Beta-lactoglobulin

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File:3blg.png
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].

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. 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.

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 ([7]) can use ELISA (enzyme linked immunoassay) methods to identify and quantify β-lactoglobulin in food products.

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.



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