Secondary Structure and Stability of the Bacterial Carbohydrate-Specific Recognition Proteins K88ab, AFA-1, NFA-1, and CFA-1

The conformation and thermodynamic stability of the four polymeric carbohydrate-specific bacterial recognition proteins K88ab, AFA-1, NFA-1, and CFA-1 and their monomeric subunits that can be obtained by variation of pH were studied by infrared spectroscopy and differential scanning microcalorimetry. For NFA-1, a pH-dependent dissociation of the polymeric form cannot be achieved due to the stronger interactions of the neighboring subunits. Generally, no alterations in secondary structure are observed between the monomeric and the polymeric proteins. All adhesins reveal a high degree of beta-sheet structure (40-55%), while the alpha-helix component is of minor importance (10-20%). The adhesins investigated in this study revealed unusually high denaturation temperatures (69-104 degrees C) and stabilizing Gibbs energies, delta G (40-125 kJ/mol), compared to common globular proteins. Statistical deconvolution of the DSC curves yields a two-state transition of K88ab, NFA-1, and the monomeric CFA-1 and the existence of intermediate states for AFA-1 and polymeric CFA-1 during the denaturation process. The irreversible denaturation of K88ab, AFA-1, and CFA-1 is explained by aggregation of the polypeptide chains forming a three-dimensional network of intermolecular beta-sheet-type structures. In contrast, denaturation of NFA-1 is completely reversible. At a physiologically relevant temperature of approximately 40 degrees C, we observe predenaturational events in the DSC curves of polymeric K88ab and NFA-1 with no concomittant changes in the secondary structure of these proteins.