Three-Dimensional Structural Aspects of Protein-Polysaccharide Interactions

The review article describes some important features that characterize the three-dimensional interactions of protein-polysaccharide interactions as elucidated by X-ray crysrtallography. Proteins that interact with linear polysaccharides have been identified or developed, such as galectins and polysaccharide-specific antibodies, respectively. These proteins are classified into two types : exo-type and endo-type. The former group specifically interacts with the terminal units of polysaccharides, whereas the latter with internal units. In this review, the structural aspects of exo-type and endo-type protein-polysaccharide interactions are described.

The example extracted from the crystal structure of anti-polysialic acid antibody single chain Fv fragment complexed with octasialic acid:illustrates how the conformational flexibility about the glycosidic linkage can generate folded conformations, far away from the previously proposed helical conformation.
The dihedral angles of the trisialic acid unit directly interacting with scFv are not uniform, indicating that the monoclonal antibody does not strictly favor the previously proposed helical conformation. The monoclonal antibody gains its apparent high affinity for a longer polysialic acid chain by recognizing every three sialic acid units in a paired manner.

The authors analyse the structural basis for affinity .and specificity enhancement in the face of inherently weak binding interactions.