C-Fucopeptides as selectin antagonists: Attachment of lipid moieties enhances the activity

Synthesis of galactose-mimicking 1H-(1,2,3-triazol-1-yl)-mannosides as selective galectin-3 and 9N inhibitors

1H-[1,2,3]-Triazol-1-yl mannosides have been synthesized as inhibitors for the beta-galactoside-binding family of galectin proteins. Easier synthetic access to C1 in mannose, as compared to C3 in galactose, for attachment of affinity-enhancing triazoles rendered a synthetic advantage. The best mannose-derived inhibitor for galectin-9N, 4-benzylaminocarbonyl-1H-[1,2,3]-triazol-1-yl beta-D-mannopyranoside, had a Kd value of 540 microM, which compares favorably with its galactoside counterpart (Kd=670 microM) and with LacNAc (Kd=500 microM).

Solid-phase synthesis for the identification of high-affinity bivalent lectin ligands

The development of carbohydrate-based therapeutics has been frustrated by the low affinities that characterize protein-carbohydrate complexation. Because of the oligomeric nature of most lectins, the use of multivalency may offer a successful strategy for the creation of high-affinity ligands. The solid-phase evaluation of libraries of peptide-linked multivalent ligands facilitates rapid examination of a large fraction of linker structure space. If such solid-phase assays are to replicate solution binding behavior, the potential for intermolecular bivalent binding on bead surfaces must be eliminated. Here we report the solid-phase synthesis and analysis of peptide-linked, spatially segregated mono- and bivalent ligands for the legume lectin concanavalin A. Bead shaving protocols were used for the creation of beads displaying spatially segregated binding sequences on the surface of Tentagel resins. The same ligands were also synthesized on PEGA resin to determine the effect of ligand presentation on solid-phase binding. While we set out to determine the lower limit of assay sensitivity, the unexpected observation that intermolecular bivalent ligand binding is enhanced for bivalent ligands relative to monovalent ligands allowed direct observation of the level of surface blocking required to prevent intermolecular bivalent ligand binding. For a protein with binding sites separated by 65 A, approximately 99.9% of Tentagel(1) surface sites and 99.99% of the total sites on a PEGA bead must be blocked to prevent intermolecular bivalent binding. We also report agglutination and calorimetric solution-phase binding studies of mono- and bivalent peptide-linked ligands.

Design and synthesis of sialyl Lewis(x) mimics as E- and P-selectin inhibitors

The selectins are a family of cell-adhesion proteins that mediate the rolling of leukocytes on activated endothelial cells through the recognition of the carbohydrate epitope sialyl Lewis(x) (sLe(x)). Control of the leukocyte-endothelial cell adhesion process may prove useful in cases where excess recruitment of leukocytes can contribute to acute diseases such as stroke and reperfusion injury and chronic diseases such as psoriasis and rheumatoid arthritis. The development of molecules that block the interactions between sLe(x) and the selectins has become an active area of research. In this review, we will highlight the various approaches taken toward the development of sLe(x) mimetics as antagonists of E- and P-selectin, including the use of structural information about the selectins and their interactions with sLe(x) that have been revealed through the use of NMR, protein crystallography and molecular modeling.

Selectin/glycoconjugate binding assays for the identification and optimization of selectin antagonists

In this study we describe ELISA-type P- and L-selectin binding assays for the analysis of selectin antagonists. A biotinylated polyacrylamide-type glycoconjugate containing sialyl Lewis A (sLe(a)-polymer) is utilized as a synthetic ligand for both selectins analogous to the E-selectin assay we have developed recently. Following precomplexation of sLe(a)-polymer with streptavidin-peroxidase, the complex is added to microtiter plates coated with the recombinant selectins. Binding of sLe(a)-polymer to the immobilized selectins is measured by the peroxidase reaction. SLe(a)-polymer was found to bind to P- and L-selectin in a cation-dependent manner. The interaction of the polymer was blocked by neutralizing anti-P- and anti-L-selectin antibody, respectively. The reference compounds heparin and fucoidan inhibited in both assays. Sialyl Lewis X (sLe(x)) blocked binding to L-selectin by 46% at 3 mM, whereas no inhibition was observed in the P-selectin assay up to 3 mM. Control polymers containing sialic acid or beta-d-glucose instead of sLe(a) weakly bound or failed to bind to the selectins. Both assays are rapid to perform and of low variability. The P-selectin assay was successfully employed to identify and optimize novel carbohydrate-based P-selectin antagonists. The P-, L-, and E-selectin assays were used to determine the fine selectivity of several sLe(x)-related selectin antagonists. These studies together suggest that sLe(a)-polymer-based selectin assays are well suited for primary screening and the characterization of selectin antagonists.

Design, synthesis and biological evaluation of aryl-substituted sialyl Lewis X mimetics prepared via cross-metathesis of C-fucopeptides

Several aryl substituted C-fucopeptides have been developed as sialyl Lewis X mimetics. Although the compounds have a much simpler structure compared to SLe(x), up to 3-times higher binding affinity toward E-selectin and > 1000 times toward P-selectin was observed. Furthermore, a convenient strategy for generating a number of analogues from a SLe(x) mimetic template at a very late stage of the synthesis was introduced, using a ruthenium catalyzed cross olefin metathesis under benchtop conditions.

Aryl C-glycosides: physiologically stable glycomimetics of sialyl Lewis X

In the course of the search for physiologically stable, structurally simple, and low molecular weight sLeX mimetics, aryl C-glycosides with carboxylic acid functionality 2 were found to be extremely potent inhibitors against L- and P-selectins with IC50 in the low microM range.

Studies on selectin blockers. 7. Structure-activity relationships of sialyl Lewis X mimetics based on modified Ser-Glu dipeptides

We have previously found that heterochiral fucodipeptides, L-Ser-D-Glu (3a) and D-Ser-L-Glu (3b), exhibited up to 20-100 times more potency than a sialyl Lewis X (sLeX, 1) and a 3'-sulfated Lewis X analogue (2) toward E-selectin binding and have also proposed, from molecular dynamics calculation, that their strong activities would depend on a possible formation of the type II and/or type II' beta-turn of compounds 3a,b (Tsukida, T.; Hiramatsu, Y.; Tsujishita, H.; Kiyoi, T.; Yoshida, M.; Kurokawa, K.; Moriyama, H.; Ohmoto, H.; Wada, Y.; Saito, T.; Kondo, H. J. Med. Chem. 1997, 40, 3534-3541). To clarify our hypothesis, we synthesized several analogues of compounds 3a,b and investigated their structure-activity relationships. As a result, it was indicated that the type II and/or type II' beta-turn conformation would be a comparatively tight form and would play important roles in favorable binding to E-selectin. These findings indicate that sLeX mimetics with type II and type II' beta-turn dipeptides could be a useful methodology for the design of an active selectin blocker.

Synthesis of sialyl Lewis X mimetics using the Ugi four-component reaction

Application of the Ugi four-component condensation to rapidly synthesize a library of glycopeptide mimics of the tetrasaccharide SLe(x) as inhibitors of E- and P-selectin, and to study the effect of varied functionality in mimics on the inhibition is described.

Inhibition of L- and P-selectin by a rationally synthesized novel core 2-like branched structure containing GalNAc-Lewisx and Neu5Acalpha2-3Galbeta1-3GalNAc sequences

The selectins interact in important normal and pathological situations with certain sialylated, fucosylated glycoconjugate ligands containing sialyl Lewisx(Neu5Acalpha2-3Galbeta1-4(Fucalpha1-3)GlcN Ac). Much effort has gone into the synthesis of sialylated and sulfated Lewisxanalogs as competitive ligands for the selectins. Since the natural selectin ligands GlyCAM-1 and PSGL-1 carry sialyl Lewisxas part of a branched Core 2 O-linked structure, we recently synthesized Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-6(SE-3Galbeta1++ +-3)GalNAc1alphaOMe and found it to be a moderately superior ligand for L and P-selectin (Koenig et al. , Glycobiology 7, 79-93, 1997). Other studies have shown that sulfate esters can replace sialic acid in some selectin ligands (Yeun et al. , Biochemistry, 31, 9126-9131, 1992; Imai et al. , Nature, 361, 555, 1993). Based upon these observations, we hypothesized that Neu5Acalpha2-3Galbeta1-3GalNAc might have the capability of interacting with L- and P-selectin. To examine this hypothesis, we synthesized Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-6(Neu5Acalpha2++ +-3Galbeta1-3)-GalNAc alpha1-OB, which was found to be 2- to 3-fold better than sialyl Lexfor P and L selectin, respectively. We also report the synthesis of an unusual structure GalNAcbeta1-4(Fucalpha1- 3)GlcNAcbeta1-OMe (GalNAc-Lewisx-O-methyl glycoside), which also proved to be a better inhibitor of L- and P-selectin than sialyl Lewisx-OMe. Combining this with our knowledge of Core 2 branched structures, we have synthesized a molecule that is 5- to 6-fold better at inhibiting L- and P-selectin than sialyl Lewisx-OMe, By contrast to unbranched structures, substitution of a sulfate ester group for a sialic acid residue in such a molecule resulted in a considerable loss of inhibition ability. Thus, the combination of a sialic acid residue on the primary (beta1-3) arm, and a modified Lexunit on the branched (beta1-6) arm on an O-linked Core 2 structure generated a monovalent synthetic oliogosaccharide inhibitor superior to SLexfor both L- and P-selectin.

Synthetic methods of glycopeptide assembly, and biological analysis of glycopeptide products

The technology of glycopeptide synthesis has recently developed into a fully mature science capable of creating diverse glycopeptides of biological interest, even in combinatorial displays. This has allowed biochemists to investigate substrate specificity in the biosynthetic processing and immunology of various protein glycoforms. The construction of all the mucin core structures and a variety of cancer-related glycopeptides has facilitated detailed analysis of the interaction between MHC-bound glycopeptides and T cell receptors. Novel dendritic neoglycopeptide ligands have been shown to demonstrate high affinity for carbohydrate receptors and these interactions are highly dendrimer specific. Large complex N-linked oligosaccharides have been introduced into glycopeptides using synthetic or chemoenzymatic procedures, both methods affording pure glycopeptides corresponding to a single glycoform in preparative quantities. The improved availability of glycosyl transferases has led to increased use of chemoenzymatic synthesis. Chemical ligation has been introduced as a method of attaching glycans to peptide templates. Combinatorial synthesis and the analysis of resin-bound glycopeptide libraries have been successfully carried out by applying the ladder synthesis principle. Direct quantitative glycosylation of peptide templates on solid phase has paved the way for the synthesis of templated glycopeptide mixtures as libraries of libraries.

Microbial aldolases and transketolases: new biocatalytic approaches to simple and complex sugars

Enzymes have become exceedingly valuable tools in organic synthesis as the reactions they catalyze generally proceed under mild conditions and in high stereo- and regioselectivity. Advances in microbiology and genetic engineering have greatly increased the availability of various enzymes. One of the most useful applications of enzyme-catalyzed chemical transformations is in the synthesis of water-soluble, polyfunctional organic molecules such as carbohydrates. As the pivotal roles that carbohydrates play in biological processes become more evident, access to these compounds becomes increasingly important. This review gives a brief overview of the use of aldolases and transketolases in the synthesis of sugars, sugar analogs, and related compounds.