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Bovine ?-lactoglobulin is dimeric under imitative physiological conditions: dissociation equilibrium and rate constants over the pH range of 2.5-7.5.

ABSTRACT: The oligomerization of ?-lactoglobulin (?Lg) has been studied extensively, but with somewhat contradictory results. Using analytical ultracentrifugation in both sedimentation equilibrium and sedimentation velocity modes, we studied the oligomerization of ?Lg variants A and B over a pH range of 2.5-7.5 in 100 mM NaCl at 25°C. For the first time, to our knowledge, we were able to estimate rate constants (k(off)) for ?Lg dimer dissociation. At pH 2.5 k(off) is low (0.008 and 0.009 s(-1)), but at higher pH (6.5 and 7.5) k(off) is considerably greater (>0.1 s(-1)). We analyzed the sedimentation velocity data using the van Holde-Weischet method, and the results were consistent with a monomer-dimer reversible self-association at pH 2.5, 3.5, 6.5, and 7.5. Dimer dissociation constants K(D)(2-1) fell close to or within the protein concentration range of ?5 to ?45 ?M, and at ?45 ?M the dimer predominated. No species larger than the dimer could be detected. The K(D)(2-1) increased as |pH-pI| increased, indicating that the hydrophobic effect is the major factor stabilizing the dimer, and suggesting that, especially at low pH, electrostatic repulsion destabilizes the dimer. Therefore, through Poisson-Boltzmann calculations, we determined the electrostatic dimerization energy and the ionic charge distribution as a function of ionic strength at pH above (pH 7.5) and below (pH 2.5) the isoelectric point (pI?5.3). We propose a mechanism for dimer stabilization whereby the added ionic species screen and neutralize charges in the vicinity of the dimer interface. The electrostatic forces of the ion cloud surrounding ?Lg play a key role in the thermodynamics and kinetics of dimer association/dissociation.

SUBMITTER: Mercadante D

PROVIDER: S-EPMC3400788 | biostudies-literature | 2012 Jul

REPOSITORIES: biostudies-literature

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Bovine β-lactoglobulin is dimeric under imitative physiological conditions: dissociation equilibrium and rate constants over the pH range of 2.5-7.5.

Mercadante Davide D Melton Laurence D LD Norris Gillian E GE Loo Trevor S TS Williams Martin A K MA Dobson Renwick C J RC Jameson Geoffrey B GB

Biophysical journal 20120717 2

The oligomerization of β-lactoglobulin (βLg) has been studied extensively, but with somewhat contradictory results. Using analytical ultracentrifugation in both sedimentation equilibrium and sedimentation velocity modes, we studied the oligomerization of βLg variants A and B over a pH range of 2.5-7.5 in 100 mM NaCl at 25°C. For the first time, to our knowledge, we were able to estimate rate constants (k(off)) for βLg dimer dissociation. At pH 2.5 k(off) is low (0.008 and 0.009 s(-1)), but at hi ...[more]

PMID: 22853908

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Project description:Although bovine beta-lactoglobulin assumes a monomeric native structure at pH 3 in the absence of salt, the addition of salts stabilizes the dimer. Thermodynamics of the monomer-dimer equilibrium dependent on the salt concentration were studied by sedimentation equilibrium. The addition of NaCl, KCl, or guanidine hydrochloride below 1 M stabilized the dimer in a similar manner. On the other hand, NaClO(4) was more effective than other salts by about 20-fold, suggesting that anion binding is responsible for the salt-induced dimer formation, as observed for acid-unfolded proteins. The addition of guanidine hydrochloride at 5 M dissociated the dimer into monomers because of the denaturation of protein structure. In the presence of either NaCl or NaClO(4), the dimerization constant decreased with an increase in temperature, indicating that the enthalpy change (DeltaH(D)) of dimer formation is negative. The heat effect of the dimer formation was directly measured with an isothermal titration calorimeter by titrating the monomeric beta-lactoglobulin at pH 3.0 with NaClO(4). The net heat effects after subtraction of the heat of salt dilution, corresponding to DeltaH(D), were negative, and were consistent with those obtained by the sedimentation equilibrium. From the dependence of dimerization constant on temperature measured by sedimentation equilibrium, we estimated the DeltaH(D) value at 20 degrees C and the heat capacity change (DeltaC(p)) of dimer formation. In both NaCl and NaClO(4), the obtained DeltaC(p) value was negative, indicating the dominant role of burial of the hydrophobic surfaces upon dimer formation. The observed DeltaC(p) values were consistent with the calculated value from the X-ray dimeric structure using a method of accessible surface area. These results indicated that monomer-dimer equilibrium of beta-lactoglobulin at pH 3 is determined by a subtle balance of hydrophobic and electrostatic effects, which are modulated by the addition of salts or by changes in temperature.

Dataset Information

Bovine ?-lactoglobulin is dimeric under imitative physiological conditions: dissociation equilibrium and rate constants over the pH range of 2.5-7.5.

Publications

Bovine β-lactoglobulin is dimeric under imitative physiological conditions: dissociation equilibrium and rate constants over the pH range of 2.5-7.5.

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