Structural elucidation of type III group B Streptococcus capsular polysaccharide using molecular dynamics simulations: the role of sialic acid

• Published in: Carbohydrate research 2005-04, Vol.340 (5), p.1007-1018
• Main Authors: González-Outeiriño, Jorge, Kadirvelraj, Renuka, Woods, Robert J.
• Format: Article
• Language: English
• Subjects: AMBER; Capsular polysaccharide; Carbohydrate Conformation; Carbohydrate Sequence; Computer Simulation; Conformational epitope; Electron Spin Resonance Spectroscopy; Epitopes - chemistry; GLYCAM; Group B Streptococcus; Helix; Hydrogen Bonding; Models, Molecular; Molecular dynamics simulation; Molecular Sequence Data; N-Acetylneuraminic Acid - chemistry; Nuclear Magnetic Resonance, Biomolecular; Polysaccharides, Bacterial - chemistry; Streptococcus agalactiae - chemistry
• ISSN: 0008-6215; 1873-426X
• DOI: 10.1016/j.carres.2004.12.034

Helical structure (diameter of 29.3 Å and a pitch 89.5 Å) of the capsular polysaccharide from group B Streptococcus (serotype III), predicted by 50 ns MD simulations. The sialylated side-chains are arrayed on the exterior surface of the helix while the immunodominant epitope lines the core. The conformational properties of the capsular polysaccharide (CPS) from group B Streptococcus serotype III (GBS III) are derived from 50 ns explicitly solvated molecular dynamics simulations of a 25-residue fragment of the CPS. The results from the simulations are shown to be consistent with experimental NMR homo- and heteronuclear J-coupling and NOE data for both the sialylated native CPS and for the chemically desialylated polysaccharide. A helical structure is predicted with a diameter of 29.3 Å and a pitch 89.5 Å, in which the sialylated side chains are arrayed on the exterior surface of the helix. The results provide an explanation for the observation that CPS antigenicity varies with carbohydrate chain length up to approximately 4 pentasaccharide repeat units. The conformation of the immunodominant region is established and shown to be independent of the presence of sialic acid. The data provide an explanation for the observation that the specificity of the determinant, associated with the major population of antibodies raised upon immunization of rabbits with GBS III, is dependent on the presence of sialic acid. In the sialylated native CPS, the antibody response is largely directed against the immunodominant core of the helix. From simulations of the desialylated CPS, a model emerges which suggests that the minor population of antibodies, whose determinant is not sialic acid dependent, recognizes the same immunodominant region, but that in the disordered CPS this region is not presented in a regular repeating motif.