Protein-carbohydrate interactions prevail in biological information exchange. It is therefore very important to understand driving forces for complex formation and the relationship of structures and affinities. Structural and biophysical data on such recognition processes are scarce, as the interaction partners often are not accessible in biochemical quantities. Moreover, thermodynamics of carbohydrate binding is difficult to assess as it mostly occurs with moderate affinities that are due to large amounts of water cooperatively rearranged during binding. This water network makes it extremely difficult to experimentally address single water molecules or define ligand binding areas. By contrast, theoretical approaches suffer from lacking experiments to validate thermodynamic predictions. We use bacterial surface carbohydrates for protein interaction studies. They are available in high quantities for structural-thermodynamic investigations. Moreover, they are intimately linked to pathogenesis and understanding their recognition by proteins is hence important for vaccine development or biofilm control. As binding partner we investigate bacteriophage tailspike proteins (TSP); model proteins providing especially large interaction sites (1,2). We have identified TSP with hydrophobic surface cavities that shield their containing solvent from the cooperative network (2). Isothermal titration calorimetry showed large enthalpic desolvation penalties. They disappear when modified oligosaccharides were used that do not fill the surface cavity. This opens the possibility to probe the thermodynamic signature of individual water molecules in these cavities and to merge these data with theoretical calculations.
1. Andres, D., Gohlke, U., Broeker, N.K., Rabsch, W., Heinemann, U., Barbirz, S. and Seckler, R. (2013) Conserved structural waters mediate recognition of Salmonella O‑antigens by phage P22 tailspike protein. Glycobiology 23, 486-494.
2. Broeker, N.K., Gohlke, U., Müller, J.J., Uetrecht, C., Heinemann, U., Seckler, R. and Barbirz, S. (2013) Single amino acid exchange in bacteriophage HK620 tailspike protein results in thousand-fold increase of its oligosaccharide affinity. Glycobiology 23, 59-68.
