β-lactoglobulin:: Structural studies, biological clues

Bovine beta-lactoglobulin (beta-Lg) is a much studied and commercially important whey protein with an as yet undetermined function, although it is of obvious nutritional value. beta-Lg binds a variety of ligands and by comparison of the general structures of these molecules together with several competition studies, it appears that there are at least 3 independent binding sites. In the absence of direct crystallographic evidence, a preliminary modelling study reveals that there is an internal cavity which can readily accommodate retinol in a manner similar to the related lipocalin, retinol-binding protein. On the outer surface, a solvent-accessible hydrophobic cleft runs between the 3-turn alpha-helix that is packed against the outer surface of the beta-barrel. This cleft can accommodate fatty acids like palmitate and stearate. There is difference electron density observed in a soaking experiment with p-nitrophenol at a third site, on the outer surface close to the conserved Trp19/Arg124. This site is large enough to accommodate larger aromatic ligands such as ellipticine, although there is yet no independent evidence for this. (C) 1998 Elsevier Science Ltd. All rights reserved.