Fullerene‐sp2‐Iminosugar Balls as Multimodal Ligands for Lectins and Glycosidases: A Mechanistic Hypothesis for the Inhibitory Multivalent Effect

Concerted functioning of lectins and carbohydrate‐processing enzymes, mainly glycosidases, is essential in maintaining life. It was commonly assumed that the mechanisms by which each class of protein recognizes their cognate sugar partners are intrinsically different: multivalency is a characteristic feature of carbohydrate–lectin interactions, whereas glycosidases bind to their substrates or substrate‐analogue inhibitors in monovalent form. Recent observations on the glycosidase inhibitory potential of multivalent glycomimetics have questioned this paradigm and led to postulate an inhibitory multivalent effect. Here the mechanisms at the origin of this phenomenon have been investigated. A D ‐gluco‐configured sp²‐iminosugar glycomimetic motif, namely 1‐amino‐5N,6O‐oxomethylydenenojirimycin (1N‐ONJ), behaving, simultaneously, as a ligand of peanut agglutinin (PNA) lectin and as an inhibitor of several glycosidases, has been identified. Both the 1N‐ONJ–lectin‐ and 1N‐ONJ–glycosidase‐recognition processes have been found to be sensitive to multivalency, which has been exploited in the design of a lectin–glycosidase competitive assay to explore the implication of catalytic and non‐glycone sites in enzyme binding. A set of isotropic dodecavalent C₆₀‐fullerene–sp²‐iminosugar balls incorporating matching or mismatching motifs towards several glycosidases (inhitopes) was synthesized for that purpose, thereby preventing differences in binding modes arising from orientational preferences. The data supports that: 1) multivalency allows modulating the affinity and selectivity of a given inhitope towards glycosidases; 2) multivalent presentation can switch on the inhibitory capacity for some inhitope–glycosidase pairs, and 3) interactions of the multivalent inhibitors with non‐glycone sites is critical for glycosidase recognition. The ensemble of results point to a shift in the binding mode on going from monovalent to multivalent systems: in the first case a typical ′′key–lock′′ model involving, essentially, the high‐affinity active site can be assumed, whereas in the second, a lectin‐like behavior implying low‐affinity non‐glycone sites probably operates. The differences in responsiveness to multivalency for different glycosidases can then be rationalized in terms of the structure and accessibility of the corresponding carbohydrate‐binding regions.     

Combining glycomimetic and multivalent strategies toward designing potent bacterial lectin inhibitors

As part of ongoing activities toward the design of potent and selective ligands against galactoside-binding proteins from animal, bacterial, and plant lectins, a systematic investigation involving the synthesis and binding evaluations of a series of original β-C-galactopyranoside mimetics is described. The multivalent presentation of partly optimized candidates on various dendritic scaffolds through Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAc) has also been achieved. Biophysical investigations based on isothermal titration calorimetry (ITC) have indicated a dissociation constant in the low micromolar range for the best optimized monovalent conjugate (K(d)=37 μM). The results thus confirmed that stable C-galactosides could represent efficient synthetic glycomimetics of natural α-linked oligosaccharidic inhibitors of PA-IL lectin (Lec A) from the pathogenic Pseudomonas aeruginosa. Striking enhancements in the avidity of the glycoconjugates were also observed for tri-, hexa-, and nonavalent derivatives, among which the most potent exhibited dissociation constants below 500 nM, corresponding to a 400-fold increase in affinity compared with the β-D-Gal-O-Me used as reference. To deepen our understanding of the binding mode of the best glycomimetics involved in the recognition process, molecular modeling studies, docking calculations, and NMR diffusion measurements have been performed. Although favorable complementary interactions induced by the addition of the hydrophobic aglycon might explain the affinity enhancement, experimental determination of the size and the topology of the multivalent conjugates further supported the formation of aggregative complexes as a major multivalent binding mode. This work represents a systematic and comprehensive study towards a thorough understanding of the protein-carbohydrate interactions involved in Pseudomonas aeruginosa infection, and as such should prove useful for the development of stable and optimized anti-adhesive agents.     

Clustering of Escherichia coli type-1 fimbrial adhesins by using multimeric heptyl α-D-mannoside probes with a carbohydrate core

Heptyl α‐D ‐mannoside (HM) is a strong inhibitor of the FimH lectin that mediates the initial adhesion of the uropathogenic Escherichia coli (E. coli) to the bladder cells. We designed a set of multivalent HM ligands based on carbohydrate cores with structural valencies that range from 1 to 7. The chemical strategy used to construct the regular hydrophilic structures consisted of the repetition of a critical glucoside fragment. A primary amino group was grafted at the sugar reducing end to couple the multimers to a fluorescent label. A one‐pot synthetic approach was developed to tether the ligands and the fluorescein isothiocyanate (FITC) probe to the scaffold simultaneously. Isothermal calorimetry with the monomeric FimH lectin revealed nanomolar affinities and saturation of all structurally available binding sites on the multivalent HM ligands. Direct titrations domain showed almost strict correlation of enthalpy–entropy compensation with increasing valency of the ligand, whereas reverse titration calorimetry demonstrated negative cooperativity between the first and the second binding site of the divalent heptyl mannoside. A multivalency effect was nevertheless observed by inhibiting the haemagglutination of type‐1 piliated UTI89 E. coli, with a titer as low as 60 nM for the heptavalent HM ligand. An FITC‐labeled HM trimer showed capture and cross‐linking of living bacteria in solution, a phenomenon not previously described with low‐valency ligands.     

Polyvalent Transition-State Analogues of Sialyl Substrates Strongly Inhibit Bacterial Sialidases**

Bacterial sialidases (SA) are validated drug targets expressed by common human pathogens such as Streptococcus pneumoniae, Vibrio cholerae, or Clostridium perfringens. Noncovalent inhibitors of bacterial SA capable of reaching the submicromolar level are rarely reported. In this work, multi- and polyvalent compounds are developed, based on the transition-state analogue 2-deoxy-2,3-didehydro-N-acetylneuraminic (DANA). Poly-DANA inhibits the catalytic activity of SA from S. pneumoniae (NanA) and the symbiotic microorganism B. thetaiotaomicron (BtSA) at the picomolar and low nanomolar levels (expressed in moles of molecules and of DANA, respectively). Each DANA grafted to the polymer surpasses the inhibitory potential of the monovalent analogue by more than four orders of magnitude, which represents the highest multivalent effect reported so far for an enzyme inhibition. The synergistic interaction is shown to operate exclusively in the catalytic domain, and not in the flanked carbohydrate-binding module (CBM). These results offer interesting perspectives for the multivalent inhibition of other SA families lacking a CBM, such as viral, parasitic, or human SA.