Structural and Thermodynamic Studies on Cation−Π Interactions in Lectin−Ligand Complexes: High-Affinity Galectin-3 Inhibitors through Fine-Tuning of an Arginine−Arene Interaction

Aryl O- and S-galactosides and lactosides as specific inhibitors of human galectins-1 and -3: Role of electrostatic potential at O-3

Phase transfer catalyzed reaction was used for the high yielding synthesis of aryl 1-thio-beta-d-galacto- and lacto-pyranosides carrying a panel of substituents on the phenyl groups. Best galectin-1 inhibitors were simple p-nitrophenyl thiogalactoside 5a for the monosaccharide and o-nitrophenyl thiolactoside 6f or napthylsulfonyl lactoside 8c, both being 20 times better relative to natural ligands. Relative inhibitory properties as low as 2500 and 40 microM were observed, respectively. The electronic effects of the lactoside aglycons directly influenced the electrostatic potential at O-3, which was associated with the inhibitory potencies against galectin-1.

Galectin-3: an open-ended story

Galectins, an ancient lectin family, are characterized by specific binding of beta-galactosides through evolutionary conserved sequence elements of carbohydrate-recognition domain (CRD). A structurally unique member of the family is galectin-3; in addition to the CRD it contains a proline- and glycine-rich N-terminal domain (ND) through which is able to form oligomers. Galectin-3 is widely spread among different types of cells and tissues, found intracellularly in nucleus and cytoplasm or secreted via non-classical pathway outside of cell, thus being found on the cell surface or in the extracellular space. Through specific interactions with a variety of intra- and extracellular proteins galectin-3 affects numerous biological processes and seems to be involved in different physiological and pathophysiological conditions, such as development, immune reactions, and neoplastic transformation and metastasis. The review attempts to summarize the existing information on structural, biochemical and intriguing functional properties of galectin-3.

Carbohydrate triazoles and isoxazoles as inhibitors of galectins-1 and -3

Galactosides and lactosides bearing triazoles or isoxazoles, regiospecifically prepared by [1,3]-dipolar cycloadditions between alkynes, azides or nitrile oxides, provided specific galectin-1 and -3 inhibitors with potencies as low as 20 microM.

A new chemical probe for the detection of the cancer-linked galectin-3

A chemical probe was developed for the detection of the emerging cancer marker galectin-3. The probe contains a benzophenone moiety which covalently attaches itself to the protein upon binding and irradiation. Introduction of a fluorescent label via'click' chemistry allows the labelled proteins to be visualized in a gel. With the probe, selective visualization of galectin-3 in protein mixtures was shown and remarkably even in cell lysates.

Thioureido N-acetyllactosamine derivatives as potent galectin-7 and 9N inhibitors

Derivatives of N-acetyllactosamine carrying structurally diverse thioureido groups at galactose C3 were prepared from a C3'-azido N-acetyllactosamine derivative in a three-step reaction sequence involving azide reduction and isothiocyanate formation by thiophosgene treatment of the C3-amine, followed by reaction of the isothiocyanate with a panel of amines. Evaluation of the N-acetyllactosamine thioureas as inhibitors against galectins-1, 3, 7, 8N (N-terminal domain), and 9N (N-terminal domain) revealed thiourea-mediated affinity enhancements for galectins-1, 3, 7, and 9N. In particular, good inhibitors were discovered against galectin-7 and 9N (K(d) 23 and 47 microM, respectively, for a 3-pyridylmethylthiourea derivative), which represents more than an order of magnitude affinity enhancement over the parent natural N-acetyllactosamine.

DC-SIGN mediates binding of dendritic cells to authentic pseudo-LewisY glycolipids of Schistosoma mansoni cercariae, the first parasite-specific ligand of DC-SIGN

During schistosomiasis, parasite-derived glycoconjugates play a key role in manipulation of the host immune response, associated with persistence of the parasite. Among the candidate host receptors that are triggered by glycoconjugates are C-type lectins (CLRs) on dendritic cells (DCs), which in concerted action with Toll-like receptors determine the balance in DCs between induction of immunity versus tolerance. Here we report that the CLR DC-SIGN mediates adhesion of DCs to authentic glycolipids derived from Schistosoma mansoni cercariae and their excretory/secretory products. Structural characterization of the glycolipids, in combination with solid phase and cellular binding studies revealed that DC-SIGN binds to the carbohydrate moieties of both glycosphingolipid species with Galbeta1-4(Fucalpha1-3)GlcNAc (Lewis(X)) and Fucalpha1-3Galbeta1-4(Fucalpha1-3)GlcNAc (pseudo-Lewis(Y)) determinants. Importantly, these data indicate that surveying DCs in the skin may encounter schistosome-derived glycolipids immediately after infection. Recent analysis of crystals of the carbohydrate binding domain of DC-SIGN bound to Lewis(X) provided insight into the ability of DC-SIGN to bind fucosylated ligands. Using molecular modeling we showed that the observed binding of the schistosome-specific pseudo-Lewis(Y) to DC-SIGN is not directly compatible with the model described. To fit pseudo-Lewis(Y) into the model, the orientation of the side chain of Phe(313) in the secondary binding site of DC-SIGN was slightly changed, which results in a perfect stacking of Phe(313) with the hydrophobic side of the galactose-linked fucose of pseudo-Lewis(Y). We propose that pathogens such as S. mansoni may use the observed flexibility in the secondary binding site of DC-SIGN to target DCs, which may contribute to immune escape.

3-(1,2,3-Triazol-1-yl)-1-thio-galactosides as small, efficient, and hydrolytically stable inhibitors of galectin-3

Copper(I)-catalyzed addition of alkynes to methyl 3-azido-3-deoxy-1-thio-beta-D-galactopyranoside afforded stable and structurally simple 3-deoxy-3-(1H-1,2,3-triazol-1-yl)-1-thio-galactosides carrying a panel of substituents at the triazole C4 in high yields. The 3-(1H-[1,2,3]-triazol-1-yl)-1-thio-galactoside collection synthesized contained inhibitors of the tumor- and inflammation-related galectin-3 with Kd values as low as 107 microM, which is as potent as the natural disaccharide inhibitors lactose and N-acetyllactosamine.

Molecular Recognition of Saccharides by Proteins. Insights on the Origin of the Carbohydrate−Aromatic Interactions

The existence of stabilizing carbohydrate-aromatic interactions is demonstrated from both the theoretical and experimental viewpoints. The geometry of experimentally based galactose-lectin complexes has been properly accounted for by using a MP2/6-31G(d,p) level of theory and by considering a counterpoise correction during optimization. In this case, the stabilizing interaction energy of the fucose-benzene complex amounts to 3.0 kcal/mol. The theoretical results obtained herein indicate that the carbohydrate-aromatic interactions are stabilizing interactions with an important dispersive component and that electronic density between the sugar hydrogens and the aromatic ring indeed exists, thus giving rise to three so-called nonconventional hydrogen bonds. Experimental evidence of the intrinsic tendency of aromatic moieties to interact with certain sugars has also been shown by simple NMR experiments in water solution. Benzene and phenol specifically interact with the clusters of C-H bonds of the alpha face of methyl beta-galactoside, without requiring the well-defined three-dimensional shape provided by a protein receptor, therefore resembling the molecular recognition features that are frequently observed in many carbohydrate-protein complexes.

Synthesis of O-galactosyl aldoximes as potent LacNAc-mimetic galectin-3 inhibitors

A panel of anomeric oxime ether derivatives of beta-galactose were synthesized via the reaction of O-beta-D-galactopyranosylhydroxylamine with aldehydes. The oxime ethers were evaluated as inhibitors against galectin-3 in a competitive fluorescence polarization assay. The best inhibitor, [E]-O-(beta-D-galactopyranosyl)-indole-3-carbaldoxime (E-52), had a Kd value of 180 microM, which is 24 times better than methyl beta-D-galactopyranoside (Kd=4400 microM) and in the same range as methyl lactoside (Kd=220 microM).

Synthesis of a phenyl thio-β-d-galactopyranoside library from 1,5-difluoro-2,4-dinitrobenzene: discovery of efficient and selective monosaccharide inhibitors of galectin-7

The galectins are a family of [small beta]-galactoside-binding proteins that have been implicated in cancer and inflammation processes. Herein, we report the synthesis of a library of 28 compounds that was tested for binding to galectins-1, -3, -7, -8N and -9N. An aromatic nucleophilic substitution reaction between 1,5-difluoro-2,4-dinitrobenzene and a galacto thiol gave 5-fluoro-2,4-dinitrophenyl 2,3,4,6-tetra-O-acetyl-1-thio-beta-D-galactopyranoside. This versatile intermediate was then modified in a two dimensional manner: either by further substitution of the second fluoride by amines or thiols, or by reduction of the nitro groups and acylation of the resulting amines, or both. Deacetylation then gave a library of aromatic beta-galactosides that showed variable inhibitory activity against the different galectins, as shown by screening with a fluorescence-polarisation assay. Particularly efficient inhibitors were found against galectin-7, while less impressive enhancements of inhibitor affinity over methyl beta-D-galactopyranoside were found for galectin-1, -3, -8N and -9N. The best inhibitors against galectin-7 showed significantly higher affinity (K(d) as low as 140 microM) than both beta-methyl galactoside (K(d) 4.8 mM) and the unsubstituted beta-phenyl thiogalactoside (non-inhibitory). The best inhibitors against galectin-7 were poor against the other galectins and thus have potential as structurally simple and selective tools for dissecting biological functions of galectin-7.