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β-Lactoglobulin

Introduction

β-Lactoglobulin (β-LG)is the major whey protein of ruminant 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 ^[1].

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. ^{[2] [3] [4]}

Bovine b-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. 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 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. ^[5]

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 bovine proteins but, while still present in porcine β-LG, is not involved in the formation of the dimer that forms at low pH.

Function:Primary component of whey, it binds retinol and is probably involved in the transport of that molecule.^[6].

Relevant background

class of protein :Belongs to the calycin superfamily. Lipocalin family. overall function of Lipocalin family: The lipocalins are a family of proteins which transport small hydrophobic molecules such as steroids, bilins, retinoids, and lipids. Lipocalins have been associated with many biological processes, among them immune response, pheromone transport, biological prostaglandin synthesis, retinoid binding, and cancer cell interactions.

short description of protein fold: They share limited regions of sequence homology and a common tertiary structure architecture.[2][3][4][5][6] This is an eight stranded antiparallel beta-barrel with a repeated + 1 topology enclosing an internal ligand binding site.[5][4] Therefore To know more abouts and the related deseases you can follow the link that leads you to the Portal to Swiss-Prot diseases and variants organisms:These proteins are found in gram negative bacteria, vertebrate cells, and invertebrate cells, and in plants.

Lipocalin Proteins

Lipocalins are typically small (160-180 residues in length), extracellular proteins sharing several common molecular recognition properties: the binding of small, principally hydrophobic molecules (such as retinol); binding to speci¢c cell-surface receptors; and the formation of covalent and non-covalent complexes with other soluble macromolecules. Although they have been classified mainly as transport proteins ^[7]

Structure of β-LG

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 monomers which preserve their native conformation. 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. However, the structural characteristics of the A and B variants of bovine b-lactoglobulin are virtually indistinguishable. In its native state, b-lactoglobulin is a predominantly b-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).^[8]

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 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 (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.^[9]

overall description of the structure of the protein: a. oligomeric state b. description of secondary structure c. description of active residues of the protein and where they are on the protein d. description of any ligands in the structure 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

Under physiological conditions beta-lactoglobulin exists as an equilibrium mixture of monomeric and dimeric forms. Subcellular location: Secreted. Tissue specificity: Synthesized in mammary gland and secreted in milk. Post-translational modification : Alternate disulfide bonds occur in equal amounts in all variants examined. Allergenic properties:Causes an allergic reaction in human. Is one of the causes of cow's milk allergy. Miscellaneous The B variant sequence is shown.

secondary structure elements

protein fold and how thats important for the function

ligands if theres ligands

the active site if relevant

features of protein that are important for function

zoom in on the active site, label the important active site residues, and hughlight those residues in a different color (make it look pretty)

Mechanism of action

how the protein function

include chemical structure of any relevant ligands, inhibitors, or important states in the reaction pathway.

Implications or possible application

describe any uses or application that have been made of the protein

External Resources

References

1. ↑ Kontopidis G, Holt C, Sawyer L. Invited review: beta-lactoglobulin: binding properties, structure, and function. J Dairy Sci. 2004 Apr;87(4):785-96. PMID:15259212 doi:http://dx.doi.org/10.3168/jds.S0022-0302(04)73222-1
2. ↑ Vijayalakshmi L, Krishna R, Sankaranarayanan R, Vijayan M. An asymmetric dimer of beta-lactoglobulin in a low humidity crystal form-Structural changes that accompany partial dehydration and protein action. Proteins. 2007 Oct 11;71(1):241-249. PMID:17932936 doi:10.1002/prot.21695
3. ↑ Sakurai K, Goto Y. Dynamics and mechanism of the Tanford transition of bovine beta-lactoglobulin studied using heteronuclear NMR spectroscopy. J Mol Biol. 2006 Feb 17;356(2):483-96. Epub 2005 Dec 1. PMID:16368109 doi:http://dx.doi.org/10.1016/j.jmb.2005.11.038
4. ↑ Qin BY, Bewley MC, Creamer LK, Baker HM, Baker EN, Jameson GB. Structural basis of the Tanford transition of bovine beta-lactoglobulin. Biochemistry. 1998 Oct 6;37(40):14014-23. PMID:9760236 doi:10.1021/bi981016t
5. ↑ http://www.sciencedirect.com/science/article/pii/S0958694698000211
6. ↑ Kontopidis G, Holt C, Sawyer L. Invited review: beta-lactoglobulin: binding properties, structure, and function. J Dairy Sci. 2004 Apr;87(4):785-96. PMID:15259212 doi:http://dx.doi.org/10.3168/jds.S0022-0302(04)73222-1
7. ↑ Flower DR, North AC, Sansom CE. The lipocalin protein family: structural and sequence overview. Biochim Biophys Acta. 2000 Oct 18;1482(1-2):9-24. PMID:11058743
8. ↑ Taulier N, Chalikian TV. Characterization of pH-induced transitions of beta-lactoglobulin: ultrasonic, densimetric, and spectroscopic studies. J Mol Biol. 2001 Dec 7;314(4):873-89. PMID:11734004 doi:http://dx.doi.org/10.1006/jmbi.2001.5188
9. ↑ Sakurai K, Konuma T, Yagi M, Goto Y. Structural dynamics and folding of beta-lactoglobulin probed by heteronuclear NMR. Biochim Biophys Acta. 2009 Jun;1790(6):527-37. doi: 10.1016/j.bbagen.2009.04.003., Epub 2009 Apr 10. PMID:19362581 doi:http://dx.doi.org/10.1016/j.bbagen.2009.04.003

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