Sandbox Reserved 779: Difference between revisions

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Bovine b-lactoglobulin (b-Lg) is a much studied and commercially important whey protein with an as yet undetermined function,

although it is of obvious nutritional value. b-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

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

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==Structure==

==Structure of BLG==

βLG 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]

(Fig. 1A). It is a predominantly β-sheet protein. The β-barrel, or socalled

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 (Fig. 1B). 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 [19]. 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. 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. 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.

overall description of the structure of the protein:

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e. methods used to solve the structure : X-ray crystallography, NMR, EM

===Molecular mechanism of the Tanford transition===

Equilibrium transition

Although βLG exists in a native state over a wide range of pH

values, it shows slight conformational changes during a change of pH

[54]. Among the pH-dependent conformational changes of βLG, the

Tanford transition is the most important because it is thought to be

related to the function of βLG. Tanford et al. [55] observed a change in optical rotatory dispersion at pH 7.0 representing a certain conformational change. Subsequently, they found that this conformational change is accompanied by a deprotonation of a carboxyl group with an anomalous pKa of 7.5 [20,55].

==Subunit structure==

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@@ Line 18: / Line 18: @@
 Bovine b-lactoglobulin (b-Lg) is a much studied and commercially important whey protein with an as yet undetermined function,
 although it is of obvious nutritional value. b-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
+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
@@ Line 55: / Line 56: @@
-==Structure==
+==Structure of BLG==
+βLG 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]
+(Fig. 1A). It is a predominantly β-sheet protein. The β-barrel, or socalled
+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 (Fig. 1B). 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 [19]. 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. 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. 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.
 overall description of the structure of the protein:
@@ Line 64: / Line 91: @@
 e. methods used to solve the structure : X-ray crystallography, NMR, EM
+===Molecular mechanism of the Tanford transition===
+Equilibrium transition
+Although βLG exists in a native state over a wide range of pH
+values, it shows slight conformational changes during a change of pH
+[54]. Among the pH-dependent conformational changes of βLG, the
+Tanford transition is the most important because it is thought to be
+related to the function of βLG. Tanford et al. [55] observed a change in optical rotatory dispersion at pH 7.0 representing a certain conformational change. Subsequently, they found that this conformational change is accompanied by a deprotonation of a carboxyl group with an anomalous pKa of 7.5 [20,55].
 ==Subunit structure==