The hydrodynamic frictional coefficient of polysaccharides: the role of the glycosidic linkage

Constructing 3-Dimensional Atomic-Resolution Models of Nonsulfated Glycosaminoglycans with Arbitrary Lengths Using Conformations from Molecular Dynamics

Glycosaminoglycans (GAGs) are the linear carbohydrate components of proteoglycans (PGs) and are key mediators in the bioactivity of PGs in animal tissue. GAGs are heterogeneous, conformationally complex, and polydisperse, containing up to 200 monosaccharide units. These complexities make studying GAG conformation a challenge for existing experimental and computational methods. We previously described an algorithm we developed that applies conformational parameters (i.e., all bond lengths, bond angles, and dihedral angles) from molecular dynamics (MD) simulations of nonsulfated chondroitin GAG 20-mers to construct 3-D atomic-resolution models of nonsulfated chondroitin GAGs of arbitrary length. In the current study, we applied our algorithm to other GAGs, including hyaluronan and nonsulfated forms of dermatan, keratan, and heparan and expanded our database of MD-generated GAG conformations. Here, we show that individual glycosidic linkages and monosaccharide rings in 10- and 20-mers of hyaluronan and nonsulfated dermatan, keratan, and heparan behave randomly and independently in MD simulation and, therefore, using a database of MD-generated 20-mer conformations, that our algorithm can construct conformational ensembles of 10- and 20-mers of various GAG types that accurately represent the backbone flexibility seen in MD simulations. Furthermore, our algorithm efficiently constructs conformational ensembles of GAG 200-mers that we would reasonably expect from MD simulations.

Efficient Construction of Atomic-Resolution Models of Non-Sulfated Chondroitin Glycosaminoglycan Using Molecular Dynamics Data

Glycosaminoglycans (GAGs) are linear, structurally diverse, conformationally complex carbohydrate polymers that may contain up to 200 monosaccharides. These characteristics present a challenge for studying GAG conformational thermodynamics at atomic resolution using existing experimental methods. Molecular dynamics (MD) simulations can overcome this challenge but are only feasible for short GAG polymers. To address this problem, we developed an algorithm that applies all conformational parameters contributing to GAG backbone flexibility (i.e., bond lengths, bond angles, and dihedral angles) from unbiased all-atom explicit-solvent MD simulations of short GAG polymers to rapidly construct models of GAGs of arbitrary length. The algorithm was used to generate non-sulfated chondroitin 10- and 20-mer ensembles which were compared to MD-generated ensembles for internal validation. End-to-end distance distributions in constructed and MD-generated ensembles have minimal differences, suggesting that our algorithm produces conformational ensembles that mimic the backbone flexibility seen in simulation. Non-sulfated chondroitin 100- and 200-mer ensembles were constructed within a day, demonstrating the efficiency of the algorithm and reduction in time and computational cost compared to simulation.

Aggrecan, an unusual polyelectrolyte: review of solution behavior and physiological implications

Aggrecan is a high-molecular-weight, bottlebrush-shaped, negatively charged biopolymer that forms supermolecular complexes with hyaluronic acid. In the extracellular matrix of cartilage, aggrecan-hyaluronic acid complexes are interspersed in a collagen meshwork and provide the osmotic properties required to resist deswelling under compressive load. In this review we compile aggrecan solution behavior from different experimental techniques, and discuss them in the context of concentration regimes that were identified in osmotic pressure experiments. At low concentrations, aggrecan exhibits microgel-like behavior. With increasing concentration, the bottlebrushes self-assemble into large complexes. In the physiological concentration range (2<c(aggrecan)<8% w/w), the physical properties of the solution are dominated by repulsive electrostatic interactions between aggrecan complexes. We discuss the consequences of the bottlebrush architecture on the polyelectrolyte characteristics of the aggrecan molecule, and its implications for cartilage properties and function.

Relationship between swelling and the electrohydrodynamic properties of functionalized carboxymethyldextran macromolecules

The electrostatic, hydrodynamic, and swelling properties of a well-defined, functionalized carboxymethyldextran (CMD) polysaccharide are investigated in aqueous NaNO3 solution over a broad ionic strength range. The impact of the polycarboxylate charge and molar mass of the CMD macromolecules on their electrohydrodynamic features is thoroughly examined by combined protolytic titration, dynamic light scattering, and electrokinetic analyses. Electrophoretic mobility data obtained for sufficiently high electrolyte concentrations reveal a typical soft particle behavior. Upon decrease of the ionic strength, mobilities strongly increase in magnitude while significant electrostatic swelling takes place, as reflected in a decrease in the diffusion coefficients. CMD entities undergo conformational transitions from compact random coil at large ionic strengths to swollen coil and possibly a wormlike structure at lower NaNO3 concentrations. The magnitude of the variations in size and mobility with electrolyte concentration strongly depends on the overall charge of the CMD entity as well as on its molar mass. These factors control the stiffness of the constituent polymer chains and thus the degree of macromolecular permeability ("softness"). Using the soft-diffuse interface formalism previously developed for the electrohydrodynamics of charged permeable macromolecules, a quantitative analysis of the electrophoretic mobility data is presented. The measured values of the diffusion coefficient and space charge density Gamma degrees, as evaluated independently from the modeling of potentiometric titration curves, are taken into account in a self-consistent manner. It is found that large CMD entities of low charge densities are the most permeable to flow penetration with a limited heterogeneous electrostatic stiffening of the chains, whereas small CMD entities of larger Gamma degrees significantly expand upon lowering the ionic strength, giving rise to a strong anisotropy for the spatial distribution of polymer chain density.

Competition between polyanions in glomerular binding and renal clearance

This study tests the hypothesis that exogenous polyanions may accumulate in the glomerular capillary wall and thereby provide fixed negative charges that will be effective, through electrostatic interactions, in governing the transcapillary transport of charged molecules. We have tested the effects of high concentrations of dextran sulfate, heparin, and orosomucoid on the fractional clearance of albumin and dextran sulfate using the isolated perfused kidney technique. We demonstrate that all these exogenous polyanions are without effect on the fractional clearance of albumin. Augmentation of glomerular charge through the binding of dextran sulfate or orosomucoid is very small compared to the charge augmentation by albumin. These results agree with previous studies which demonstrate that electrostatic interactions governing glomerular transcapillary wall transport are negligible. This study also demonstrates that there is no plausible correlation between the fixed negative charge contributed by endogenous metabolically labeled glomerular heparan sulfate and the manifestation of apparent charge selectivity as determined with dextran sulfate. The results are also consistent with previous studies that the renal processing of both dextran sulfate and albumin is associated with specific cellular uptake.