β-lactoglobulin binds palmitate within its central cavity

Bovine beta-lactoglobulin (beta-Lg) has been studied extensively in both the isolated and the naturally occurring states. It is a commercially important whey protein of obvious nutritional value but, so far, one that has no clearly identified biological function. In common with many of the other members of the lipocalin family to which it belongs, beta-Lg binds hydrophobic ligands, and it appears possible that there are at least two distinct binding sites per monomer for a variety of ligands, By comparison with other members of the family, there is a probable binding site in the central cavity of the molecule that is formed by the eight antiparallel beta-strands that are typical of the lipocalins, We have now cocrystallized beta-Lg with palmitic acid, and the refined structure (R = 0.204, R-free = 0.240 for 6,888 reflections to 2.5-Angstrom resolution) reveals that the ligand binds in the central cavity in a manner similar to the binding of retinol to the related lipocalin, serum retinol-binding protein. The carboxyl group binds to both Lys-60 and Lys-69 at the entrance to the cavity. The hydrophobic tail stretches in an almost fully extended conformation into the center of the protein. This is the first direct observation of a ligand binding to beta-Lg.