β-Lactoglobulin: Nomenclature, Structure, and Function

β-Lactoglobulin is a small globular protein found in ruminant milk and many, but not all, other mammals. The cow protein is the major whey protein synthesized as a 180-amino-acid polypeptide from which an 18-residue signal peptide is removed on secretion. Genetic variants involving a few amino acid positions have been reported for many species. The cow protein is stable between pH 3 and 7 but undergoes conformational change and unfolding as the pH is raised further. The structure is a conical barrel, open at one end, comprising eight antiparallel β-strands with a three-turn α-helix on the outer surface of the barrel. While the ruminant forms of the protein are dimeric, those from other species appear to be monomeric. The protein belongs to a widespread family of small, mostly secretory proteins called the lipocalins, since most members appear to carry small hydrophobic ligands internally. The function of the protein is unknown, although it is a good nutritional source of amino acids, and a transport role of some kind has been suggested. β-Lactoglobulin binds a wide variety of ligands and has been studied intensively not only as a model protein but also because it affects milk stability during commercial processing, its five cysteine residues allowing disulfide interchange to play a key role.