Sandbox Reserved 779: Difference between revisions
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<Structure load='1beb' size='400' frame='true' align='right' caption='3D model_The dimer interface of β-lactoglobulin lattice X_1BEB' scene='Insert optional scene name here' /> | <Structure load='1beb' size='400' frame='true' align='right' caption='3D model_The dimer interface of β-lactoglobulin lattice X_1BEB' scene='Insert optional scene name here' /> | ||
== β-Lactoglobulin == | == β-Lactoglobulin == | ||
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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)for each monomer 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)for each monomer 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. | ||
===Lipocalin Proteins=== | ===Lipocalin Proteins=== | ||
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The lipocalin family is a large and diverse family of proteins with functions varying from insect camouflage to small hydrophobic molecule transport typified by the serum retinol-binding protein <ref>http://www.biochemj.org/bj/318/bj3180001.htm</ref> The crystal structures so far determined reveal the typical lipocalin to be an eight-stranded antiparallel β-barrel arranged to form a conical central calyx or cavity in which the hydrophobic ligand is located.<ref>PMID:6540172</ref> | The lipocalin family is a large and diverse family of proteins with functions varying from insect camouflage to small hydrophobic molecule transport typified by the serum retinol-binding protein <ref>http://www.biochemj.org/bj/318/bj3180001.htm</ref> The crystal structures so far determined reveal the typical lipocalin to be an eight-stranded antiparallel β-barrel arranged to form a conical central calyx or cavity in which the hydrophobic ligand is located.<ref>PMID:6540172</ref> | ||
===Biological Function=== | ===Biological Function=== | ||
Binding of variety of small hydrophobic molecules | Binding of variety of small hydrophobic molecules | ||
<Structure load='1b0o' size='300' frame='true' align='right' caption='3D model_β-lactoglobulin complexed with Palmitate, lattice Z_1B0O' scene='Insert optional scene name here' /> | <Structure load='1b0o' size='300' frame='true' align='right' caption='3D model_β-lactoglobulin complexed with Palmitate, lattice Z_1B0O' scene='Insert optional scene name here' /> | ||
Retinol and Palmitate Binding | ====Retinol and Palmitate Binding==== | ||
β-lactoglobulin (β-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> | β-lactoglobulin (β-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 (Fig. 2), 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 (Fig. 3)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>. | A cocrystallized β-Lg with palmitic acid (Fig. 2), 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 (Fig. 3)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>. | ||
====Transport Protein==== | |||
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== Structure of β-Lactoglobulin == | == Structure of β-Lactoglobulin == | ||
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.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. | ||
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. | |||
===Active sites=== | |||
βLG contains two tryptophan residues, Trp 19 on the A strand and Trp 61 on the C strand. The former is buried in the hydrophobic core whereas the latter is exposed to the solvent in the native structure, making them useful probes for monitoring site-specific conformational changes.<ref>PMID:19362581</ref> | |||
In addition, studies on the monomer–dimer equilibrium [30,32,42,43] and the reactivity of the thiol group of Cys121 which deeply buried between the α-helix and H strand [44–48] revealed other important properties of β-LG.<ref>PMID:19362581</ref> the stability of the structure also depend so heavily upon the external loop around residue 64 or the β 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 which deeply buried between the α-helix and H strand [44–48] revealed other important properties of β-LG.<ref>PMID:19362581</ref> the stability of the structure also depend so heavily upon the external loop around residue 64 or the β strand with the free thiol.<ref>PMID:9115437</ref> | ||
===Dimer/Monomer=== | |||
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 monomers (Fig. 1) 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 monomers (Fig. 1) which preserve their native conformation.<ref>PMID:11734004</ref> | ||
[[Image:Beta_lac_2Q2M.png|thumb|right|320px|Fig. 1 Bovine β-Lactoglobulin Native_Monomer]] | [[Image:Beta_lac_2Q2M.png|thumb|right|320px|Fig. 1 Bovine β-Lactoglobulin Native_Monomer]] | ||
===Variants=== | |||
Genetically, β-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, β-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> | ||
However, the structural characteristics of the A and B variants of bovine b-lactoglobulin are virtually indistinguishable. In its native state, β-lactoglobulin is a predominantly β-sheet protein containing nine b-strands and three a-helices. The core of the protein is formed by a flattened b-barrel (a calyx) composed of eight antiparallel b-strands (A to H).<ref>PMID:11734004</ref> | However, the structural characteristics of the A and B variants of bovine b-lactoglobulin are virtually indistinguishable. In its native state, β-lactoglobulin is a predominantly β-sheet protein containing nine b-strands and three a-helices. The core of the protein is formed by a flattened b-barrel (a calyx) composed of eight antiparallel b-strands (A to H).<ref>PMID:11734004</ref> | ||
==Ligands== | ===Ligands=== | ||
Most lipocalins bind small hydrophobic molecules within the central cup or calyx. The true function of β-Lg is unknown, but it has been suggested that it is involved in the transport of retinol and/or fatty acids [8,50]. It binds retinol with a higher affinity than does RBP [51] and, as with RBP, specific binding of retinol to β-Lg has been observed in the small intestine of the neonatal calf [3]. The structure of RBP with retinol bound within the hydrophobic calyx has been solved [2] and retinol was successfully modelled into our previous β-Lg structure [3]. β-Lg contains two tryptophans, Trp19 and Trp61, and their fluorescence is altered when retinol is bound [51].<ref>PMID:9115437</ref> | Most lipocalins bind small hydrophobic molecules within the central cup or calyx. The true function of β-Lg is unknown, but it has been suggested that it is involved in the transport of retinol and/or fatty acids [8,50]. It binds retinol with a higher affinity than does RBP [51] and, as with RBP, specific binding of retinol to β-Lg has been observed in the small intestine of the neonatal calf [3]. The structure of RBP with retinol bound within the hydrophobic calyx has been solved [2] and retinol was successfully modelled into our previous β-Lg structure [3]. β-Lg contains two tryptophans, Trp19 and Trp61, and their fluorescence is altered when retinol is bound [51].<ref>PMID:9115437</ref> | ||
