Structural basis of the abscess-modulating polysaccharide A2 from Bacteroides fragilis

Zwitterionic capsular polysaccharides from pathogenic bacteria have peculia r immunological properties. They are capable of eliciting T-cell proliferat ion and modulating the course of abscess formation. To understand the molec ular basis of this characteristic immune response, we are conducting detail ed structure-function studies on these polysaccharides. We have identified, purified, and characterized an abscess-modulating polysaccharide, PS A2, f rom the clinical strain Bacteroides fragilis 638R. Here, we report the eluc idation of both the chemical and three-dimensional structures of PS AZ by N MR spectroscopy, chemical methods, gas chromatography-mass spectrometry, an d restrained molecular dynamics calculations. PS A2 consists of a pentasacc haride repeating unit containing mannoheptose, N-acetylmannosamine, 3-aceta mido-3,6-dideoxyglucose, 2-amino-4-acetamido-2,4,6-trideoxygalactose, fucos e, and 3-hydroxybutanoic acid. PS A2 is zwitterionic and carries one cation ic free amine and one anionic carboxylate in each repeating unit. It forms an extended right-handed helix with two repeating units per turn and a pitc h of 20 Angstrom. Positive and negative charges are exposed on the outer su rface of the polymer in a regularly spaced pattern, which renders them easi ly accessible to other molecules. The helix is characterized by repeated la rge grooves whose lateral boundaries are occupied by the charges. The three -dimensional structure of PS A2 explicitly suggests mechanisms of interacti on between zwitterionic polysaccharides and proteins.