when large macromolecular complexes are formed, the NMR signals disappear from the NMR spectrum, due to the existence of fast transverse relaxation, related to the large size and exchange features. Indeed, at the heart of the molecular recognition event, the associated free-bound chemical exchange process for both partners takes place in a particular timescale. Thus, these factors have to be considered and overcome. In this review article, we have distinguished, in a subjective manner, the existence of multivalent presentations in the glycan or in the lectin. From the glycan perspective, we have also considered whether multiple epitopes of a given ligand are presented in the same linear chain of a saccharide (i.e., poly-LacNAc oligosaccharides) or decorating different arms of a multiantennae scaffold, either natural (as in multiantennae N-glycans) or synthetic (of dendrimer or polymer nature). From the lectin perspective, the presence of an individual binding site at every monomer of a multimeric lectin may also have key consequences for the binding event at different levels of complexity.

Structural Analysis of Oligosaccharides and Glycoconjugates Using NMR

Carbohydrate chains play critical roles in cellular recognition and subsequent signal transduction in the nervous system. Furthermore, gangliosides are targets for various amyloidogenic proteins associated with neurodegenerative disorders. To better understand the molecular mechanisms underlying these biological phenomena, atomic views are essential to delineate dynamic biomolecular interactions. Nuclear magnetic resonance (NMR) spectroscopy provides powerful tools for studying structures, dynamics, and interactions of biomolecules at the atomic level. This chapter describes the basics of solution NMR techniques and their applications to the analysis of 3D structures and interactions of glycoconjugates in the nervous system.

Contribution of Molecular Dynamics in pNMR for the Structural Determination of AnV and AnVI Complexes in Solution

In this study, we propose to use classical molecular dynamics (MD) coupled with ¹H NMR spectroscopy to study the conformations of different actinyl An^VI (An = U, Np, and Pu) and An^V (An = Np) complexes with tetra-ethyl dyglicolamide (TEDGA) ligands in order to have a better representation of such complexes in solution. Molecular dynamics simulations showed its effectiveness in interpreting the experiments by the calculation of geometric factors needed for the determination of magnetic properties of these complexes. We demonstrated that different conformations of the An^V and An^VI complexes with TEDGA exist in solution with different coordination modes, which is experimentally confirmed by ¹H NMR and EXAFS spectroscopies. Furthermore, MD simulations provide additional insights into the structures of complexes in solution since conformations with fast exchanges, which are not accessible from NMR experiments, have been observed by MD simulations.

Molecular simulations of complex carbohydrates and glycoconjugates

Complex carbohydrates (glycans) are the most abundant and versatile biopolymers in nature. The broad diversity of biochemical functions that carbohydrates cover is a direct consequence of the variety of 3D architectures they can adopt, displaying branched or linear arrangements, widely ranging in sizes, and with the highest diversity of building blocks of any other natural biopolymer. Despite this unparalleled complexity, a common denominator can be found in the glycans' inherent flexibility, which hinders experimental characterization, but that can be addressed by high-performance computing (HPC)-based molecular simulations. In this short review, I present and discuss the state-of-the-art of molecular simulations of complex carbohydrates and glycoconjugates, highlighting methodological strengths and weaknesses, important insights through emblematic case studies, and suggesting perspectives for future developments.

Paramagnetic Chemical Probes for Studying Biological Macromolecules

Paramagnetic chemical probes have been used in electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopy for more than four decades. Recent years witnessed a great increase in the variety of probes for the study of biological macromolecules (proteins, nucleic acids, and oligosaccharides). This Review aims to provide a comprehensive overview of the existing paramagnetic chemical probes, including chemical synthetic approaches, functional properties, and selected applications. Recent developments have seen, in particular, a rapid expansion of the range of lanthanoid probes with anisotropic magnetic susceptibilities for the generation of structural restraints based on residual dipolar couplings and pseudocontact shifts in solution and solid state NMR spectroscopy, mostly for protein studies. Also many new isotropic paramagnetic probes, suitable for NMR measurements of paramagnetic relaxation enhancements, as well as EPR spectroscopic studies (in particular double resonance techniques) have been developed and employed to investigate biological macromolecules. Notwithstanding the large number of reported probes, only few have found broad application and further development of probes for dedicated applications is foreseen.

Experimental and computational characterization of dynamic biomolecular interaction systems involving glycolipid glycans

On cell surfaces, carbohydrate chains that modify proteins and lipids mediate various biological functions, which are exerted not only through carbohydrate-protein interactions but also through carbohydrate-carbohydrate interactions. These glycans exhibit considerable degrees of conformational variability and often form clusters providing multiple binding sites. The integration of nuclear magnetic resonance spectroscopy and molecular dynamics simulation has made it possible to delineate the dynamical structures of carbohydrate chains. This approach has facilitated the remodeling of oligosaccharide conformational space in the prebound state to improve protein-binding affinity and has been applied to visualize dynamic carbohydrate-carbohydrate interactions that control glycoprotein-glycoprotein complex formation. Functional glycoclusters have been characterized by experimental and computational approaches applied to various model membranes and artificial self-assembling systems. This line of investigation has provided dynamic views of molecular assembling on glycoclusters, giving mechanistic insights into physiological and pathological molecular events on cell surfaces as well as clues for the design and creation of molecular systems exerting improved glycofunctions. Further development and accumulation of such studies will allow detailed understanding and artificial control of the "glycosynapse" foreseen by Dr. Sen-itiroh Hakomori.

Rigid, Highly Reactive and Stable DOTA-like Tags Containing a Thiol-Specific Phenylsulfonyl Pyridine Moiety for Protein Modification and NMR Analysis*

Site specific installation of a paramagnetic ion with magnetic anisotropy in a biomolecule generates valuable structural restraints, such as pseudocontact shifts (PCSs) and residual dipolar couplings (RDCs). These paramagnetic effects can be used to characterize the structures, interactions and dynamics of biological macromolecules and their complexes. Two single-armed DOTA-like tags, BrPSPy-DO3M(S)A-Ln and BrPSPy-6M-DO3M(S)A-Ln, each containing a thiol-specific reacting group, that is, a phenylsulfonyl pyridine moiety, are demonstrated as rigid, reactive and stable paramagnetic tags for protein modification by formation of a reducing resistant thioether bond between the protein and the tag. The two tags present high reactivity with the solvent exposed thiol group in aqueous solution at room temperature. The introduction of Br at the meta-position in pyridine enhances the reactivity of 4-phenylsulfonyl pyridine towards the solvent exposed thiol group in a protein, whereas the ortho-methyl group in pyridine increases the rigidity of the tag in the protein conjugates. The high performance of these two tags has been demonstrated in different cysteine mutants of ubiquitin and GB1. The high reactivity and rigidity of these two tags can be added in the toolbox of paramagnetic tags suitable for the high-resolution NMR measurements of biological macromolecules and their complexes.

Sparse isotope labeling for nuclear magnetic resonance (NMR) of glycoproteins using 13C-glucose

Preparation of samples for nuclear magnetic resonance (NMR) characterization of larger proteins requires enrichment with less abundant, NMR-active, isotopes such as 13C and 15N. This is routine for proteins that can be expressed in bacterial culture where low-cost isotopically enriched metabolic substrates can be used. However, it can be expensive for glycosylated proteins expressed in mammalian culture where more costly isotopically enriched amino acids are usually used. We describe a simple, relatively inexpensive procedure in which standard commercial media is supplemented with 13C-enriched glucose to achieve labeling of all glycans plus all alanines of the N-terminal domain of the highly glycosylated protein, CEACAM1. We demonstrate an ability to detect partially occupied N-glycan sites, sites less susceptible to processing by an endoglycosidase, and some unexpected truncation of the amino acid sequence. The labeling of both the protein (through alanines) and the glycans in a single culture requiring no additional technical expertise past standard mammalian expression requirements is anticipated to have several applications, including structural and functional screening of the many glycosylated proteins important to human health.

Lanthanide Tagging of Oligonucleotides to Nucleobase for Paramagnetic NMR

Although lanthanide tags, which have large anisotropic magnetic susceptibilities, have already been introduced to enrich NMR parameters by long‐range pseudoconact shifts (PCSs) and residual dipolar couplings (RDCs) of proteins, their application to nucleotides has so far been limited to one previous report, due to the high affinities of lanthanides for the phosphodiester backbone of nucleotides and difficult organic synthesis. Herein, we report successful attachment of a lanthanide tag to a chemically synthesized oligonucleotide via a disulfide bond. NMR experiments reveal PCSs of up to 1 ppm and H−H RDCs of up to 8 Hz at 950 MHz. Although weaker magnetic alignment was achieved than with proteins, the paramagnetic data could be fitted to the known structure of the DNA, taking the mobility of the tag into account. While further rigidification of the tag is desirable, this tag could also be used to measure heteronuclear RDCs of ¹³C,¹⁵N‐labeled chemically synthesized DNA and RNA.

Biophysical characterization of dynamic structures of immunoglobulin G

Immunoglobulin G (IgG) is a major antibody and functions as a hub linking specific antigen binding and recruitment of effector molecules typified by Fcγ receptors (FcγRs). These activities are associated primarily with interactions involving its Fab and Fc sites, respectively. An IgG molecule is characterized by a multiple domain modular structure with conserved N-glycosylation in Fc. The molecule displays significant freedom in internal motion on various spatiotemporal scales. The consequent conformational flexibility and plasticity of IgG glycoproteins are functionally significant and potentially important factors for design and engineering of antibodies with enhanced functionality. In this article, experimental and computational approaches are outlined for characterizing the conformational dynamics of IgG molecules in solution. In particular, the importance of integration of these approaches is highlighted, as illustrated by dynamic intramolecular interactions between the pair of N-glycans and their proximal amino acid residues in Fc. These interactions can critically affect effector functions mediated by human IgG1 and FcγRIII. Further improvements in individual biophysical techniques and their integration will advance understanding of dynamic behaviors of antibodies in physiological and pathological conditions. Such understanding will provide opportunities for engineering antibodies through controlling allosteric networks in IgG molecules.

Remodeling of the Oligosaccharide Conformational Space in the Prebound State To Improve Lectin-Binding Affinity

We developed an approach to improve the lectin-binding affinity of an oligosaccharide by remodeling its conformational space in the precomplexed state. To develop this approach, we used a Lewis X-containing oligosaccharide interacting with RSL as a model system. Using an experimentally validated molecular dynamics simulation, we designed a Lewis X analogue with an increased population of conformational species that were originally very minor but exclusively accessible to the target lectin without steric hindrance by modifying the nonreducing terminal galactose, which does not directly contact the lectin in the complex. This Lewis X mimetic showed 17 times higher affinity for the lectin than the native counterpart. Our approach, complementing the lectin-bound-state optimizations, offers an alternative strategy to create high-affinity oligosaccharides by increasing populations of on-pathway metastable conformers.

Design and applications of lanthanide chelating tags for pseudocontact shift NMR spectroscopy with biomacromolecules

In this review, lanthanide chelating tags and their applications to pseudocontact shift NMR spectroscopy as well as analysis of residual dipolar couplings are covered. A complete overview is presented of DOTA-derived and non-DOTA-derived lanthanide chelating tags, critical points in the design of lanthanide chelating tags as appropriate linker moieties, their stability under reductive conditions, e.g., for in-cell applications, the magnitude of the anisotropy transferred from the lanthanide chelating tag to the biomacromolecule under investigation and structural properties, as well as conformational bias of the lanthanide chelating tags are discussed. Furthermore, all DOTA-derived lanthanide chelating tags used for PCS NMR spectroscopy published to date are displayed in tabular form, including their anisotropy parameters, with all employed lanthanide ions, C_B-Ln distances and tagging reaction conditions, i.e., the stoichiometry of lanthanide chelating tags, pH, buffer composition, temperature and reaction time. Additionally, applications of lanthanide chelating tags for pseudocontact shifts and residual dipolar couplings that have been reported for proteins, protein-protein and protein-ligand complexes, carbohydrates, carbohydrate-protein complexes, nucleic acids and nucleic acid-protein complexes are presented and critically reviewed. The vast and impressive range of applications of lanthanide chelating tags to structural investigations of biomacromolecules in solution clearly illustrates the significance of this particular field of research. The extension of the repertoire of lanthanide chelating tags from proteins to nucleic acids holds great promise for the determination of valuable structural parameters and further developments in characterizing intermolecular interactions.

What are the methodological and theoretical prospects for paramagnetic NMR in structural biology? A glimpse into the crystal ball

NMR spectroscopy is very sensitive to the presence of unpaired electrons, which perturb the NMR chemical shifts, J splittings and nuclear relaxation rates. These paramagnetic effects have attracted increasing attention over the last decades, and their use is expected to increase further in the future because they can provide structural information not easily achievable with other techniques. In fact, paramagnetic data provide long range structural restraints that can be used to assess the accuracy of crystal structures in solution and to improve them by simultaneous refinements with the X-ray data. They are also precious for obtaining information on the conformational variability of biomolecular systems, possibly in conjunction with SAXS and/or DEER data. We foresee that new tools will be developed in the next years for the simultaneous analysis of the paramagnetic data with data obtained from different techniques, in order to take advantage synergistically of the information content of all of them. Of course, the use of the paramagnetic data for structural purposes requires the knowledge of the relationship between these data and the molecular coordinates. Recently, the equations commonly used, dating back to half a century ago, have been questioned by first principle quantum chemistry calculations. Our prediction is that further theoretical/computational improvements will essentially confirm the validity of the old semi-empirical equations for the analysis of the experimental paramagnetic data.

Biophysical Analyses for Probing Glycan-Protein Interactions

Glycan-protein interactions occur at many physiological events, and the analyses are of considerable importance for understanding glycan-dependent mechanisms. Biophysical approaches including 3D structural analysis are essential for revealing glycan-protein interactions at the atomic level. The inherent diversity of glycans suits them to function as identification tags, e.g., distinguish self from the nonself components of pathogens. However, the complexity of glycans and poor affinities for interaction partners limit the usefulness of conventional analyses. To cope with such troublesome glycans, a logical sequence of biophysical analyses need to be developed. In this chapter, we introduce a workflow of glycan-protein interaction analysis consisting of six steps: preparation of lectin and glycan, screening of glycan ligand, determination of binding epitope, quantitative interaction analysis, 3D structural analysis, and molecular dynamics simulation. Our increasing knowledge and understanding of lectin-glycan interactions will hopefully lead to the design of glyco-based medicines and vaccines.

The recognition of glycans by protein receptors. Insights from NMR spectroscopy

Carbohydrates (glycans, saccharides, sugars) are everywhere. In fact, glycan-protein interactions are involved in many essential processes of life and disease. The understanding of the key structural details at the atomic and molecular level is of paramount importance to effectively design molecules for therapeutic purposes. Different approximations may be employed to decipher these molecular recognition processes with high resolution. Advances in cryo-electron microscopy are providing exquisite details on different biological mechanisms involving sugars, while better and better protocols for structural refinement in the application of X-ray methods for protein-sugar complexes and glycoproteins are also permitting fantastic advances in the glycoscience arena. Alternatively, NMR spectroscopy remains as one of the most rewarding techniques to explore protein-carbohydrate interactions. In fact, given the intrinsic dynamic nature of saccharides, NMR can afford exquisite structural information at the atomic detail, not accessible by other techniques. However, the access to this information is sometimes intricate, and requires careful analysis and well-defined strategies. In this review, we have highlighted these issues and presented an overview of different modern NMR approaches with a focus on the latest developments and challenges.

Stable isotope labeling approaches for NMR characterization of glycoproteins using eukaryotic expression systems

Glycoproteins are characterized by the heterogeneous and dynamic nature of their glycan moieties, which hamper crystallographic analysis. NMR spectroscopy provides potential advantages in dealing with such complicated systems, given that the target molecules can be isotopically labeled. Methods of metabolic isotope labeling in recombinant glycoproteins have been developed recently using a variety of eukaryotic production vehicles, including mammalian, yeast, insect, and plant cells, each of which has a distinct N-glycan diversification pathway. Yeast genetic engineering has enabled the overexpression of homogeneous high-mannose-type oligosaccharides with ¹³C labeling for NMR characterization of their conformational dynamics. The utility of stable isotope-assisted NMR spectroscopy has also been demonstrated using the Fc fragment of immunoglobulin G (IgG) as a model glycoprotein, providing useful information regarding intramolecular carbohydrate-protein interactions. Transverse relaxation optimization of intact IgG with a molecular mass of 150 kDa has been achieved by tailored deuteration of selected amino acid residues using a mammalian expression system. This offers a useful probe for the characterization of molecular interaction networks in multimolecular crowded systems typified by serum. Perspectives regarding the development of techniques for tailoring glycoform designs and isotope labeling of recombinant glycoproteins are also discussed.

Molecular Dynamics of Gangliosides

Computational methodologies have immense potential to delineate the dynamic conformations of glycoconjugates including gangliosides, thereby characterizing the conformational adaptability of their glycans upon interacting with various target proteins. Replica-exchange molecular dynamics simulations have been employed to effectively explore the vast conformational spaces of large, branched carbohydrate moieties. When experimentally validated using NMR, molecular simulations can provide dynamical views of molecular recognition events involving the ganglioside glycans.

Structural Aspects of ER Glycoprotein Quality-Control System Mediated by Glucose Tagging

N-linked oligosaccharides attached to proteins act as tags for glycoprotein quality control, ensuring their appropriate folding and trafficking in cells. Interactions with a variety of intracellular lectins determine glycoprotein fates. Monoglucosylated glycoforms are the hallmarks of incompletely folded glycoproteins in the protein quality-control system, in which glucosidase II and UDP-glucose/glycoprotein glucosyltransferase are, respectively, responsible for glucose trimming and attachment. In this review, we summarize a recently emerging view of the structural basis of the functional mechanisms of these key enzymes as well as substrate N-linked oligosaccharides exhibiting flexible structures, as revealed by applying a series of biophysical techniques including small-angle X-ray scattering, X-ray crystallography, high-speed atomic force microscopy , electron microscopy , and computational simulation in conjunction with NMR spectroscopy.

Conformational effects of N-glycan core fucosylation of immunoglobulin G Fc region on its interaction with Fcγ receptor IIIa

Antibody-dependent cellular cytotoxicity (ADCC) is promoted through interaction between the Fc region of immunoglobulin G1 (IgG1) and Fcγ receptor IIIa (FcγRIIIa), depending on N-glycosylation of these glycoproteins. In particular, core fucosylation of IgG1-Fc N-glycans negatively affects this interaction and thereby compromises ADCC activity. To address the mechanisms of this effect, we performed replica-exchange molecular dynamics simulations based on crystallographic analysis of a soluble form of FcγRIIIa (sFcγRIIIa) in complex with IgG1-Fc. Our simulation highlights increased conformational fluctuation of the N-glycan at Asn162 of sFcγRIIIa upon fucosylation of IgG1-Fc, consistent with crystallographic data giving no interpretable electron density for this N-glycan, except for the innermost part. The fucose residue disrupts optimum intermolecular carbohydrate-carbohydrate interactions, rendering this sFcγRIIIa glycan distal from the Fc glycan. Moreover, our simulation demonstrates that core fucosylation of IgG1-Fc affects conformational dynamics and rearrangements of surrounding amino acid residues, typified by Tyr296 of IgG1-Fc, which was more extensively involved in the interaction with sFcγRIIIa without Fc core fucosylation. Our findings offer a structural foundation for designing and developing therapeutic antibodies with improved ADCC activity.

Conformational Analysis of a High-Mannose-Type Oligosaccharide Displaying Glucosyl Determinant Recognised by Molecular Chaperones Using NMR-Validated Molecular Dynamics Simulation

Exploration of the conformational spaces of flexible oligosaccharides is essential to gain deeper insights into their functional mechanisms. Here we characterised dynamic conformation of a high-mannose-type dodecasaccharide with a terminal glucose residue, a critical determinant recognised by molecular chaperones. The dodecasaccharide was prepared by our developed chemoenzymatic technique, which uses ¹³ C labelling and lanthanide tagging to detect conformation-dependent paramagnetic effects by NMR spectroscopy. The NMR-validated molecular dynamics simulation produced the dynamic conformational ensemble of the dodecasaccharide. This determined its spatial distribution as well as the glycosidic linkage conformation of the terminal glucose determinant. Moreover, comparison of our results with previously reported crystallographic data indicates that the chaperone binding to its target oligosaccharides involves an induced-fit mechanism.

Structural Aspects of Heparan Sulfate Binding to Robo1-Ig1-2

Roundabout 1, or Robo1, is a cell surface signaling molecule important in axon guidance. Its interaction with heparan sulfate (HS) and members of the Slit protein family is essential to its activity, making characterization of these interactions by structural methods, such as NMR, highly desirable. However, the fact that Robo1 is a glycosylated protein prevents employment of commonly used bacterial hosts for expression of properly glycosylated forms with the uniform 15N, 13C, and 2H labeling needed for NMR studies. Here, we apply an alternative methodology, based on labeling with a single amino acid type and high structural content NMR data, to characterize a two-domain construct of glycosylated Robo1 (Robo1-Ig1-2) interacting with a synthetic HS tetramer (IdoA-GlcNS6S-IdoA2S-GlcNS6S-(CH2)5NH2). Significant chemical shift perturbations of the crosspeak from K81 on titration with the tetramer provide initial evidence for the location of a binding site and allow determination of a 255 μM disassociation constant. The binding epitopes, bound conformation, and binding site placement of the HS tetramer have been further characterized by saturation transfer difference (STD), transferred nuclear Overhauser effect (trNOE), and paramagnetic perturbation experiments. A model of the complex has been generated using constraints derived from the various NMR experiments. Postprocessing energetic analysis of this model provides a rationale for the role each glycan residue plays in the binding event, and examination of the binding site in the context of a previous Robo-Slit structure provides a rationale for modulation of Robo-Slit interactions by HS.

Identifying and Visualizing Macromolecular Flexibility in Structural Biology

Structural biology comprises a variety of tools to obtain atomic resolution data for the investigation of macromolecules. Conventional structural methodologies including crystallography, NMR and electron microscopy often do not provide sufficient details concerning flexibility and dynamics, even though these aspects are critical for the physiological functions of the systems under investigation. However, the increasing complexity of the molecules studied by structural biology (including large macromolecular assemblies, integral membrane proteins, intrinsically disordered systems, and folding intermediates) continuously demands in-depth analyses of the roles of flexibility and conformational specificity involved in interactions with ligands and inhibitors. The intrinsic difficulties in capturing often subtle but critical molecular motions in biological systems have restrained the investigation of flexible molecules into a small niche of structural biology. Introduction of massive technological developments over the recent years, which include time-resolved studies, solution X-ray scattering, and new detectors for cryo-electron microscopy, have pushed the limits of structural investigation of flexible systems far beyond traditional approaches of NMR analysis. By integrating these modern methods with powerful biophysical and computational approaches such as generation of ensembles of molecular models and selective particle picking in electron microscopy, more feasible investigations of dynamic systems are now possible. Using some prominent examples from recent literature, we review how current structural biology methods can contribute useful data to accurately visualize flexibility in macromolecular structures and understand its important roles in regulation of biological processes.