Effects of pH and protein conformation on in-solution complexation between bovine α-lactalbumin and oleic acid: Binding trend analysis by using SPR and ITC

To analyze the trends in molecular interaction between bovine α-LA (BLA) and oleic acid, we investigated the effects of pH and protein conformation on oleate binding to BLA and compared the order-of-magnitude differences in binding behavior. Both isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR) were used. For the ITC experiments, a solution of holo- or apo-BLA was titrated with sodium oleate at pH 4.0 and 10.0 to look for protein surface charge effects. In the SPR experiments, BLA was immobilized on the chip surface and sodium oleate solutions at various pHs (4.0, 5.5, 7.0, 8.5 and 10.0) were injected. Binding stoichiometry of ca. 5.2 molecules of oleate per unit molecule of BLA was observed. apo-BLA at a lower pH (lower than the pI of α-LA) yielded stronger binding avidity and affinity, which indicated that electrostatic interactions between the basic residues and the negatively charged carboxyl groups may play a major role in the complexation, in addition to hydrophobic interactions. The complexes formed at pH 4.0 and 5.5 were relatively unstable and rapidly dissociated when exposed to pH 7.0. In summary, we propose that the complexation can be regarded as a very weak (or, transient) and multivalent binding, requiring both electrostatic and hydrophobic interactions, probably in series and also in a dynamic equilibrium.