Molecular interaction between apo or holo α-lactalbumin and lysozyme: Formation of heterodimers as assessed by fluorescence measurements

In a previous work, we reported that contrary to native calcium-loaded α-lactalbumin (holo α-LA), calcium-depleted form (apo α-LA) has the ability to self-assemble with lysozyme (LYS) to form different supramolecular structures in temperature-dependent manner. In this study, we examine what happens at molecular scale using fluorescence techniques. Fluorescence anisotropy coupled with fluorescence lifetime measurements provides a means to measure intermolecular interactions. We showed that LYS interacts with both apo α-LA and holo α-LA to form oligomers, assumed to be heterodimers, at 10 °C and 45 °C. The dissociation constants for dimerization were found to be in the μM range and increased significantly with increasing ionic strength from 39 to 124 mM. Although the binding constants of holo α-LA–LYS and apo α-LA–LYS complexes were of the same order of magnitude, the shape or conformation of formed heterodimers differed as assessed by fluorescence parameters in particular correlation time calculations. Such conformation differences could explain why holo α-LA–LYS complexes are trapped as heterodimers while the apo α-LA–LYS complexes have the ability to further self-assemble into various supramolecular structures.