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β-Lactoglobulin (β-LG)is the major whey protein of cow’s milk with a concentration of 0.3 g/100 mL <ref>PMID:14254409</ref> and was first isolated in 1934 <ref>http://www.jbc.org/content/104/2/359.citation</ref>. It is a globular protein consisting of 162 AA with a relative molecular mass of 18.4 kDa.

 

Its amino-acid sequence and 3-dimensional structure show that it is a lipocalin, a widely diverse family, most of which bind small hydrophobic ligands and thus may act as specific transporters, as does serum retinol binding protein <ref>PMID:15259212</ref>.

 

 

Bovine b-lactoglobulin (β-Lg) is a much studied and commercially important whey protein with an as yet undetermined function,although it is of obvious nutritional value. b-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 a-helix that is packed against the outer surface of the b-barrel. This cleft can accommodate fatty acids like palmitate and stearate.

β-Lactoglobulin is a small protein, soluble in dilute salt solution as befits a globulin, with 162 amino acid residues (Mr ∼18,400) that fold up into an 8-stranded, antiparallel β-barrel with a 3-turn α-helix on the outer surface and a ninth β-strand flanking the first strand.  

===Background ===

Lipocalins are typically small (160-180 residues in length), extracellular proteins sharing several common molecular recognition properties: the binding of small, principally hydrophobic molecules (such as retinol); binding to specific cell-surface receptors; and the formation of covalent and non-covalent complexes with other soluble macromolecules. Although they have been classified mainly as transport proteins  

βLG consists of 162 amino acid residues (18 kDa), containing two disulfide bonds (Cys 66–Cys 160 and Cys 106–Cys 119) and a free thiol (Cys 121). Structures of βLG have been reported by several groups with X-ray crystallography [19–21] and solution NMR [29,40,41] (Fig. 1A). It is a predominantly β-sheet protein. The β-barrel, or so called calyx, is conical and is made of two β-sheets: the B–D strands and N-terminal half of the A strand (denoted AN) form one sheet, and the E–H strands and C-terminal half of the A strand (denoted AC)form the other (Fig. 1B). On the outer surface of the β-barrel, between the G and H strands, is the 3-turn α-helix. The loops that connect the β-strands at the closed end of the calyx, BC, DE, and FG,are generally quite short, whereas those at the open end, AB, CD, EF,and GH, are significantly longer and more flexible [19]. In the calyx,there is a large central cavity which is surrounded by hydrophobic residues and is accessible to solvent. This cavity provides the principal ligand-binding site. βLG contains two tryptophan residues,Trp 19 on the A strand and Trp 61 on the C strand. The former is buried in the hydrophobic core whereas the latter is exposed to the

solvent in the native structure, making them useful probes for monitoring site-specific conformational changes. In addition, studies on the monomer–dimer equilibrium [30,32,42,43] and the reactivity of the thiol group of Cys121 deeply buried between the α-helix and H strand [44–48] revealed other important properties of βLG.<ref>PMID:19362581</ref>

Ever since the fortuitous observation that beta-lactoglobulin (beta-Lg), the major whey protein in the milk of ruminants, bound retinol, the details of the binding have been controversial. beta-Lg is a lipocalin, like plasma retinol-binding protein, so that ligand association was expected to make use of the central cavity in the protein.<ref>PMID:12054801</ref>