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'''β-Lactoglobulin'''

<Structure load='1beb' size='320' frame='true' align='left' caption='3D model 1._The dimer interface of β-lactoglobulin lattice X_1BEB' scene='Insert optional scene name here' />

=='''β-Lactoglobulin'''==

β-Lactoglobulin (β-LG) was first isolated in 1934.<ref>http://www.jbc.org/content/104/2/359.citation</ref> It is the main globular protein of 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, 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 commercially important whey protein with undetermined biological function, although it is of obvious nutritional value.  

β-Lg binds a variety of ligands, and 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 ligands and 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 belongs to the calycin superfamily and Lipocalin family. Lipocalins are typically small (160-180 residues in length), extracellular proteins and able to bind small hydrophobic molecules (such as retinol); bind to specific cell-surface receptors; and form of covalent and non-covalent complexes with other soluble macromolecules. Lipocalin proteins have also been classified mainly as transport proteins.

[[Image:Beta_lac_Residues.jpg|thumb|right|320px|Figure 1. Primary structure of β-LG. β-Lactoglobulin comprises 162 AA, including 5 Cys residues. Two disulfide linkages are located at residues Cys-106 to Cys-119 and Cys-66 to Cys-160. One free Cys is at position 121.(Taken from Liu, et al. 2007)<ref>PMID:17235131</ref>]]

[[Image:General_view_of_beta_lactoglobulin.jpg|thumb|left|320px|Figure 2. A general view of b-lactoglobulin, a typical lipocalin. The binding site (filled atoms) is shown in the central calyx, and the putative binding site (open atoms) is indicated on the outer surface of the protein. The structurally conserved regions are at the rear of the molecule on strand A, the FG loop, and the loop before the a-helix.(Wu S et al. J. Biol. Chem. 1999;274:170-174)<ref>PMID:9867826</ref>]]

=='''Structure of β-Lactoglobulin'''==

β-Lactoglobulin is a small globulin protein, soluble in dilute salt solution with 162 amino acid residues (Mr ∼18,400 Daltons) 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 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, that it is 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>

At physiological conditions, majority of bovine b-lactoglobulin forms a dimer (Fig. 3). Below pH 3, the dimer dissociates into monomers which maintain their native conformation.  

===Ligands and Active sites===

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. It binds retinol with a higher affinity than does RBP. β-Lg contains two tryptophans, 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> and their fluorescence is altered when retinol is bound.<ref>PMID:9115437</ref>

Co-crystallized β-Lg with palmitic acid (3D Model 2._1B0O) and the refined structure 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. The carboxyl group binds to both Lys-60 and Lys-69 at the entrance to the cavity. The hydrophobic tail stretches in an almost fully extended conformation into the center of the protein.<ref>PMID:9867826</ref>  

====Biological function====

=='''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. 5) 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>

===Food Industry and Pharmacy===

β-Lactoglobulin interaction with hydrophobic molecules and with other proteins, and its sensitivity to chemical, thermal and baric denaturation, all with a view to establishing relationships among structure, properties and functionality and its potential use in food and pharmacy industry.<ref>http://www.sciencedirect.com/science/article/pii/S0958694698000211</ref>

Food Industry has been widely utilize β-LG to increase functionality (sensory, texture, nutritional) in commercial food and beverages application, and also as inexpensive source of protein to increase the overall protein content of their product. Pharmacy Industry is recently investigating and developing design of drugs that utilize β-LG as vehicle to transport antioxidants, drugs and vitamins to the gut.<ref>http://www.faqs.org/patents/app/20110038942</ref>

In the dairy industry, bovine milk is frequently heated for pasteurization (62.5°C for 30 min) and sterilization. This heating process may induce oxidative losses of proteins, unsaturated lipids, vitamins, active enzymes, and immunological factors. Cross-linking the free thiol groups of β-LG by heating (100 °C for 2 min), or chemically modifying the β-LG by carboxymethylation to block the thiol groups resulted in a substantial loss of antioxidant activity. The data suggest that Cys-121 plays an essential role in the antioxidant nature of β-LG. Because β-LG is extremely sensitive to thermal denaturation, to maintain its antioxidant nature, dairy products consumed daily should not be overheated in order to maintain its antioxidant nature.<ref>PMID:17235131</ref>

Whey Protein whose majority constituent is β-Lactoglobulin, gives abundant source of branched chain amino acids especially Leucine which plays a key role in initiating transcription of protein synthesis that speed recovery and adaptation to stress (exercise) and also a free thiol Cys which is (postulated) utilized in the body to generate antioxidant glutathione.<ref>http://www.kettlercise.com/members-area/supplements/whey-protein.html</ref>

=='''Other β-Lactoglobulin related 3D structures and complexes'''==

=='''References'''==

=='''Proteopedia Page Contributors and Editors'''==