Sandbox Reserved 779: Difference between revisions
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<Structure load='1beb' size='400' frame='true' align='right' caption='The dimer interface of β-lactoglobulin lattice X' scene='Insert optional scene name here' /> | <Structure load='1beb' size='400' frame='true' align='right' caption='Fig. 1 The dimer interface of β-lactoglobulin lattice X' scene='Insert optional scene name here' /> | ||
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== Introduction == | == Introduction == | ||
β-Lactoglobulin (β-LG) is the | β-Lactoglobulin (β-LG) is the primary component of 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>. | ||
Under physiological conditions beta-lactoglobulin exists as an equilibrium mixture of monomeric and dimeric forms (Fig. 1) | |||
Its amino-acid sequence and 3-dimensional structure show that it is a member of 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> β-Lactoglobulin is synthesized in mammary gland and secreted in milk. It causes an allergic reaction in human and is one of the causes of cow's milk allergy. | |||
Bovine β-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. 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. 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. | |||
Bovine | |||
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. | |||
<ref>http://www.sciencedirect.com/science/article/pii/S0958694698000211</ref> | <ref>http://www.sciencedirect.com/science/article/pii/S0958694698000211</ref> | ||
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. | 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 === | ===Background === | ||
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== Structure of β-Lactoglobulin == | == Structure of β-Lactoglobulin == | ||
<Structure load='1beb' size='400' frame='true' align='right' caption='β- | <Structure load='1beb' size='400' frame='true' align='right' caption='Fig.1 The dimer interface of β-lactoglobulin lattice X' scene='Insert optional scene name here' /> | ||
β-Lactoglobulin 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].<ref>PMID:19362581</ref> | β-Lactoglobulin 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].<ref>PMID:19362581</ref> | ||
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.<ref>PMID:19362581</ref> | 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.<ref>PMID:19362581</ref> | ||
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. | 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. | ||
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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> the stability of the structure should depend so heavily upon the external loop around residue 64 or the beta strand with the free thiol<ref>PMID:9115437</ref> | 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> the stability of the structure should depend so heavily upon the external loop around residue 64 or the beta strand with the free thiol<ref>PMID:9115437</ref> | ||
At physiological conditions, bovine b-lactoglobulin forms a dimer, with each monomer consisting of 162 amino acid residues and characterized by a molecular mass of 18,350. Below pH 3, the dimer dissociates into <scene name='56/564055/Native_beta-lactoglobulin/1'>monomers</scene> which preserve their native conformation.<ref>PMID:11734004</ref> | At physiological conditions, bovine b-lactoglobulin forms a dimer (Fig. 1), with each monomer consisting of 162 amino acid residues and characterized by a molecular mass of 18,350. Below pH 3, the dimer dissociates into <scene name='56/564055/Native_beta-lactoglobulin/1'>monomers (click here)</scene> which preserve their native conformation.<ref>PMID:11734004</ref> | ||
Genetically, b-lactoglobulin may exist as one of several variants, among which the variants A and B are the most abundant. The A and B variants of the protein differ from each other by amino acid residues at positions Asp64 (Gly64 in variant B) and Val118 (Ala118 in variant B). These differences in primary structure render the two variants slightly different with respect to isoelectric point, solubility, self-association properties, as well as pressure and temperature stability.<ref>PMID:11734004</ref> | Genetically, b-lactoglobulin may exist as one of several variants, among which the variants A and B are the most abundant. The A and B variants of the protein differ from each other by amino acid residues at positions Asp64 (Gly64 in variant B) and Val118 (Ala118 in variant B). These differences in primary structure render the two variants slightly different with respect to isoelectric point, solubility, self-association properties, as well as pressure and temperature stability.<ref>PMID:11734004</ref> | ||
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==Molecular mechanism of the Tanford transition== | ==Molecular mechanism of the Tanford transition== | ||
Above pH 6.5, b-lactoglobulin undergoes the so-called Tanford transition which is triggered by protonation of Glu89 exhibiting an anomalously | Above pH 6.5, b-lactoglobulin undergoes the so-called Tanford transition which is triggered by protonation of Glu89 exhibiting an anomalously | ||
high pKa value. The Tanford transition involves displacement of the loop EF (residues 85 to 90) that acts as a lid which closes the protein interior/binding site below pH 7.3 and opens it at higher pH. The Tanford transition may involve some other structural changes as well. For example, the transition is accompanied by a change in the microenvironment of Tyr428 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. It should be noted that all transitions that take place between pH 2 and pH 9 do not cause any appreciable changes in the nativelike b-barrel conformation of b-lactoglobulin. | high pKa value. The Tanford transition involves displacement of the loop EF (residues 85 to 90) that acts as a lid which closes the protein interior/binding site below pH 7.3 and opens it at higher pH. The Tanford transition may involve some other structural changes as well. For example, the transition is accompanied by a change in the microenvironment of Tyr428 and causes an alteration in the relative orientation of <scene name='56/564055/Native_beta-lactoglobulin/1'>monomers (click here)</scene> in the dimer by as much as 5 degrees, which breaks a number of intersubunit hydrogen bonds. It should be noted that all transitions that take place between pH 2 and pH 9 do not cause any appreciable changes in the nativelike b-barrel conformation of b-lactoglobulin. | ||
<ref>PMID:11734004</ref> | <ref>PMID:11734004</ref> | ||
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==Biological Function== | ==Biological Function== | ||
Binding of variety of small hydrophobic molecules | Binding of variety of small hydrophobic molecules | ||
Retinol and Palmitate Binding | Retinol and Palmitate Binding | ||
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> | 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> | ||
A cocrystallized β-Lg with palmitic acid, and the refined structure (R = 0.204, R free = 0.240 for 6,888 reflections to 2.5-Å resolution) reveals that the ligand binds in the central cavity in a manner similar to the binding of retinol to the related lipocalin, serum retinol-binding protein.<ref>PMID:9867826</ref> | A cocrystallized β-Lg with palmitic acid, and the refined structure (R = 0.204, R free = 0.240 for 6,888 reflections to 2.5-Å resolution) reveals that the ligand binds in the central cavity in a manner similar to the binding of retinol to the related lipocalin, serum retinol-binding protein.<ref>PMID:9867826</ref> It is probably also involved in the transport of that molecule.<ref>PMID:15259212</ref>. | ||
==Implications or possible application== | ==Implications or possible application== | ||
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[[Image:Beta_lac_2Q2M.png|thumb|right|320px|Bovine Beta-Lactoglobulin Native]] | |||
==Other β-Lactoglobulin 3D Structures and complexes== | ==Other β-Lactoglobulin 3D Structures and complexes== | ||
