Beta-lactoglobulin

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The structure of β-lactoglobulin from PDB entry 3BLG
The ribbons denote the secondary structure. Rendered with Kinemage

β-lactoglobulin is the major whey protein of cow's milk (~3 g/l), and is also present in many other mammalian species; a notable exception being humans. Its structure, properties and biological role have been reviewed many times ^[1][2][3].

[edit] Structure and role

Unlike the other main whey protein, α-lactalbumin, no clear function has been identified for β-lactoglobulin, although it binds to several hydrophobic molecules, suggesting its role in their transport. The strong suggestion is that the molecule exists primarily as a food source. Several genetic variants have been identified, the main ones in the cow being labelled A and B. Because of its abundance and ease of purification, it has been subjected to a wide range of biophysical studies. Its structure has been determined several times by X-ray crystallography and NMR. One such structure is shown on the right (from http://www.pdb.org entry 3BLG). β-lactoglobulin is of direct interest to the food industry since its properties can variously be advantageous or disadvantageous in dairy products and processing ^[4].

Bovine β-lactoglobulin is a relatively small protein of 162 residues, with an 18.4 kDa molecular weight (1 Dalton being defined as 1 molecular weight unit). In physiological conditions it is predominantly dimeric, but dissociates to a monomer below about pH 3. Nevertheless, its native state remains fairly intact at lower pH values, as determined using NMR ^[5].

β-lactoglobulin solutions form gels in various conditions, when the native structure is sufficiently destabilised to allow aggregation ^[6]. Under prolonged heating at low pH and low ionic strength, a transparent `fine-stranded' gel is formed, in which the protein molecules assemble into long stiff fibres.

Folding intermediates for this protein can be studied using light spectroscopy and denaturant. Such experiments interestingly show an unusual but important intermediate composed purely of alpha helices, despite the fact that the native structure is beta sheet. Evolution has probably selected for the helical intermediate to avoid aggregation during the folding process. ^[7]

As milk is a known allergen (as listed in Annex IIIa of Directive 2000/13/EC), manufacturers need to prove the presence or absence of β-lactoglobulin to ensure their labelling satisfies the requirements of the aforementioned directive. Food testing laboratories such as Genon Laboratories Ltd ([8]) can use ELISA (enzyme linked immunoassay) methods to identify and quantify β-lactoglobulin in food products.

[edit] Footnotes

1. ^  Hambling, S. G., A. S. McAlpine, and L. Sawyer. 1992. Advanced Dairy Chemistry: 1. Proteins, chapter: Beta-lactoglobulin. Elsevier Applied Science, 141–190.
2. ^  Sawyer, L., and G. Kontopidis. 2000. The core lipocalin, bovine beta-lactoglobulin. Biochim Biophys Acta 1482:136–48.
3. ^  Kontopidis, G., C. Holt, and L. Sawyer. 2004. Invited review: beta-lactoglobulin: binding properties, structure, and function. J Dairy Sci 87:785–96.
4. ^  Jost, R. 1993. Functional characteristics of dairy proteins. Trends in Food Science & Technology 4:283–288.
5. ^  Uhrinova, S., M. H. Smith, G. B. Jameson, D. Uhrin, L. Sawyer, and P. N. Barlow. 2000. Structural changes accompanying ph-induced dissociation of the beta-lactoglobulin dimer. Biochemistry 39:3565–74.
6. ^  Bromley, E. H. C., M. R. H. Krebs, and A. M. Donald. 2005. Aggregation across the length scales in beta-lactoglobulin. Faraday Discussions. 128:13–27.
7. ^  Kuwajima K., Yamaya H. & Sugai S. 1996. The Burst-phase Intermediate in the Refolding of beta-Lactoglobulin Studied by Stopped-flow Circular Dichroism and Absorption Spectroscopy. Journal of Molecular Biology, 264:806-822.