Bovine beta-lactoglobulin: Interaction studies with palmitic acid

Bovine beta-lactoglobulin (BLG) in vivo has been found complexed with fatty acids, especially palmitic and oleic acid. To elucidate the still unknown structure-function relationship in this protein, the interactions between C -13 enriched palmitic acid (PA) and BLG were investigated by means of one-, two-, and three-dimensional NMR spectroscopy in the pH range 8.4-2.1. The NMR spectra revealed that at neutral pH the ligand is bound within the cent ral cavity of BLG, with the methyl end deeply buried within the protein. Th e analysis of C-13 spectra of the hole protein revealed the presence of con formational variability of bound PA carboxyl end in the pH range 8.4-5.9, r elated to the Tanford transition. The release of PA starts at pH lower than 6.0, and it is nearly complete at acidic pH. This finding is relevant in r elation to the widely reported hypothesis that this protein can act as a tr ansporter through the acidic gastric tract. Ligand binding and release is s hown to be completely reversible over the entire pH range examined, differe ntly from other fatty acid binding proteins whose behavior is analyzed thro ughout the paper. The mode of interaction of BLG is compatible with the pro posed function of facilitating the digestion of milk fat during the neonata l period of calves.