ABSTRACT
The water-urea-beta-lactoglobulin interaction was studied by means of principal component analysis (PCA) and two-dimensional correlation spectroscopy applied to the urea concentration-dependent FTIR spectra of aqueous urea-protein solutions. The two nu(CO) and nu(as)(CN) bands coming from urea's absorbance, instead of the amide bands arising from protein, were employed in the analysis. To get a precise view of the changes induced by the urea concentration-controlled unfolding process, the absorbance variations developed in the ternary water-urea-protein system were compared with those observed in a binary water-urea system [Y.M. Jung et al., J. Phys. Chem. B 2004, 108, 13008]. The comparative studies enabled to detect apparent differences between the absorbance changes caused solely by urea's concentration increase and by the urea-dependent unfolding process. Urea's ability to unfold protein was discussed in context of the indirect and the direct mechanism depending on urea's concentration. It was shown that both mechanisms are relevant, that is, the indirect for solutions below 3 M and the direct for solutions above 3 M concentration. The character of the mechanism is strictly correlated with the association level of urea molecules.
Subject(s)
Lactoglobulins/chemistry , Urea/chemistry , Computer Simulation , Lactoglobulins/classification , Principal Component Analysis , Protein Denaturation/drug effects , Spectrum Analysis , Urea/pharmacologyABSTRACT
Nano-electrospray-ionization mass spectrometry (nano-ESI-MS) is applied to comparison of bovine and porcine beta-lactoglobulin (BLG and PLG). The conformational and oligomeric properties of the two proteins under different solvent and experimental conditions are analyzed. The pH-dependence of dimerization is described for the pH range 2-11. The results indicate maximal dimer accumulation at pH 6 for BLG and pH 4 for PLG, as well as a lower stability of the PLG dimer at pH 4 compared to BLG at pH 6. Conformational stability appears to be higher for BLG at acidic pH, but higher for PLG at basic pH. The higher stability of BLG at low pH is revealed by means of either chemical or thermal denaturation. Equilibrium folding intermediates of both proteins are detected. Finally, conditions are found that promote dissociation of the BLG dimer at pH 6 into folded monomers.
Subject(s)
Lactoglobulins/chemistry , Lactoglobulins/ultrastructure , Spectrometry, Mass, Electrospray Ionization/methods , Animals , Cattle , Hydrogen-Ion Concentration , Lactoglobulins/classification , Protein Conformation , Solvents/chemistry , Species Specificity , SwineABSTRACT
The lipocalin family became established shortly after the structural similarity was noted between plasma retinol binding protein and the bovine milk protein, beta-lactoglobulin. During the past 60 years, beta-lactoglobulin has been studied by essentially every biochemical technique available and so there is a huge literature upon its properties. Despite all of these studies, no specific biological function has been ascribed definitively to the protein, although several possibilities have been suggested. During the processing of milk on an industrial scale, the unpredictable nature of the process has been put down to the presence of beta-lactoglobulin and certainly the whey protein has been implicated in the initiation of aggregation that leads to the fouling of heat exchangers. This short review of the properties of the protein will concentrate mainly on studies carried out under essentially physiological conditions and will review briefly some of the possible functions for the protein that have been described.
Subject(s)
Lactoglobulins/chemistry , Lactoglobulins/classification , Animals , Cattle , Lactoglobulins/metabolism , Ligands , Models, Molecular , Phylogeny , Protein Conformation , Protein Denaturation , Protein Folding , Tissue DistributionABSTRACT
The complete primary structure of the minor beta-lactoglobulin II component from donkey milk is presented. It has been established by amino-acid sequencing and mass-spectrometry analysis of intact protein and peptides obtained after enzymatic and chemical cleavages. The molecular mass and the pI of the protein are calculated to be 18,261 Da and 4.5 respectively. Despite the close structural similarity of the donkey and horse major beta-lactoglobulin I components, their minor beta-lactoglobulin II components show substantial differences in sequence. Most observed exchanges are clustered at residues 78-106 where only 6 amino-acid residues are conserved. The primary structure of donkey beta-lactoglobulin II reveals some unusual features of minor beta-lactoglobulins II and gives new light to the evolution of beta-lactoglobulins and other lipocalins involved in retinol binding or reproductive functions.
