NMR of glycans: shedding new light on old problems

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.

Structure elucidation of metabolite x17299 by interpretation of mass spectrometric data

INTRODUCTION

A major bottleneck in metabolomic studies is metabolite identification from accurate mass spectrometric data. Metabolite x17299 was identified in plasma as an unknown in a metabolomic study using a compound-centric approach where the associated ion features of the compound were used to determine the true molecular mass.

OBJECTIVES

The aim of this work is to elucidate the chemical structure of x17299, a new compound by de novo interpretation of mass spectrometric data.

METHODS

An Orbitrap Elite mass spectrometer was used for acquisition of mass spectra up to MS4 at high resolution. Synthetic standards of N,N,N-trimethyl-l-alanyl-l-proline betaine (l,l-TMAP), a diastereomer, and an enantiomer were chemically prepared.

RESULTS

The planar structure of x17299 was successfully proposed by de novo mechanistic interpretation of mass spectrometric data without any laborious purification and nuclear magnetic resonance spectroscopic analysis. The proposed structure was verified by deuterium exchanged mass spectrometric analysis and confirmed by comparison to a synthetic standard. Relative configuration of x17299 was determined by direct chromatographic comparison to a pair of synthetic diastereomers. Absolute configuration was assigned after derivatization of x17299 with a chiral auxiliary group followed by its chromatographic comparison to a pair of synthetic standards.

CONCLUSION

The chemical structure of metabolite x17299 was determined to be l,l-TMAP.

Magnetic Resonance Spectroscopy as a Tool for Assessing Macromolecular Structure and Function in Living Cells

Investigating the structure, modification, interaction, and function of biomolecules in their native cellular environment leads to physiologically relevant knowledge about their mechanisms, which will benefit drug discovery and design. In recent years, nuclear and electron magnetic resonance (NMR) spectroscopy has emerged as a useful tool for elucidating the structure and function of biomacromolecules, including proteins, nucleic acids, and carbohydrates in living cells at atomic resolution. In this review, we summarize the progress and future of in-cell NMR as it is applied to proteins, nucleic acids, and carbohydrates.

Glycan OH Exchange Rate Determination in Aqueous Solution: Seeking Evidence for Transient Hydrogen Bonds

Hydrogen bonds (Hbonds) are important stabilizing forces in biomolecules. However, for glycans in aqueous solution, direct NMR detection of Hbonds is elusive because of their transient nature. Here, we present Isotope-based Natural-abundance TOtal correlation eXchange SpectroscopY (INTOXSY), a new ¹H-¹³C heteronuclear single quantum coherence-total correlation spectroscopy based method, to extract OH groups' exchange rate constants (k_ex) for molecules in natural ¹³C abundance and show that OH Hbonds can be inferred from "slower" H/D k_ex. We evaluate k_ex measured with INTOXSY in light of those extracted with line-shape analysis. Subsequently, we use a set of common glycans to establish a k_ex reference basis set and to infer the existence of transient Hbonds involving OH donor groups. Then, we report k_ex values for a series of mono- and disaccharides, as well as for oligosaccharides sialyl Lewis X and β-cyclodextrin, and compare the results with those from the reference set to extract Hbond information. Finally, we utilize NMR experimental data in conjunction with molecular dynamics simulations to establish donor and acceptor Hbond pairs. Our exchange rate measurements indicate that OH/OD exchange rates, k_HD, values <10 s^-1 are consistent with transient Hbond OH groups and potential acceptor groups can be uncovered through MD simulations.

Direct NOE simulation from long MD trajectories

A software package, MD2NOE, is presented which calculates Nuclear Overhauser Effect (NOE) build-up curves directly from molecular dynamics (MD) trajectories. It differs from traditional approaches in that it calculates correlation functions directly from the trajectory instead of extracting inverse sixth power distance terms as an intermediate step in calculating NOEs. This is particularly important for molecules that sample conformational states on a timescale similar to molecular reorientation. The package is tested on sucrose and results are shown to differ in small but significant ways from those calculated using an inverse sixth power assumption. Results are also compared to experiment and found to be in reasonable agreement despite an expected underestimation of water viscosity by the water model selected.

Differences in Dynamics between Crosslinked and Non-Crosslinked Hyaluronates Measured by using Fast Field-Cycling Relaxometry

The dynamic properties of water molecules in gels containing linear and crosslinked hyaluronic acid polymers are investigated by using an integrated approach that includes relaxometry, solid-state NMR spectroscopy, and scanning electron microscopy. A model-free analysis of field-dependent nuclear relaxation is applied to obtain information on mobility and the population of different pools of water molecules in the gels. Differences between linear and crosslinked hyaluronic acid polymers are observed, indicating that crosslinking increases both the fraction and the correlation time of water molecules with slow dynamics.

Synthesis and Physicochemical Characterization of D-Tagatose-1-Phosphate: The Substrate of the Tagatose-1-Phosphate Kinase in the Phosphotransferase System-Mediated D-Tagatose Catabolic Pathway of Bacillus licheniformis

We report the first enzymatic synthesis of D-tagatose-1-phosphate (Tag-1P) by the multicomponent phosphoenolpyruvate:sugar phosphotransferase system (PEP-PTS) present in tagatose-grown cells of Klebsiella pneumoniae. Physicochemical characterization by (31)P and (1)H nuclear magnetic resonance spectroscopy reveals that, in solution, this derivative is primarily in the pyranose form. Tag-1P was used to characterize the putative tagatose-1-phosphate kinase (TagK) of the Bacillus licheniformis PTS-mediated D-tagatose catabolic pathway (Bli-TagP). For this purpose, a soluble protein fusion was obtained with the 6 His-tagged trigger factor (TF(His6)) of Escherichia coli. The active fusion enzyme was named TagK-TF(His6). Tag-1P and D-fructose-1-phosphate are substrates for the TagK-TF(His6) enzyme, whereas the isomeric derivatives D-tagatose-6-phosphate and D-fructose-6-phosphate are inhibitors. Studies of catalytic efficiency (kcat/Km) reveal that the enzyme specificity is markedly in favor of Tag-1P as the substrate. Importantly, we show in vivo that the transfer of the phosphate moiety from PEP to the B. licheniformis tagatose-specific Enzyme II in E. coli is inefficient. The capability of the PTS general cytoplasmic components of B. subtilis, HPr and Enzyme I to restore the phosphate transfer is demonstrated.