Sites for Dynamic Protein-Carbohydrate Interactions of O- and C-Linked Mannosides on the E. coli FimH Adhesin.

Antagonists of the Escherichia coli type-1 fimbrial adhesin FimH are recognized as attractive alternatives for antibiotic therapies and prophylaxes against acute and recurrent bacterial infections. In this study α-d-mannopyranosides O- or C-linked with an alkyl, alkene, alkyne, thioalkyl, amide, or sulfonamide were investigated to fit a hydrophobic substituent with up to two aryl groups within the tyrosine gate emerging from the mannose-binding pocket of FimH. The results were summarized into a set of structure-activity relationships to be used in FimH-targeted inhibitor design: alkene linkers gave an improved affinity and inhibitory potential, because of their relative flexibility combined with a favourable interaction with isoleucine-52 located in the middle of the tyrosine gate. Of particular interest is a C-linked mannoside, alkene-linked to an ortho-substituted biphenyl that has an affinity similar to its O-mannosidic analog but superior to its para-substituted analog. Docking of its high-resolution NMR solution structure to the FimH adhesin indicated that its ultimate, ortho-placed phenyl ring is able to interact with isoleucine-13, located in the clamp loop that undergoes conformational changes under shear force exerted on the bacteria. Molecular dynamics simulations confirmed that a subpopulation of the C-mannoside conformers is able to interact in this secondary binding site of FimH.

Fluorinated carbohydrates as lectin ligands: dissecting glycan-cyanovirin interactions by using 19F†NMR spectroscopy.

NMR spectroscopy and isothermal titration calorimetry (ITC) are powerful methods to investigate ligand-protein interactions. Here, we present a versatile and sensitive fluorine NMR spectroscopic approach that exploits the (19)F nucleus of (19)F-labeled carbohydrates as a sensor to study glycan binding to lectins. Our approach is illustrated with the 11 kDa Cyanovirin-N, a mannose binding anti-HIV lectin. Two fluoro-deoxy sugar derivatives, methyl 2-deoxy-2-fluoro-α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside and methyl 2-deoxy-2-fluoro-α-D-mannopyranosyl-(1→2)-α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside were utilized. Binding was studied by (19)F NMR spectroscopy of the ligand and (1)H-(15)N HSQC NMR spectroscopy of the protein. The NMR data agree well with those obtained from the equivalent reciprocal and direct ITC titrations. Our study shows that the strategic design of fluorinated ligands and fluorine NMR spectroscopy for ligand screening holds great promise for easy and fast identification of glycan binding, as well as for their use in reporting structural and/or electronic perturbations that ensue upon interaction with a cognate lectin.

Synthesis of four (4"-, 2"-, 2'-, and 6-) monodeoxy analogs of the trisaccharide α-D-Glcp-(1 → 3)-α-D-Manp-(1 → 2)-α-D-ManpOMe recognized by Calreticulin/Calnexin.

Routes for efficient synthesis of four (4"-, 2"-, 2'-, and 6-) monodeoxy analogs of the trisaccharide α-D-Glcp-(1 → 3)-α-D-Manp-(1 → 2)-α-D-ManpOMe have been developed. For the introduction of the 2'- and 2"-deoxy motifs the most efficient way was to use a 1,2-di-bromo-mannosyl donor in silver triflate-promoted couplings to construct the α-glycosidic linkage followed by reduction of the 2-bromo-function to a 2-deoxy motif at the di- or trisaccharide level. In contrast, the 4"- and 6-deoxy functions were introduced already at the monosaccharide stage. The most challenging part of the syntheses was the stereoselective formation of the non-reducing end cis-α-D-glucosidic linkages. The established α-directing effect of a 3-O-acetyl group in glucosyl donors was explored but the magnitude of the effect was variable and dependent on donor/acceptor structure and nature of promoter and an optimization had to be made for each individual glycosylation.