CONICET | Buscador de Institutos y Recursos Humanos

Carbohydrate-protein interactions are involved in cellular recognition processes that include viral and bacterial infections, inflammation, and tumor metastasis. Recognition events, occurring in the active sites of glycosyltransferases and glycosyl hydrolases, account for the metabolism of carbohydrates. In some natural systems, the sometimes weak binding affinity of carbohydrates to their receptors is overcome by a multivalent display of sugar residues at the surface of cells, which leads to the so-called glycoside clustering effect. During the last years, multivalent ligands having â-galactoside or â-lactoside epitopes attached to a variety of scaffolds have been synthesized. Most of them present O-linked saccharides and their preparation involves classic glycosylating methods. To connect the epitope to the scaffold, oligoethyleneglycol (EG) linkers are usually employed, due to their wide biomedical applications, flexibility, and ability to prevent nonspecific adsorption to proteins. The preparation of a variety of multivalent ligands differing in EG linker lengths can contribute to elucidate the mechanisms involved in the recognition processes. The cluster effect has been extensively studied for several lectin systems, but little is known about the influence of the multivalency on the activity of glycosidases. So far, we have shown that a trivalent iminosugar displayed a 6-fold affinity enhancement toward Jack bean R-mannosidase. Also, a recent report on the multivalent effect of fullerene iminosugar balls on the inhibition of R-glucosidases suggests that alternative binding mechanisms are operative. The interaction of lactose-functionalized gold glyconanoparticles with a lectin and with the â-galactosidase from Escherichia coli has been evaluated. The proper selection of ligand densities and spacers led to an increased resistance of the lactose moiety of the nanoparticle to hydrolysis by the enzyme. The â-galactosidase from E. coli is highly specific for â-galactopyranosyl nonreducing moieties, which may be linked to a variety of aglycons. The enzyme is able to recognize C- and S-galactosyl residues that, in turn, may act as moderate to good inhibitors of its hydrolytic activity. The interaction of this â-galactosidase with several substrates has been studied by X-ray and NMR experiments, and we have synthesized a family of thiodisaccharides that display an important inhibitory activity against the enzyme. In contrast to natural O-linked sugars, the S-carbohydrate mimetics are usually resistant to metabolic processes and are seen as potential precursors of promising carbohydrate-based therapeutics. Particularly, the resistance of glycoclusters to enzyme hydrolysis is crucial to circumvent degradation when they are internalized in the cell. When designing glycoclusters to study biological processes, specificity, affinity, and stability against glycolytic enzymes are important factors to be considered. We assumed that the replacement of the interglycosidic oxygen atom by a sulfur atom in glycoclusters could increase the resistance to hydrolytic degradation by enzymes and can even cause inhibition of this activity. Therefore, we report here the synthesis of a family of glycoclusters based on carbohydrate scaffolds bearing one to four S-galactoside epitopes and EG spacers of different lengths. The copper-catalyzed azide_alkyne cycloaddition (CuAAC) reaction was employed as the key step to connect the recognition elements to the scaffolds. The inhibition behavior against E. coli â-galactosidase was studied in order to determine the existence of cluster effects.