Conformational analysis of blood group A-active glycosphingolipids using HSEA-calculations. The possible significance of the core oligosaccharide chain for the presentation and recognition of the A-determinant.

Conformational analysis of four different A-active glycosphingolipids, A types 1-4, was carried out using HSEA-calculations with the GESA-program. In their minimum energy conformations the oligosaccharide chains are more or less curved; in particular the type 3 and 4 have a strongly bent shape. When the carbohydrate structures are linked to ceramide, using the conformational features predominantly observed in crystal structures of membrane lipids, rather drastic differences in the orientation of the oligosaccharide chains are obtained. For the type 1 glycosphingolipid the model study indicates that the A-determinant extends almost perpendicularly to the membrane plane whereas for type 2, 3 and 4 the terminal part of the oligosaccharide chains is more parallel to the membrane. The fucose branch on type 3 and type 4 thereby appears directed towards the environment whereas for type 2 it would face the membrane. Due to restrictions imposed by the membrane layer this core specific orientation is largely preserved even if the flexibility of the saccharide-ceramide linkage is taken into account. Hydrophilic and hydrophobic sites on the surface of the different oligosaccharide chains in their minimum energy conformation were located using the GRID-program. It is suggested that the core-dependent presentation of the A-determinant might explain the chain type specificity observed for different monoclonal anti-A antibodies. The results further suggest that assay systems ensuring a membrane-like presentation of the glycolipid antigen should be used in studies of glycolipid/protein interactions.