Structural elucidation of the backbone oligosaccharide from the lipopolysaccharide of Moraxella catarrhalis serotype A

Abstract

The cell-surface lipopolysaccharide produced by Moraxella catarrhalis serotype A is composed of a hydrophobic lipid A moiety and an oligosaccharide, but lacks high-molecular-weight O-polysaccharide chains. The oligosaccharide component is composed of D-glucose, D-galactose, D-glucosamine, and 3-deoxy-D-manno-octulosonic acid. The carbohydrate backbone was obtained from the lipopolysaccharide by employing a reaction sequence involving deacylation, dephosphorylation, and reduction of the reducing D-glucosamine terminus of the lipid A moiety. Structural analysis of the backbone oligosaccharide employed a combination of microanalytical methods and nuclear magnetic resonance spectroscopy. Homo- and hetero-nuclear chemical shift correlation techniques and nuclear Overhauser enhancement (NOE) experiments led to the unambiguous assignment of the 1H and l3C resonances associated with each of the component glycosyl residues and established their sequence within the backbone oligosaccharide as shown,[Formula: see text]This lipopolysaccharide was found to consist of a highly branched, D-glucose-containing inner-core region which possesses a unique and unusual solution conformation. This was established by measurement of transglycosidic NOEs and three-bond 1H−13C coupling constants, in conjunction with molecular modeling. A D-galactose-containing disaccharide identified as a terminal group of the lipopolysaccharide was structurally identical to antigenic epitopes expressed by certain mammalian epithelial cells and may be related to the virulence potential of this human pathogen.