Sandbox Reserved 779: Difference between revisions
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β-Lactoglobulin (β-LG) was first isolated in 1934.<ref>http://www.jbc.org/content/104/2/359.citation</ref> It is a main globular protein which was isolated from whey, a by-product from cow milk-cheese manufacture. Cow milk itself contains 20% whey proteins and 80% casein protein. β-Lactoglobulin constitutes 50-65% dry solids whey protein or 12% of whole cow milk proteins. | β-Lactoglobulin (β-LG) was first isolated in 1934.<ref>http://www.jbc.org/content/104/2/359.citation</ref> It is a main globular protein which was isolated from whey, a by-product from cow milk-cheese manufacture. Cow milk itself contains 20% whey proteins and 80% casein protein. β-Lactoglobulin constitutes 50-65% dry solids whey protein or 12% of whole cow milk proteins. | ||
Due to its abundance, cheap and relatively easy to isolate nature, β-Lactoglobulin used widely in Industry to increase the protein contents of the food and beverage products. | Due to its abundance, cheap and relatively easy to isolate nature, β-Lactoglobulin used widely in Industry to increase the protein contents of the food and beverage products. | ||
Bovine β-lactoglobulin (β-Lg) is a much biochemically and structurally studied and commercially important whey protein with an as yet undetermined function, although it is of obvious nutritional value. β-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: | Bovine β-lactoglobulin (β-Lg) is a much biochemically and structurally studied and commercially important whey protein with an as yet undetermined function, although it is of obvious nutritional value. β-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: calyx, putative grove, and dimer interface (Fig. 2).<ref>http://www.sciencedirect.com/science/article/pii/S0958694698000211</ref><ref>PMID:9867826</ref> | ||
β-Lactoglobulin amino-acid sequence and 3-dimensional structure show that it belongs to [[Lipocalin]] family which capable of binding hydrophobic ligandsand thus may act as specific transporters, as does serum retinol binding protein. <ref>PMID:15259212</ref> Bovine β-Lactoglobulin is synthesized in cow mammary gland and secreted in the milk. It causes an allergic reaction in human and is one of the causes of cow's milk allergy. | β-Lactoglobulin amino-acid sequence and 3-dimensional structure show that it belongs to [[Lipocalin]] family which capable of binding hydrophobic ligandsand thus may act as specific transporters, as does serum retinol binding protein. <ref>PMID:15259212</ref> Bovine β-Lactoglobulin is synthesized in cow mammary gland and secreted in the milk. It causes an allergic reaction in human and is one of the causes of cow's milk allergy. | ||
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===Residues and secondary structures=== | ===Residues and secondary structures=== | ||
β-Lactoglobulin contains two disulfide bonds (Cys 66–Cys 160 and Cys 106–Cys 119) and a free thiol (Cys 121)(Fig. 1). Structures of βLG have been reported by several groups with X-ray crystallography and solution NMR 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. 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. 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. <ref>PMID:19362581</ref> | β-Lactoglobulin contains two disulfide bonds (Cys 66–Cys 160 and Cys 106–Cys 119) and a free thiol (Cys 121)(Fig. 1). Structures of βLG have been reported by several groups with X-ray crystallography and solution NMR 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. 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. 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 (Fig. 2). <ref>PMID:19362581</ref> | ||
===Dimer/Monomer=== | ===Dimer/Monomer=== | ||
At physiological conditions, majority of bovine b-lactoglobulin forms a dimer (Fig. | At physiological conditions, majority of bovine b-lactoglobulin forms a dimer (Fig. 3). Below pH 3, the dimer dissociates into monomers (Fig. 4) which maintain their native conformation. | ||
[[Image:BLG_Dimer_1BEB_Chain_A&B.png|thumb|right| | [[Image:BLG_Dimer_1BEB_Chain_A&B.png|thumb|right|240px|Figure 3. Bovine β-Lactoglobulin Dimer_[[1BEB]]]] | ||
Dimeric Lactoglobulin molecules exist in the open conformation at basic pH, whereas they exist in the closed conformation at acidic pH, after undergoing Tanford transition around neutral pH.<ref>PMID:17932936</ref> | Dimeric Lactoglobulin molecules exist in the open conformation at basic pH, whereas they exist in the closed conformation at acidic pH, after undergoing Tanford transition around neutral pH.<ref>PMID:17932936</ref> | ||
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=='''Molecular mechanism of the Tanford transition'''== | =='''Molecular mechanism of the Tanford transition'''== | ||
The Tanford transition is a conformational change of bovine β-lactoglobulin occurring at around pH 7, identified originally on the basis of optical rotatory dispersion and the accessibility of a thiol group. X-ray analysis has suggested that a conformational change to the EF-loop (residues I84 -N90) (Fig. | The Tanford transition is a conformational change of bovine β-lactoglobulin occurring at around pH 7, identified originally on the basis of optical rotatory dispersion and the accessibility of a thiol group. X-ray analysis has suggested that a conformational change to the EF-loop (residues I84 -N90) (Fig. 4) is responsible for the Tanford transition, with the loop closing the hydrophobic cavity of the β-barrel of the β-LG molecule below pH 7 and flipping to open the cavity above pH 7.<ref>PMID:16368109</ref>Tanford transition is triggered by protonation of Glu89 exhibiting an anomalously high pKa value. | ||
The Tanford transition may involve some other structural changes as well. For example, the transition is accompanied by a change in the microenvironment of Tyr42 and causes an alteration in the relative orientation of monomers in the dimer by as much as 5 degrees, which breaks a number of intersubunit hydrogen bonds. <ref>PMID:11734004</ref> | The Tanford transition may involve some other structural changes as well. For example, the transition is accompanied by a change in the microenvironment of Tyr42 and causes an alteration in the relative orientation of monomers in the dimer by as much as 5 degrees, which breaks a number of intersubunit hydrogen bonds. <ref>PMID:11734004</ref> | ||
[[Image:BLG_monomer_showing_EF_Loop.png|thumb|left| | [[Image:BLG_monomer_showing_EF_Loop.png|thumb|left|240px|Figure 4. Bovine β-Lactoglobulin Monomer showing EF loop (colored yellow)]] | ||
The structures of the trigonal crystal form of bovine β-lactoglobulin variant A at pH 6.2, 7.1, and 8.2 have been determined by X-ray diffraction methods. 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 84-90, provides a structural basis for a variety of pH-dependent chemical, physical, and spectroscopic phenomena, collectively known as the Tanford transition.<ref>PMID:9760236</ref>It was found that the stereochemical environment of Tyr42 changes significantly with pH variation between pH 7 and pH 8. This may provide a structural explanation for an as yet unexplained feature of the Tanford transition, namely the increase in exposure of a tyrosine residue.<ref>PMID:11168385</ref> | The structures of the trigonal crystal form of bovine β-lactoglobulin variant A at pH 6.2, 7.1, and 8.2 have been determined by X-ray diffraction methods. 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 84-90, provides a structural basis for a variety of pH-dependent chemical, physical, and spectroscopic phenomena, collectively known as the Tanford transition.<ref>PMID:9760236</ref>It was found that the stereochemical environment of Tyr42 changes significantly with pH variation between pH 7 and pH 8. This may provide a structural explanation for an as yet unexplained feature of the Tanford transition, namely the increase in exposure of a tyrosine residue.<ref>PMID:11168385</ref> | ||
