Predicting the molecular shape of polysaccharides from dynamic interactions with water

How simple monosaccharides, once polymerized, become the basis for structural materials remains a mystery. A framework is developed to investigate the role of water in the emergence of dynamic structure in polysaccharides, using the important beta(1-->4) linkage as an example. This linkage is studied within decasaccharide fragments of cellulose, chitin, mannan, xylan, and hyaluronan, using molecular simulations in the presence of explicit water solvent. Although cellulose, mannan, chitin, and xylan are chemically similar, their intramolecular hydrogen-bond dynamics and interaction with water are predicted to differ. Cellulose, mannan, and chitin favor relatively static intramolecular hydrogen bonds, xylan prefers dynamic water bridges, and multiple water configurations are predicted at the beta(1-->4) linkages of hyaluronan. With such a variety of predicted dynamics, the hypothesis that the beta(1-->4) linkage is stabilized by intramolecular hydrogen bonds was rejected. Instead, it is proposed that favored molecular configurations are consistent with maximum rotamer and water degrees of freedom, explaining observations made previously by X-ray diffraction. Furthermore, polysaccharides predicted to be conformationally restricted in simulations (cellulose, chitin, and mannan) prefer the solid state in reality, even as oligosaccharides. Those predicted to be more flexible (xylan and hyaluronan) are known to be soluble, even as high polymers. Therefore an intriguing correlation between chemical composition, water organization, polymer properties, and biological function is proposed.

Physical interpretation of residual dipolar couplings in neutral aligned media

A novel method is described for rapidly calculating alignment tensors from hydrodynamic shape, required for the prediction of residual dipolar couplings in neutral aligned media. Simulations of alignment were used to show that for steric restriction at a planar surface, the alignment process is dependent on linear hydrodynamic length. However, as discussed, previous methods are not in agreement with this observation. Therefore, the method presented here is the first to provide simple, accurate predictions of the alignment tensor for neutral and dilute media, while being consistent with simulations of alignment. It provides predictions in a fraction of the time of a simulation approach, while aiding physical intuition by providing a direct link between shape and alignment. Not only is this physically gratifying, but it also permits residual dipolar couplings to be applied in demanding situations where simulations of alignment are not desirable, such as in studies of molecular dynamics.

Quantitative conformational analysis of the core region of N-glycans using residual dipolar couplings, aqueous molecular dynamics, and steric alignment

A method is described for quantitatively investigating the dynamic conformation of small oligosaccharides containing an alpha(1 --> 6) linkage. It was applied to the oligosaccharide Man-alpha(1 --> 3) [Man-alpha(1 --> 6)] Man-alpha-O-Me, which is a core region frequently observed in N-linked glycans. The approach tests an aqueous molecular dynamics simulation, capable of predicting microscopic dynamics, against experimental residual dipolar couplings, by assuming that alignment is caused purely by steric hindrance. The experimental constraints were heteronuclear and homonuclear residual dipolar couplings, and in particular those within the alpha(1 --> 6) linkage itself. Powerful spin-state-selective pulse sequences and editing schemes were used to obtain the most relevant couplings for testing the model. Molecular dynamics simulations in water over a period of 50 ns were not able to predict the correct rotamer population at the alpha(1 --> 6) linkage to agree with the experimental data. However, this sampling problem could be corrected using a simple maximum likelihood optimisation, indicating that the simulation was modelling local dynamics correctly. The maximum likelihood prediction of the residual dipolar couplings was found to be an almost equal population of the gg and gt rotamer conformations at the alpha(1--> 6) linkage, and the tg conformation was predicted to be unstable and unpopulated in aqueous solution. In this case all twelve measured residual dipolar couplings could be satisfied. This conformer population could also be used to make predictions of scalar couplings with the use of a previously derived empirical equation, and is qualitatively in agreement with previous predictions based on NMR, X-ray crystallography and optical data.

Comparison of aqueous molecular dynamics with NMR relaxation and residual dipolar couplings favors internal motion in a mannose oligosaccharide

An investigation has been performed to assess how aqueous dynamical simulations of flexible molecules can be compared against NMR data. The methodology compares state-of-the-art NMR data (residual dipolar coupling, NOESY, and (13)C relaxation) to molecular dynamics simulations in water over several nanoseconds. In contrast to many previous applications of residual dipolar coupling in structure investigations of biomolecules, the approach described here uses molecular dynamics simulations to provide a dynamic representation of the molecule. A mannose pentasaccharide, alpha-D-Manp-(1-->3)-alpha-D-Manp-(1-->3)-alpha-D-Manp-(1-->3)-alpha-D-Manp-(1-->2)-D-Manp, was chosen as the model compound for this study. The presence of alpha-linked mannan is common to many glycopeptides, and therefore an understanding of the structure and the dynamics of this molecule is of both chemical and biological importance. This paper sets out to address the following questions. (1) Are the single structures which have been used to interpret residual dipolar couplings a useful representation of this molecule? (2) If dynamic flexibility is included in a representation of the molecule, can relaxation and residual dipolar coupling data then be simultaneously satisfied? (3) Do aqueous molecular dynamics simulations provide a reasonable representation of the dynamics present in the molecule and its interaction with water? In summary, two aqueous molecular dynamics simulations, each of 20 ns, were computed. They were started from two distant conformations and both converged to one flexible ensemble. The measured residual dipolar couplings were in agreement with predictions made by averaging the whole ensemble and from a specific single structure selected from the ensemble. However, the inclusion of internal motion was necessary to rationalize the relaxation data. Therefore, it is proposed that although residual dipolar couplings can be interpreted as a single-structure, this may not be a correct interpretation of molecular conformation in light of other experimental data. Second, the methodology described here shows that the ensembles from aqueous molecular dynamics can be effectively tested against experimental data sets. In the simulation, significant conformational motion was observed at each of the linkages, and no evidence for intramolecular hydrogen bonds at either alpha(1-->2) or alpha(1-->3) linkages was found. This is in contrast to simulations of other linkages, such as beta(1-->4), which are often predicted to maintain intramolecular hydrogen bonds and are coincidentally predicted to have less conformational freedom in solution.

Glycosaminoglycan conformation: do aqueous molecular dynamics simulations agree with x-ray fiber diffraction?

Glycosaminoglycan-protein interactions are biologically important and require an appreciation of glycan molecular shape in solution, which is presently unavailable. In previous studies we found strong similarity between aqueous molecular dynamics (MD) simulations and published x-ray diffraction refinements of hyaluronan. We have applied a similar approach here to chondroitin and dermatan, attempting to clarify some of the issues raised by the x-ray diffraction literature relating to chondroitin and dermatan sulfate. We predict that chondroitin has the same beta(1-->4) linkage conformation as hyaluronan, and that their average beta(1-->3) conformations differ. This is explained by changes in hydrogen-bonding across this linkage, resulting from its axial hydroxyl, causing a different sampling of left-handed helices in chondroitin (2.5- to 3.5-fold) as compared with hyaluronan (3.0- to 4.0-fold). Few right-handed helices, which lack intramolecular hydrogen-bonds, were sampled during our MD simulations. Thus, we propose that the 8-fold helix observed in chondroitin-6-sulfate, represented in the literature as an 8(3) helix (right-handed), though it has never been refined, is more likely to be 8(5) (left-handed) helix. Molecular dynamics simulations implied that (4)C(1) and (2)S(O), but not (1)C(4), forms of iduronate could be used in refinements of dermatan x-ray fiber diffraction patterns. Current models of 8-fold dermatan sulfate chains containing (4)C(1) iduronate refine to right-handed helices, which possess no intramolecular hydrogen-bonds. However, MD simulations predict that models containing (2)S(O) iduronate could provide better (8(5) helix) starting structures for refinement. Thus, the 8-fold dermatan sulfate refinement (8(3) helix) could be in error.

Deducing polymeric structure from aqueous molecular dynamics simulations of oligosaccharides: predictions from simulations of hyaluronan tetrasaccharides compared with hydrodynamic and X-ray fibre diffraction data

Molecular dynamics simulations of the two hyaluronan tetrasaccharides in water predict that over a period of 500 ps, their central linkages populate a single primary minima. Over the same period the peripheral linkages explore this minima, but also a secondary minima. Structures constructed using the primary minima were found to be extended left-handed helices of axial rise per disaccharide (h) 0.8 to 1.0 nm and 2.8 to 4.5 disaccharides per turn (n), in good agreement with n=3 and n=4 helices found by X-ray fibre diffraction studies. We have used the predicted average conformation from molecular dynamics to calculate the translational diffusion coefficients of the oligosaccharide series up to decasaccharide, and compared these with experimental measurements obtained using the method of capillary dispersion. Our calculated values are found to be in good agreement with experiment beyond the size of a tetrasaccharide. A partial digest of hyaluronan in the molecular mass range 10 to 100 kDa was fractionated by gel chromatography. Molecular weights were determined by in-line laser light-scattering measurements, and the translational diffusion coefficients of selected fractions were determined by dynamic laser light-scattering. A similar experiment was performed on hyaluronan with a molecular mass greater than 1MDa. The data suggest a change from rod-like to stiff coil behaviour beyond a molecular weight of 10 kDa. We have also examined the conformations available using the secondary minima, found at the peripheral linkages. In contrast to the extended structures previously described we have found left and right-handed helices with high values of n (5-10) and low values of h. Although there is no experimental evidence for these structures, they are of interest as, over short stretches, they would introduce folds, loops, and turns into the hyaluronan molecule. Such shapes may play an important role in the hydrodynamics of hyaluronan and its interaction with lipids and proteins.

Dynamic exchange between stabilized conformations predicted for hyaluronan tetrasaccharides: comparison of molecular dynamics simulations with available NMR data

Studies of the hyaluronan (HA) tetrasaccharides are important for understanding hydrogen-bonding in the HA polymer, as they are probably the smallest oligomers in which characteristics of the constituent monosaccharides and the polymer are simultaneously exhibited. Here we present extensive molecular dynamics simulations of the two tetrasaccharides of HA in dilute aqueous solution. These simulations have confirmed the existence of intramolecular hydrogen-bonds between the neighboring sugar residues of HA in solution, as proposed by Scott (1989). However, our simulations predict that these intramolecular hydrogen-bonds are not static as previously proposed, but are in constant dynamic exchange on the sub-nanosecond time-scale. This process results in discrete internal motion of the HA tetrasaccharides where they rapidly move between low energy conformations. Specific interactions between water and intramolecular hydrogen-bonds involving the hydroxymethyl group were found to result in differing conformations and dynamics for the two alternative tetrasaccharides of HA. This new observation suggests that this residue may play a key role in the entropy and stability of HA in solution, allowing it to stay soluble up to high concentration. The vicinal coupling constants3 J NHCH of the acetamido groups have been calculated from our aqueous simulations of HA. We found that high values of 3J NHCH approximately 8 Hz, as experimentally measured for HA, are consistent with mixtures of both trans and cis conformations, and thus3 J NHCH cannot be used to imply a purely trans conformation of the acetamido. The rapid exchange of intramolecular hydrogen-bonds indicates that although the structure is at any moment stabilized by these hydrogen-bonds, no one hydrogen-bond exists for an extended period of time. This could explain why NMR often fails to provide evidence for intramolecular hydrogen-bonds in HA and other aqueous carbohydrate structures.

Molecular dynamics simulations of the two disaccharides of hyaluronan in aqueous solution

Hyaluronan is an unusually stiff polymer when in aqueous solution, which has important consequences for its biological function. Molecular dynamics simulations of hyaluronan disaccharides have been performed, with explicit inclusion of water, to determine the molecular basis of this stiffness, and to investigate the dynamics of the glycosidic linkages. Our simulations reveal that stable sets of hydrogen bonds frequently connect the neighboring residues of hyaluronan. Water caging around the glycosidic linkage was observed to increase the connectivity between sugars, and further constrain them. This, we propose, explains the unusual stiffness of polymeric hyaluronan. It would allow the polysaccharide to maintain local secondary structure, and occupy large solution domains consistent with the visco-elastic nature of hyaluronan. Simulations in water showed no significant changes on inclusion of the exoanomeric effect. This, we deduced, was due to hyaluronan disaccharides ordering first shell water molecules. In some cases these waters were observed to transiently induce conformational change, by breaking intramolecular hydrogen bonds.

Structural characterisation of two forms of procyclic acidic repetitive protein expressed by procyclic forms of Trypanosoma brucei

A procyclic acidic repetitive protein (PARP) fraction was purified from long-term cultures of Trypanosoma brucei procyclic forms by a solvent-extraction and reverse phase chromatography procedure. The PARP fraction yielded small quantities of a single N-linked oligosaccharide with the structure Man alpha1-6(Man alpha1-3)Man alpha1-6(Man alpha1-3)Manbeta1-4GlcNAcbeta1-4GlcNAc (Man5GlcNAc2). Fractionation of PARP on Con A-Sepharose revealed that the majority (80 to 90%) of the PARP fraction did not bind to Con A and was composed of the parpA alpha gene product that contains repeats of -Glu-Pro-Pro-Thr- (GPEET-PARP) and that lacks an N-glycosylation site. This form of PARP has not been previously identified at the protein-level. The minor Con-A-binding fraction was shown to be rich in the previously described form of PARP, encoded by the parpAbeta and/or parpB alpha genes, that contains a -Glu-Pro- repeat domain (EP-PARP) and an N-glycosylation site. Analysis of longer and shorter-term cultures suggested that procyclic cells initially express predominantly EP-PARP that is gradually replaced by GPEET-PARP. Both forms of PARP were shown to contain indistinguishable glycosylphosphatidylinositol (GPI) membrane anchors, where the conserved GPI core structure is substituted by heterogeneous sialylated branched polylactosamine-like structures that are predicted to form a dense surface glycocalyx above which the polyanionic -Glu-Pro-Pro-Thr- and -Glu-Pro- repeat domains are displayed. The phosphatidylinositol (PI) component of the GPI anchor was shown to be a mixture of 2-O-acyl-myo-inositol-1-HPO4-(sn-1-stearoyl-2-lyso-glycerol) and 2-O-acyl-myo-inositol-1-HPO4-(sn-1-octadecyl-2-lyso-glycerol), where the acyl chain substituting the inositol ring showed considerable heterogeneity. Mass spectrometric and light scattering experiments both suggested an average mass of approximately 15 kDa for GPEET-PARP, with individual glycoforms ranging from about 12 kDa to 20 kDa, that is consistent with its amino acid and carbohydrate composition. A measured translational diffusion coefficient of 3.9 x 10(7) cm2 s(-1) indicates that this molecule has a highly elongated shape. The possible functions of these unusual glycoproteins are discussed.