Circular dichroism study on the early folding events of β-lactoglobulin entrapped in wet silica gels

Investigation into predominant peptide and potential allergenicity of ultrasonicated β-lactoglobulin digestion products

The effect of ultrasonicated β-lactoglobulin on the allergenic potential of predominant peptide was studied in vitro digestion. Gastrointestinal (GI) digestion of ultrasonicated β-lg was fractionated into four fractions, which have different molecular weight and allergenic potentials. The lowest allergenicity of fraction was produced by ultrasonicated β-Lg after GI digestion, depending on the changes in the structure of β-Lg by ultrasonic and the promotion of its proteolysis, resulting in the production of numerous small peptides with significantly reduced IgE activity and basophil histamine release. Mass spectrometry analysis showed that ultrasonic can promote the further hydrolysis of large intermediate peptides, Y42, L54, L57/L58, L95, L104/F105, L122 were target residues that became more available to protease by the pretreatment of ultrasonic, thus have a smaller molecular weight with reduced allergenic potential. Ultrasonic processing of milk products alone could reduce the risk of an allergenic reaction in milk allergy patients to some extent.

The Molten Globule, and Two-State vs. Non-Two-State Folding of Globular Proteins

From experimental studies of protein folding, it is now clear that there are two types of folding behavior, i.e., two-state folding and non-two-state folding, and understanding the relationships between these apparently different folding behaviors is essential for fully elucidating the molecular mechanisms of protein folding. This article describes how the presence of the two types of folding behavior has been confirmed experimentally, and discusses the relationships between the two-state and the non-two-state folding reactions, on the basis of available data on the correlations of the folding rate constant with various structure-based properties, which are determined primarily by the backbone topology of proteins. Finally, a two-stage hierarchical model is proposed as a general mechanism of protein folding. In this model, protein folding occurs in a hierarchical manner, reflecting the hierarchy of the native three-dimensional structure, as embodied in the case of non-two-state folding with an accumulation of the molten globule state as a folding intermediate. The two-state folding is thus merely a simplified version of the hierarchical folding caused either by an alteration in the rate-limiting step of folding or by destabilization of the intermediate.

Comparative study on the effects of nystose and fructofuranosyl nystose in the glycation reaction on the antigenicity and conformation of β-lactoglobulin

Our previous work indicated that the antigenicity of bovine β-lactoglobulin (β-LG) decreased after conjugation with fructo-oligosaccharides (FOS) which was related to its conformational changes. In attempt to unravel further changes of β-LG antigenicity, nystose (GF3) and 1(F)-β-fructofuranosyl nystose (GF4) of FOS were used to investigate the relationship between conformation and antigenicity. The antigenicity of β-LG after conjugated with GF3 and GF4 decreased from 143.4 to 29.5 and 31.6 μg/mL, respectively. The results of mass spectrometry revealed that the molecular weight of β-LG increased from 18.4 to 19.8 and 19.1 kDa after conjugation with GF3 and GF4, respectively. It was shown that the conformational changes of β-LG after conjugation with GF3 were bigger than that with GF4, including quenching of fluorescence intensity, the red-shift of fluorescence spectra, and the increase in sulfhydryl content. However, there was no significant difference in the antigenicity between β-LG-GF3 and β-LG-GF4 conjugates (P>0.05).

Delineation of solution burst-phase protein folding events by encapsulating the proteins in silica gels

Many studies have shown that during the early stages of the folding of a protein, chain collapse and secondary structure formation lead to a partially folded intermediate. Thus, direct observation of these early folding events is crucial if we are to understand protein-folding mechanisms. Notably, these events usually manifest as the initial unresolvable signals, denoted the burst phase, when monitored during conventional mixing experiments. However, folding events can be substantially slowed by first trapping a protein within a silica gel with a large water content, in which the trapped native state retains its solution conformation. In this study, we monitored the early folding events involving secondary structure formation of five globular proteins, horse heart cytochrome c, equine β-lactoglobulin, human tear lipocalin, bovine α-lactalbumin, and hen egg lysozyme, in silica gels containing 80% (w/w) water by CD spectroscopy. The folding rates decreased for each of the proteins, which allowed for direct observation of the initial folding transitions, equivalent to the solution burst phase. The formation of each initial intermediate state exhibited single exponential kinetics and Arrhenius activation energies of 14-31 kJ/mol.

Ionic liquid-induced formation of the α-helical structure of β-lactoglobulin

Structural modification of bovine milk β-lactoglobulin (β-LG) in aqueous 1-butyl-3-methylimidazolium nitrate ([bmim][NO3]) and ethylammonium nitrate ([EAN][NO3]) solutions has been investigated by Fourier transform infrared and circular dichroism spectroscopy. Remarkably, high ionic liquid (IL) concentrations (>15 mol %IL) caused formation of a non-native α-helical structure of β-LG and disruption of its tertiary structure. Furthermore, while [bmim][NO3] promoted protein aggregation, [EAN][NO3] inhibited it probably owing to differences in the unique solution structure (nanoheterogeneity) of the ILs by the different cationic species. The IL-induced α-helical formation of β-LG shows a behavior similar to the alcohol denaturation, but a disordered structure-rich state was observed in the β-α transition process by adding IL, in contrast to the case of an aqueous alcohol solution of protein. We propose that the molten salt-like property of aqueous IL solutions strongly support α-helical formation of proteins.

Relationship between heat-induced fibrillogenicity and hemolytic activity of thermostable direct hemolysin and a related hemolysin of Vibrio parahaemolyticus

The formation of nonspecific ion channels by small oligomeric amyloid intermediates is toxic to the host's cellular membranes. Thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH) are major virulence factors of Vibrio parahaemolyticus. We have previously reported the crystal structure of TDH tetramer with the central channel. We have also identified the molecular mechanism underlying the paradoxical responses to heat treatment of TDH, known as the Arrhenius effect, which is the reversible amyloidogenic property. In the present report, we describe the biophysical properties of TRH, which displays 67% amino acid similarity with TDH. Molecular modeling provided a good fit of the overall structure of TDH and TRH. Size-exclusion chromatography, ultracentrifugation, and transmission electron microscopy revealed that TRH formed tetramer in solution. These toxins showed similar hemolytic activity on red blood cells. However, TRH had less amyloid-like structure than TDH analyzed by thioflavin T-binding assay and far-UV circular dichroism spectra. These data indicated that amyloidogenicity upon heating is not essential for the membrane disruption of erythrocytes, but the maintenance of tetrameric structure is indispensable for the hemolytic activity of the TDH and TRH.