Synthetic triacyl glucosamine monosaccharide lipid A part structures corresponding to the non-reducing moiety of enterobacterial lipid A with an acyloxyacyl chain linked to position 3 of the glucosamine and an unbranched chain linked to position 2 (group 1) and vice versa (group 2) were analyzed biophysically: Fourier-transform infrared spectroscopy was performed to characterize the gel-to-liquid crystalline phase transition, the phosphate band contour, and the orientation of the glucosamine with respect to the membrane surface. Small-angle x-ray diffraction was applied for the elucidation of the supramolecular aggregate structure and, with that, of the molecular shape. With fluorescence resonance energy transfer the lipopolysaccharide-binding protein (LBP)-mediated intercalation of the lipid A partial structures into phospholipid liposomes was monitored. The physical data clearly exhibit a classification of the synthetic compounds into two groups: group 1 compounds have sharp phase transitions, indicating dense acyl chain packing and an inclination of the glucosamine backbone with respect to the membrane surface of 30 degrees with the phosphate buried in the membrane. Group 2 compounds have a very broad phase transition, indicating poorly packed acyl chains, and an inclination of -30 degrees with the phosphate group sticking outward. For the first group unilamellar phases are observed superimposed by a non-lamellar structure, and for the second one only multilamellar aggregate structures. The cytokine-inducing capacity in human mononuclear cells is relatively high for the first group and low or absent for the second group. Based on these data a model of the intra and intermolecular conformations is proposed which also extends the concept of "endotoxic conformation."
Sialic acids are one of the important constituents of glycoconjugates in the deuterostome lineage of animals and microorganisms. Siglecs (Sialic acid-binding immunoglobulin like lectins) are a family of cell-surface receptor proteins that recognize sialylated glycoconjugates as ligands. To date, 15 Siglecs have been described in humans and are mainly known as regulators of the immune system. Several of the Siglecs are emerging as potential targets for the treatment of some inflammatory, autoimmune, allergic, neurodegenerative and infectious diseases. In addition to antibody mediated therapy, high-affinity ligand-based probes of Siglec receptors would represent invaluable tools to effectively address therapeutic opportunities of Sialic acid-mediated Siglec recognition. This review discusses some aspects of structure and function of Siglec receptors, and concisely summarizes up-to-date progress on the identification of sialic acid based high-affinity ligands of certain well explored Siglec receptors.
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