STRUCTURAL FEATURES AND REVERSIBLE ASSOCIATION OF DIFFERENT QUATERNARY STRUCTURES OF BETA-LACTOGLOBULIN

Structural and functional features were studied on the native dimeric form of beta-lactoglobulin at neutral pH and on the monomeric forms ob tained either by raising the pH to 9.0 or by blocking the thiol group of Cys121 with iodoacetamide under bland dissociating conditions. The thiol blocked monomer did not reassociate to native-like dimers, it re tained retinol-binding ability, and it was found to display many of th e structural features of the monomer obtained at pH 9, but differed in several structural features from the native dimer. The supramolecular associative properties of the proteins were studied by measuring conc entration dependence of the accessibility of the backbone exchangeable amide protons in H-1 NMR H/D exchange experiments, of their ligand-bi nding properties, and of their intrinsic fluorescence features. Eviden ce of reversible association was found for all the proteins with a ver y similar concentration dependence, indicating that the weak forces in volved in this association were different from those stabilizing the n ative dimer.