NMR AND MOLECULAR-DYNAMICS STUDIES OF THE CONFORMATIONAL EPITOPE OF THE TYPE-III GROUP-B STREPTOCOCCUS CAPSULAR POLYSACCHARIDE AND DERIVATIVES

The conformational epitope of the type III group B Streptococcus capsu lar polysaccharide (GBSP III) exhibits unique properties which can be ascribed to the presence of sialic acid in its structure and the requi rement for an extended binding site. By means of NMR and molecular dyn amics studies on GBSP III and its fragments, the extended epitope of G BSP III was further defined. The influence of sialic acid on the confo rmational properties of GBSP III was examined by performing conformati onal analysis on desialylated GBSP III, which is identical to the poly saccharide of Streptococcus pneumoniae type 14, and also on oxidized a nd reduced GBSP III. Conformational changes were gauged by H-1 and C-1 3 chemical shift analysis, NOE, ID selective TOCSY-NOESY experiments, J(HH) and J(CH) variations, and NOE of OH resonances. Changes in mobil ity were examined by C-13 T-1 and T-2 measurements. Unrestrained molec ular dynamics simulations with explicit water using the AMBER force fi eld and the GLYCAM parameter set were used to assess static and dynami c conformational models, simulate the observable NMR parameters and ca lculate helical parameters. GBSP III was found to be capable of formin g extended helices. Hence, the length dependence of the conformational epitope could be explained by its location on extended helices within the random coil structure of GBSP III. The interaction of sialic acid with the backbone of the PS was also found to be important in definin g the conformational epitope of GBSP III.