Monomeric bovine beta-lactoglobulin adopts a beta-barrel fold at pH 2

We have determined a crude structure of the apo form of bovine beta-lactoglobulin, a protein of 162 amino acid residues with a molecular mass of 18 kDa, at a Low pH on the basis of data collected using only homonuclear H-1 NMR spectroscopy, An ensemble of protein conformations was calculated with the distance-geometry algorithm for NMR applications (DYANA). The monomeric protein at low pH adopts a beta-barrel fold, well-superimposable on the structure determined by X-ray crystallography for the dimer at physiological pH, NMR evidence suggests the presence of disordered loop regions and terminal segments. Structural differences between the monomer at pH 2 and the dimer at pH 7, obtained by X-ray crystallography, are discussed, paying particular attention to surface electrostatic properties, in view of the high charge state of the protein at low pH, (C) 1998 Federation of European Biochemical Societies.