The solution structure of the Link module from human TSG-6, a hyaladherin with important roles in inflammation and ovulation, has been determined in both its free and hyaluronan-bound conformations. This reveals a well defined hyaluronan-binding groove on one face of the Link module that is closed in the absence of ligand. The groove is lined with amino acids that have been implicated in mediating the interaction with hyaluronan, including two tyrosine residues that appear to form essential intermolecular hydrogen bonds and two basic residues capable of supporting ionic interactions. This is the first structure of a non-enzymic hyaladherin in its active state, and identifies a ligand-induced conformational change that is likely to be conserved across the Link module superfamily. NMR and isothermal titration calorimetry experiments with defined oligosaccharides have allowed us to infer the minimum length of hyaluronan that can be accommodated within the binding site and its polarity in the groove; these data have been used to generate a model of the complex formed between the Link module and a hyaluronan octasaccharide.Hyaluronan (HA), 1 a high molecular weight polysaccharide with a central role in extracellular matrix organization and cell adhesion in mammals (1), is essential to a wide range of normal physiological processes including development, immunology, and reproduction (2-4). Alterations in the metabolism and localization of this molecule underlie the progression of many diseases, for instance arthritis, pulmonary/vascular disorders, and cancer (5, 6). These diverse biological activities may seem surprising for a linear polymer composed entirely of a repeating disaccharide (i.e. -glucuronic acid-␤Ϫ1,3-N-acetylglucosamine-␤Ϫ1,4-; up to 10 7 Da) that, unlike other glycosaminoglycans, is neither attached to a core protein nor sulfated. This functional complexity is thought to arise from the interaction of HA with a large number of specific HA-binding proteins (7), which can form structurally diverse complexes (see Ref. 8). The majority of these "hyaladherins" belong to a superfamily of proteins that share a common ϳ100 amino acid domain, termed a Link module, that mediates the interaction with HA.Previously we have determined the solution structure of the Link module from human TSG-6 (the protein product of the tumor necrosis factor-stimulated gene-6 (9)), thereby defining the consensus fold for this superfamily (10). In TSG-6, a 35-kDa secreted protein composed mainly of contiguous Link and CUB modules, the Link module is sufficient to mediate a high affinity interaction with HA (10, 11); this has been termed a "type A" HA-binding domain (7). The HA receptor CD44, which has an important role in mediating lymphocyte migration, however, requires N-and C-terminal extensions to its Link module for correct folding and functional activity of its type B interaction domain. Most other members of the superfamily, such as link proteins and chondroitin-sulfate proteoglycans (critical for extracellular matrix organiza...

The pyranose ring of l-iduronic acid (IdoA), a major constituent of the anticoagulant heparin, is an equilibrium of multiple ring puckers that have evaded quantification by experiment or computation. In order to resolve this enigma, we have calculated the free energy landscape of IdoA and two related monosaccharides from extensive microsecond simulations. After establishing that the simulated puckers had reached equilibrium, hypotheses were confirmed that (a) IdoA 1 C 4- and 4 C 1-chair conformations exchange on the microsecond time scale, (b) C5 epimerization leads to a 4 C 1-chair, and (c) IdoA 2-O-sulfation (IdoA2S) stabilizes the 1 C 4 conformer. The IdoA and IdoA2S 1 C 4 conformers were isoenergetic and computed to be 0.9 and 2.6 kcal mol−1 lower in free energy than their respective 4 C 1-chair conformations. The simulations also predicted that the IdoA 2 S O-skew-boat was less populated than previously thought. Novel chemical synthesis and ultra-high-field NMR supported these observations, but slight discrepancies in observed and predicted NMR vicinal couplings implied that the simulation overestimated the population of the IdoA 4 C 1-chair with respect to 1 C 4-chair due to small force field inaccuracies that only manifest in long simulations. These free-energy calculations drive improvements in computational methods and provide a novel route to carbohydrate mimetic biomaterials and pharmaceuticals.

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Understanding microsecond-timescale dynamics is crucial to establish three-dimensional (3D) structure–activity relationships in sugars but has been intractable to experiments and simulations. As a consequence, whether arguably the most important chemical scaffold in glycobiology, N-acetyl-d-glucosamine (GlcNAc), deviates from a rigid 4C1 chair is unknown. Here, conformer populations and exchange kinetics were quantified from the longest aqueous carbohydrate simulations to date (0.2 ms total) of GlcNAc, four derivatives from heparan sulfate and their methylglycosides. Unmodified GlcNAc took 3–5 μs to reach a conformational equilibrium, which comprised a metastable 4C1 chair that underwent 4C1 ↔ 1C4 transitions at a predicted forward rate of 0.8 μs−1 with an average 1C4-chair lifetime of 3 ns. These predictions agree with high-resolution crystallography and nuclear magnetic resonance but not with the hypothesis that GlcNAc is a rigid 4C1 chair, concluded from previous experimental analyses and non-aqueous modeling. The methylglycoside was calculated to have a slower forward rate (0.3 μs−1) and a more stable 4C1 conformer (0.2 kcal mol−1), suggesting that pivotal 3D intermediates (particularly 2SO, 1S5 and B2,5) increased in energy, and water was implicated as a major cause. Sulfonation (N-, 3-O and 6-O) significantly augmented this effect by blocking pseudorotation, but did not alter the rotational preferences of hydroyxl or hydroxymethyl groups. We therefore propose that GlcNAc undergoes puckering exchange that is dependent on polymerization and sulfo substituents. Our analyses, and 3D model of the equilibrium GlcNAc conformer in water, can be used as dictionary data and present new opportunities to rationally modify puckering and carbohydrate bioactivity, with diverse applications from improving crop yields to disease amelioration.

The polysaccharide hyaluronan is an essential component of the vertebrate extracellular matrix and also produced by viruses, bacteria and fungi. Although the hyaluronan polymer is simply a disaccharide that repeats many thousands of times, it has an amazing array of biological functions and medical uses. For example, it is an efficient space filler that maintains hydration, serves as a substrate for assembly of proteoglycans and cellular locomotion, regulates cellular function and development, and is involved in tumor progression, inflammation and wound healing. Its physical properties and biocompatibility also make it of considerable importance in the development of engineered tissue, biomaterials and in clinical applications.

The Link module from human TSG-6, a hyaladherin with roles in ovulation and inflammation, has a hyaluronan (HA)-binding groove containing two adjacent tyrosine residues that are likely to form CH-stacking interactions with sequential rings in the sugar. We have used this observation to construct a model of a protein⅐HA complex, which was then tested against existing experimental information and by acquisition of new NMR data sets of [ 13 C, 15 N]HA (8-mer) complexed with unlabeled protein. A major finding of this analysis was that acetamido side chains of two GlcNAc rings fit into hydrophobic pockets on either side of the adjacent tyrosines, providing a selectivity mechanism of HA over other polysaccharides. Furthermore, two basic residues have a separation that matches that of glucuronic acids in the sugar, consistent with the formation of salt bridges; NMR experiments at a range of pH values identified protein groups that titrate due to their proximity to a free carboxylate in HA. Sequence alignment and construction of homology models for all human Link modules in their HA-bound states revealed that many of these features are conserved across the superfamily, thus allowing the prediction of functionally important residues. In the case of cartilage link protein, its two Link modules were docked together (using bound HA as a guide), identifying hydrophobic residues likely to form an intra-Link module interface as well as amino acids that could be involved in supporting intermolecular interactions between link proteins and chondroitin sulfate proteoglycans. Here, we propose a mechanism for ternary complex formation that generates higher order helical structures, as may exist in cartilage aggregates.

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