Structural mechanism of complex assemblies: characterisation of beta-lactoglobulin and pectin interactions

Knowledge of how proteins and polysaccharides interact is the key to understanding encapsulation and emulsification in these composite systems and ultimately to understanding the structures of many biological network systems. As a model system we have studied beta-lactoglobulin A (beta LgA) interacting with pectins of various amounts and distribution patterns of charge. The studies were conducted at pH 4 at minimal ionic strength, where the beta LgA and the pectins are oppositely charged, resulting in an electrostatic attraction to each other. Isothermal titration calorimetry (ITC) experiments were performed to determine the thermodynamics associated with beta LgA-pectin titration. It was found that beta LgA only interacted with pectins with an adequate amount of charge, and that the complexation between beta LgA and pectin was a two-step process initially involving binding of the protein to available sites on the pectin, and subsequently binding of the protein onto the bound protein that has previously adsorbed. Circular dichroism (CD) and intrinsic tryptophan fluorescence were also measured of beta LgA during its interaction with the pectin samples, and show that the binding leads to significant conformational changes in beta LgA. An increase in the turbidity of the solution of the resultant complexes indicates the formation of large-scale interpolymer associations of the primary complexes mediated by protein-rich domains.