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== Beta Lactoglobulin ==
== Beta Lactoglobulin ==
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[[Image:1b8e_bio_r_500.jpg]]
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[[Image:1b8e_bio_r_500.jpg|thumb|left|260px|]]
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β-Lactoglobulin (β-LG) is the major whey protein of ruminant species and is also present in the milks of many, but not all, other species. 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. Bovine β-LG binds a wide range of ligands, but this may not be the reason for its presence in milk. In reviewing the structure and physicochemical properties of the protein, we present the structures of the ligands cholesterol (at a resolution of 2.0Å, R = 0.221; Rfree = 0.295) and vitamin D2 (at a resolution of 2.4Å, R = 0.212; Rfree = 0.297) each bound to the central binding cavity of bovine β-LG at pH 7.3. Neither ligand is fully visible in the electron density maps, and the less well-ordered regions are the polar end groups at the mouth of the binding site. In a separate experiment, a mercury ion was bound to the free Cys121 (at a resolution of 2.2Å, R = 0.218; Rfree = 0.288) in a way that transmitted a small structural change through Asp137 via Arg148 to the dimer interface. It is not clear if the known dissociation that arises from the reaction of β-LG with HgCl2 results from this perturbation.
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β-Lactoglobulin(β-LG) is the bulk constituent of whey protein in most ruminant species and is also present in the milks of most mammalian species, although not in rodent, human, and lagomorph milks. It has been studied extensively for some time ( Tilley, 1960; Lyster, 1972; Kinsella and Whitehead, 1987; Hambling et al., 1992; Sawyer, 2003). β-LG is a lipocalin as is seen through its amino-acid sequence and structure (Flower et al., 2000). Lipocalins are a varied family which bind small hydrophobic ligands, and often act as transporters in vivo. The purpose of β-LG, however, is not clear. Genetic variants of β-LG expressing genes have been reported, especially in cows and several nonruminant species (Kontopidis, 2004).
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In reviewing the structural studies that reveal the ligand binding sites for long-chain fatty acids, retinoids, and steroids, only the central location, common to all lipocalins so far examined, is occupied under the conditions used. We find that there is no crystallographic evidence of another ligand binding site in our crystals grown in approximately 1.3 M citrate, although low ionic strength studies in solution indicate the possible presence of at least one other low affinity site. The apparent ability of the binding site to accommodate a wide range of ligands may point to a possible physiological function. However, by considering the lipocalin family in general, and the species distribution of β-LG in particular, some speculation as to the physiological function can be made. β-Lactoglobulin has been reported as being implicated, inter alia, in hydrophobic ligand transport and uptake, enzyme regulation, and the neonatal acquisition of passive immunity. However, these functions do not appear to be consistent between species. Sequence comparisons among members of the lipocalin family reveal that glycodelin, found in the human endometrium during early pregnancy, is the most closely related to β-LG. Although the function of glycodelin is also unknown, it appears to have effects on the immune system and/or to be involved in differentiation. It is proposed that β-LG, over-expressed in the lactating mammary gland of many, but not all, species, is primarily an important source of amino acids for the offspring of those animals that produce it, but that this function arose by gene duplication from the physiologically essential glycodelin. The other functions that have been associated with β-LG in the neonate are, therefore, fortuitous.
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Whey is an often used food ingredient in the food industry and has many useful functional characteristics including foaming, gelling, thermal stability, emulsification, and protein fortification. It has also been suggested that it has many useful health benefits including exercise recovery, weight management, cardiovascular health, anti-cancer effects, anti-infection activity, wound repair, and infant nutrition.
 +
Human consumption of whey ingredients has steadily increased and has become more and more important to the dairy industry.
 +
An issue with whey ingredients is the subtle off flavor often associated with whey proteins. During the
 +
Cheddar cheese making process, norbixin leaches into the whey during the draining of the curd. Norbixin, a carotenoid extracted from the bixa orellana seed and added to cheese milk to give Cheddar its expected yellow flavor, would also contribute yellow color to the final product into which the whey ingredient is added. To prevent discoloration of the final product, whey is bleached by either hydrogen peroxide or benzoyl peroxide. In addition to a white color, this also causes lipid oxidation and associated off flavors.
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[[Image:Picture2.jpg|thumb|right|260px|frame|Norbixin]]
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[[Image:Picture6.jpg|thumb|right|260px|frame|Whey production process]]
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[[Image:Picture4.png|thumb|left|260px|frame|Whey protein powder]]
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[[Image:Picture3.png|thumb|center|610px|]]
<StructureSection load='1B8E' size='350' side='right' caption='Structure of Beta Lactoglobulin (PDB entry [[1B8E]])' scene=''>
<StructureSection load='1B8E' size='350' side='right' caption='Structure of Beta Lactoglobulin (PDB entry [[1B8E]])' scene=''>
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Anything in this section will appear adjacent to the 3D structure and will be scrollable.
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β-Lactoglobulin is a small protein with 162 amino acid residues (Mr ∼18,400). β-LG folds 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. The ninth β-strand forms the dimer interface in cow and sheep proteins. The calyx, or β-barrel, is conical and serves as the ligand binding sight for small hydrophobic ligands. It is made of β-strands A-D forming one sheet, and strands E-H forming a second. Strand A turns at a right angle and the C-terminal end forms an antiparallel strand with H. A 3-turn helix between strands G and H is located on the outer surface of the β-barrel between strands G and H. 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 are significantly longer and more flexible (Kontopidis, 2004). This becomes especially important when considering conformational changes at different pH values. Specifically, the EF loops can act as a “gate”, opening or closing access to and from the internal binding site. At acidic pH the “gate” closes while at neutral and basic pH values the “gate” is open, allowing for ligand binding at the hydrophobic binding site. The gate “hinge” is Glu89 (Kontopidis, 2004).
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The free Cys121 has a reactive thiol group, whose activity parallels the pH dependency of the Tanford transition and is involved with the denaturation and aggregation of the system (Havea et al., 2001).. It is situated on the outer surfaced of the β-barrel and underneath the α-helix, which offers it some protection from the solvent and pH effects.
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</StructureSection>
</StructureSection>
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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 (see Figure 1). It is this strand that forms a significant part of the dimer interface in the bovine and ovine proteins but, while still present in porcine β-LG, is not involved in the formation of the dimer that forms at low pH. The genetic variants referred to above in the ruminant species result in relatively minor amino acid differences, but the processing properties appear to be significantly affected even by these small changes, such that consideration has been given to removing the less favorable variants by selective breeding (Hill et al., 1997; Harris, 1997). The so-called calyx, or β-barrel, is conical and is made of β-strands A-D forming one sheet, and strands E-H forming a second. Strand A bends through a right angle such that the C-terminal end forms an antiparallel strand with H; strands D and E also form a less significant interaction completely closing the calyx. It is this central cavity, the calyx, that provides the ligand-binding site. On the outer surface of the β-barrel, between strands G and H, 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 are significantly longer and more flexible.
 
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In particular, the EF loop acts as a gate over the binding site. At low pH, it is in the “closed” position, and binding is inhibited or impossible, whereas at high pH it is open, allowing ligands to penetrate into the hydrophobic binding site. The “latch” for this gate is Glu89, the residue implicated in the Tanford transition observed, although not identified, as having an abnormally high pKa (Tanford et al., 1959; Brownlow et al., 1997; Qin et al., 1998a).
 
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The free Cys121, with its reactive thiol group, has a pH-dependent activity that parallels that of the Tanford transition, and its involvement in the denaturation and aggregation behavior is now firmly established (Havea et al., 2001). The residue is situated on the outer surface of the β-barrel on strand H, under the α-helix, and quite some distance from the EF loop. The effect of this is to mask its accessibility to solvent, particularly at low pH. Reaction with heavy metals like Hg2+ or Cd2+ does occur at most pH values and leads to a dissociation of the dimer. Indeed, reaction of the Cys with almost any thiol reagent appears to enhance dissociation into monomers (McKenzie and Sawyer, 1967; Hambling et al., 1992).
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===Physiological Function of β-LG===
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<StructureSection load='2gj5' size='280' side='right' caption='Structure of Beta Lactoglobulin with Vitamin D3 (PDB entry [[2gj5]])' scene=''>
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<StructureSection load='1bso' size='280' side='right' caption='Structure of Beta Lactoglobulin with 12-BROMODODECANOIC ACID (PDB entry [[1bso]])' scene=''>
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While no definite physiological function has been attributed to β-LG, several possibilities do exist. As a lipocalin it only stands to reason that most suggestions involve a role in molecular transport of small hydrophobic ligand transport (Kontopidis et al., 2002). Most research concentrates on the lactating cell or the neonate. β-Lactoglobulin does bind to fatty acids of intermediate to long length and retinol. It is very similar in structure to plasma retinol binding protein. Genetic variation does throw some doubt on these suggestions however, as β-Lactoglobulin in some species do not bind with fatty acids (Perez et al., 1989, 1993). This would suggest that its main function is in fact, not as a fatty acid transporter. It also seems unnecessary as a retinol binding transporting seeing as retinol associates more strongly with the fat in milk and does not depend on β-LG as its primary transporter. Data on β-LG’s role as an activity-modulator tends to be circumstantial. It has been suggested that the function of β-LG is directly related to maternal physiology (Kontopidis et al., 2002).
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Nutritional value seems to be a valid purpose for β-LG. In the case of many species, the gene determining β-LG production has undergone a gene duplication and is involved more with nutrition than other functional purposes. Some species, like rodents, humans, and lagomorphs lost the ability to produce β-LG through formation of a pseudo-gene. While ligand binding and even the inhibition of harmful bacterial adhesion in the intestine (Ouwehand et al., 1997) are certainly useful properties, but neonatal nutrition continues to be the primary function.
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</StructureSection>
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===Tanford Transition of β-LG===
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<StructureSection load='3blg' size='280' side='right' caption='Structure of Beta Lactoglobulin at pH 6.2 (PDB entry [[3blg]])' scene=''>
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<StructureSection load='1bsy' size='280' side='right' caption='Structure of Beta Lactoglobulin at pH 7.1 (PDB entry [[1bsy]])' scene=''>
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<StructureSection load='2blg' size='280' side='right' caption='Structure of Beta Lactoglobulin at pH 8.2 (PDB entry [[2blg]])' scene=''>
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The structures of the trigonal crystal form of bovine beta-lactoglobulin variant A at pH 6.2, 7.1, and 8.2 have been determined by X-ray diffraction methods at a resolution of 2.56, 2. 24, and 2.49 A, respectively. The corresponding values for R (Rfree) are 0.192 (0.240), 0.234 (0.279), and 0.232 (0.277). The C and N termini as well as two disulfide bonds are clearly defined in these models. The glutamate side chain of residue 89 is buried at pH 6.2 and becomes exposed at pH 7.1 and 8.2. This conformational change, involving the loop 85-90, provides a structural basis for a variety of pH-dependent chemical, physical, and spectroscopic phenomena, known as the Tanford transition.
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</StructureSection>
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===Implications===
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For the dairy industry, functionality and nutritional value are the primary issues when dealing with dairy proteins such as beta-lactoglobulin. The genetic variants referred to above in the ruminant species result in relatively minor amino acid differences, but the processing properties appear to be significantly affected even by these small changes, such that consideration has been given to removing the less favorable variants by selective breeding (Hill et al., 1997; Harris, 1997, Kontopidis et al., 2004).
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===References===
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G. Kontopidis, C. Holt, L. Sawyer. The ligand binding site of bovine β-lactoglobulin—Evidence for a function? J. Mol. Biol., 318 (2002), pp. 1043–1055
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J.P. Hill, M. Boland, A.F. Smith. The effect of β-lactoglobulin variants on milk powder manufacture and properties
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J.P. Hill, M. Boland (Eds.), Milk Protein Polymorphism, International Dairy Federation, Brussels, Belgium (1997), pp. 372–394
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B.L. Harris. Selection to increase β-lactoglobulin B variant in New Zealand dairy cattle. J.P. Hill, M. Boland (Eds.), Milk Protein Polymorphism, International Dairy Federation, Brussels, Belgium (1997), pp. 467–473
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P. Havea, H. Singh, L.K. Creamer. Characterization of heat-induced aggregates of β-lactoglobulin, α-lactalbumin, and bovine serum albumin in a whey protein concentrate environment. J. Dairy Res., 68 (2001), pp. 483–497
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D.R. Flower, A.C.T. North, C.E. Sansom
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The lipocalin protein family—structural and sequence overview. Biochim. Biophys. Acta, 1482 (2000), pp. 9–24
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A.C. Ouwehand, S.J. Salminen, M. Skurnik, P.L. Conway
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Inhibition of pathogen adhesion by beta-lactoglobulin. Int. Dairy J., 7 (1997), pp. 685–692
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J.M.A. Tilley. The chemical and physical properties of bovine β-lactoglobulin
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Dairy Sci. Abstr., 22 (1960), pp. 111–125
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R.L.J. Lyster. Reviews of the progress of dairy science. Section C. Chemistry of milk proteins. J. Dairy Res., 39 (1972), pp. 279–318
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J.E. Kinsella, D.M. Whitehead. Proteins in whey: Chemical, physical and functional properties. Adv. Food Nutr. Res., 33 (1989), pp. 343–438
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The CD and GH loops in many of the crystallographic analyses are indistinct, and the similarly indistinct nature of the extended AB loop led to the original misthreading of the amino acid sequence into the first electron density map (Papiz et al., 1986; Brownlow et al., 1997). β-Strand I is on the opposite side of strand A from strand H and forms an antiparallel interaction with the same strand of the second monomer. The other dimer-stabilizing interaction comes from the AB loop, where there is an ion pair from Asp33 of one subunit to Arg40 of the other. It is interesting to note that although strand I also exists in the porcine and equine β-LG structures, albeit with a slightly modified sequence, as do the Asp and Arg residues, no similar dimerization occurs at physiological pH. Indeed, site-directed mutagenesis designed to disrupt one or the other of these interactions, or better, both, produces a monomeric bovine protein (Sakurai and Goto, 2002; Wilson and Holt, to be published). There is no direct interaction between the α-helixes, which are antiparallel on the outer surface of the molecule with the polypeptide chains, some 12 Å apart. The closest approach of any atoms of the helix in the A subunit from the triclinic lattice X structure to the helix in the B subunit is 5.8Å, and this involves the flexible side chains of Asp137A and Arg148B and vice versa.
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S.G. Hambling, A.S. McAlpine, L. Sawyer. β-Lactoglobulin. P.F. Fox (Ed.), Advanced Dairy Chemistry-I: Proteins, Elsevier, London, UK (1992), pp. 140–191
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The structure of β-LG has also been determined by NMR techniques by several groups (Belloque and Smith, 1998; Fogolari et al., 1998; Kuwata et al., 1998; Uhrinova et al., 2000) and includes the equine structure (Kobayashi et al., 2000). The bovine protein is monomeric at pH values below 3, and the structure has been determined at this pH. Although there is no matching crystal structure at this pH where flat hexagonal plates of poor diffraction quality can be grown, the structure of the core of the protein is well conserved. However, significant differences do exist, and these have been critically reviewed (Jameson et al., 2002); it is not surprising that it is the external loops that contain these major differences.
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L. Sawyer. β-Lactoglobulin. P.F. Fox, P. McSweeney (Eds.), Advanced Dairy Chemistry I (3rd ed.), Kluwer, Amsterdam, The Netherlands (2003), pp. 319–386

Current revision

Contents

Beta Lactoglobulin

β-Lactoglobulin(β-LG) is the bulk constituent of whey protein in most ruminant species and is also present in the milks of most mammalian species, although not in rodent, human, and lagomorph milks. It has been studied extensively for some time ( Tilley, 1960; Lyster, 1972; Kinsella and Whitehead, 1987; Hambling et al., 1992; Sawyer, 2003). β-LG is a lipocalin as is seen through its amino-acid sequence and structure (Flower et al., 2000). Lipocalins are a varied family which bind small hydrophobic ligands, and often act as transporters in vivo. The purpose of β-LG, however, is not clear. Genetic variants of β-LG expressing genes have been reported, especially in cows and several nonruminant species (Kontopidis, 2004).

Whey is an often used food ingredient in the food industry and has many useful functional characteristics including foaming, gelling, thermal stability, emulsification, and protein fortification. It has also been suggested that it has many useful health benefits including exercise recovery, weight management, cardiovascular health, anti-cancer effects, anti-infection activity, wound repair, and infant nutrition. Human consumption of whey ingredients has steadily increased and has become more and more important to the dairy industry. An issue with whey ingredients is the subtle off flavor often associated with whey proteins. During the Cheddar cheese making process, norbixin leaches into the whey during the draining of the curd. Norbixin, a carotenoid extracted from the bixa orellana seed and added to cheese milk to give Cheddar its expected yellow flavor, would also contribute yellow color to the final product into which the whey ingredient is added. To prevent discoloration of the final product, whey is bleached by either hydrogen peroxide or benzoyl peroxide. In addition to a white color, this also causes lipid oxidation and associated off flavors.

Norbixin
Norbixin
Whey production process
Whey production process
Whey protein powder
Whey protein powder

Structure of Beta Lactoglobulin (PDB entry 1B8E)

Drag the structure with the mouse to rotate


Physiological Function of β-LG

Structure of Beta Lactoglobulin with Vitamin D3 (PDB entry 2gj5)

Drag the structure with the mouse to rotate

Tanford Transition of β-LG

Structure of Beta Lactoglobulin at pH 6.2 (PDB entry 3blg)

Drag the structure with the mouse to rotate

Implications

For the dairy industry, functionality and nutritional value are the primary issues when dealing with dairy proteins such as beta-lactoglobulin. The genetic variants referred to above in the ruminant species result in relatively minor amino acid differences, but the processing properties appear to be significantly affected even by these small changes, such that consideration has been given to removing the less favorable variants by selective breeding (Hill et al., 1997; Harris, 1997, Kontopidis et al., 2004).


References

G. Kontopidis, C. Holt, L. Sawyer. The ligand binding site of bovine β-lactoglobulin—Evidence for a function? J. Mol. Biol., 318 (2002), pp. 1043–1055

J.P. Hill, M. Boland, A.F. Smith. The effect of β-lactoglobulin variants on milk powder manufacture and properties J.P. Hill, M. Boland (Eds.), Milk Protein Polymorphism, International Dairy Federation, Brussels, Belgium (1997), pp. 372–394

B.L. Harris. Selection to increase β-lactoglobulin B variant in New Zealand dairy cattle. J.P. Hill, M. Boland (Eds.), Milk Protein Polymorphism, International Dairy Federation, Brussels, Belgium (1997), pp. 467–473

P. Havea, H. Singh, L.K. Creamer. Characterization of heat-induced aggregates of β-lactoglobulin, α-lactalbumin, and bovine serum albumin in a whey protein concentrate environment. J. Dairy Res., 68 (2001), pp. 483–497

D.R. Flower, A.C.T. North, C.E. Sansom The lipocalin protein family—structural and sequence overview. Biochim. Biophys. Acta, 1482 (2000), pp. 9–24

A.C. Ouwehand, S.J. Salminen, M. Skurnik, P.L. Conway Inhibition of pathogen adhesion by beta-lactoglobulin. Int. Dairy J., 7 (1997), pp. 685–692

J.M.A. Tilley. The chemical and physical properties of bovine β-lactoglobulin Dairy Sci. Abstr., 22 (1960), pp. 111–125

R.L.J. Lyster. Reviews of the progress of dairy science. Section C. Chemistry of milk proteins. J. Dairy Res., 39 (1972), pp. 279–318

J.E. Kinsella, D.M. Whitehead. Proteins in whey: Chemical, physical and functional properties. Adv. Food Nutr. Res., 33 (1989), pp. 343–438

S.G. Hambling, A.S. McAlpine, L. Sawyer. β-Lactoglobulin. P.F. Fox (Ed.), Advanced Dairy Chemistry-I: Proteins, Elsevier, London, UK (1992), pp. 140–191

L. Sawyer. β-Lactoglobulin. P.F. Fox, P. McSweeney (Eds.), Advanced Dairy Chemistry I (3rd ed.), Kluwer, Amsterdam, The Netherlands (2003), pp. 319–386

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