Denaturation and aggregation of β-lactoglobulin—a preliminary molecular dynamics study

Effects of Exidia yadongensis polysaccharide as emulsifier on the stability, aroma, and antioxidant activities of fat-free stirred mango buffalo yogurt

Because of the poor stability and rheological properties of fat-free stirred yogurt fortified with fruit pulp, new functional polysaccharides as a natural emulsifier, which can increase viscosity in the aqueous phase, may be needed. This study aimed to evaluate the effects of Exidia yadongensis polysaccharide (EYP) as emulsifier on the stability, aroma, and antioxidant activities of mango buffalo yogurt at 4 °C for 25 days. The yogurt with 15 g/L EYP gave a higher content of 215 g/L total solids, 11.3 g/L exopolysaccharides, 0.10 g/L total polyphenols, 630.5 g/L water-holding capacity, and 11.43 g/kg total free amino acids, and maintained better texture, DPPH scavenging activity of 54.05 % and OH scavenging rates of 67.16 %. Moreover, the EYP exhibited the expected ability to weaken postacidification, syneresis, and growth of microorganism, and greatly promote the textural, rheological properties, suspension stability, microstructure, and aroma profiles of stirred mango-flavored buffalo yogurt (p < 0.05). In addition, the addition of 15 g/L EYP can inhibit protein degradation and improve the stability of secondary structure of the protein complex in mango yogurt during 25 days of storage. Therefore, EYP (15 g/L) could be used as natural positive functional factors and emulsifiers in such fat-free stirred yogurt industry.

Amyloid-like aggregation of recombinant β-lactoglobulin at pH 3.5 and 7.0: Is disulfide bond removal the key to fibrillation?

Functional nanofibrils from globular proteins are usually formed by heating for several hours at pH 2.0, which induces acidic hydrolysis and consecutive self-association. The functional properties of these micro-metre-long anisotropic structures are promising for biodegradable biomaterials and food applications, but their stability at pH > 2.0 is low. The results presented here show that modified β-lactoglobulin can also form nanofibrils by heating at neutral pH without prior acidic hydrolysis; the key is removing covalent disulfide bonds. The aggregation behaviour of various recombinant β-lactoglobulin variants was systemically studied at pH 3.5 and 7.0. The suppression of intra- and intermolecular disulfide bonds by eliminating one to three out of the five cysteines makes the non-covalent interactions more prevalent and allow for structural rearrangement. This stimulated the linear growth of worm-like aggregates. Full elimination of all five cysteines led to the transformation of worm-like aggregates into actual fibril structures (several hundreds of nanometres long) at pH 7.0. This understanding of the role of cysteine in protein-protein interactions will help to identify proteins and protein modifications to form functional aggregates at neutral pH.

Amyloidogenic and non-amyloidogenic molten globule conformation of β-lactoglobulin in self-crowded regime

Molecular insights on the β-lactoglobulin thermal unfolding and aggregation are derived from FTIR and UV Resonance Raman (UVRR) investigations. We propose an in situ and in real-time approach that thanks to the identification of specific spectroscopic markers can distinguish the two different unfolding pathways pursued by β-lactoglobulin during the conformational transition from the folded to the molten globule state, as triggered by the pH conditions. For both the investigated pH values (1.4 and 7.5) the greatest conformational variation of β-lactoglobulin occurs at 80 °C and a high degree of structural reversibility after cooling is observed. In acidic condition β-lactoglobulin exposes to the solvent its hydrophobic moieties in a much higher extent than in neutral solution, resulting on a highly open conformation. Moving from the diluted to the self-crowded regime, the solution pH and consequently the different molten globule conformation select the amyloid or non-amyloid aggregation pathway. At acidic condition the amyloid aggregates form during the heating cycle leading to the formation of transparent hydrogel. On the contrary, in neutral condition the amyloid aggregates never form. Information on the secondary structure conformational change of β-lactoglobulin and the formation of amyloid aggregates are obtained by FTIR spectroscopy and are related to the information of the structural changes localized around the aromatic amino acid sites by UVRR technique. Our results highlight a strong involvement of the chain portions where tryptophan is located on the formation of amyloid aggregates.

Ultrasound-induced protein restructuring and ordered aggregation to form amyloid crystals

Amyloid crystals, a form of ordered protein aggregates documented relatively recently, have not been studied as extensively as amyloid fibres. This study investigates the formation of amyloid crystals with low frequency ultrasound (20 kHz) using β-lactoglobulin, as a model protein for amyloid synthesis. Acoustic cavitation generates localised zones of intense shear, with extreme heat and pressure that could potentially drive the formation of amyloid structures at ambient bulk fluid temperatures (20 ± 1 °C). Thioflavin T fluorescence and electron microscopy showed that low-frequency ultrasound at 20 W/cm³ input power induced β-stacking to produce amyloid crystals in the mesoscopic size range, with a mean length of approximately 22 µm. FTIR spectroscopy indicated a shift towards increased intermolecular antiparallel β-sheet content. An increase in sonication time (0–60 min) and input power (4–24 W/cm³) increased the mean crystal length, but this increase was not linearly proportional to sonication time and input power due to the delayed onset of crystal growth. We propose that acoustic cavitation causes protein unfolding and aggregation and imparts energy to aggregates to cross the torsion barrier, to achieve their lowest energy state as amyloid crystals. The study contributes to a further understanding of protein chemistry relating to the energy landscape of folding and aggregation. Ultrasound presents opportunities for practical applications of amyloid structures, presenting a more adaptable and scalable approach for synthesis.

Structural and functional changes of the protein β-lactoglobulin under thermal and electrical processing conditions

In the present study we have tried to explore the effect of static external electric field of strength 3.0 V/nm on the conformational changes adopted by the protein β-lactoglobulin. We have chosen different temperatures viz. 300 K, 400 K and 450 K to evaluate the temperature dependent effect of electric field. We have observed that combined effect of high temperature and static external electric field show significant changes on the structural conformation of the protein which in turn may affect the functional properties of the protein. Calculations of root mean square deviations reveal that both helical and β-sheet regions of the protein are noticeably affected at high temperature. We have used solvent accessible surface area (SASA) and dipole moment values to explain that there is changes in hydrophobicity of the protein surface due to presence of external electric field. The study reveals that electric field in combination with high temperature can be used to alter the conformation of the protein and the effect of external electric field is more pronounced at high temperature than that of low temperature. The study provides a better understanding of the conformational changes adopted by the protein under the stress of external electric field and high temperature and provide guidance to choose optimum conditions for processing without loss of nutritional properties.

Formation of β-Lactoglobulin Aggregates from Quite, Unfolded Conformations upon Heat Activation

In presence of calcium ions, β-lactoglobulin (BLG) unfolds and subsequently aggregates after heating. This process has important pharmaceutical and agroalimentary applications. Nowadays, the molecular mechanism of unfolding and BLG aggregation, and the role of calcium in the mechanism, is poorly understood. Actually, in most studies, data have been acquired at room temperature, after heating and after aggregation, which makes it difficult to establish a clear causal-temporal relation between calcium binding, heat, and aggregation. Thus, the goal of the present study is to get accurate, nanoscale data about the molecular events leading to BLG unfolding and calcium-dependent aggregation. The molecular transformation of BLG during heating has been investigated, using the NMR pulse field gradient technique, operating in a high field (900 MHz). Thanks to this technique, the molecular conformation of newly formed unfolded BLG molecules can be distinguished in a large pool of native ones. The present work shows that BLG at neutral pH at 65 °C displays fast, cooperative-like unfolding, in which no long-lived intermediary state (as a molten globule one) is detected, before aggregation. These data also indicate that calcium ions bind unfolded BLG in specific sites which might be a necessary feature to form the aggregate. Finally, these data also provide an NMR-based methodology to monitor the rate of protein unfolding using NMR.

Molecular Dynamics Simulation for Mechanism Elucidation of Food Processing and Safety: State of the Art

Molecular dynamics (MD) simulation is a useful technique to study the interaction between molecules and how they are affected by various processes and processing conditions. This review summarizes the application of MD simulations in food processing and safety, with an emphasis on the effects that emerging nonthermal technologies (for example, high hydrostatic pressure, pulsed electric field) have on the molecular and structural characteristics of foods and biomaterials. The advances and potential projection of MD simulations in the science and engineering aspects of food materials are discussed and focused on research work conducted to study the effects of emerging technologies on food components. It is expected by showing key case studies that it will stir novel developments as a valuable tool to study the effects of emerging food technologies on biomaterials. This review is useful to food researchers and the food industry, as well as researchers and practitioners working on flavor and nutraceutical encapsulations, dietary carbohydrate product developments, modified starches, protein engineering, and other novel food applications.

Deciphering β-Lactoglobulin Interactions at an Oil-Water Interface: A Molecular Dynamics Study

Protein adsorption at liquid-liquid interfaces is of immense relevance to many biological processes and dairy-based functional foods. Due to experimental limitations, however, there is still a remarkable lack of understanding of the adsorption mechanism, particularly at a molecular level. In this study, atomistic molecular dynamics simulations were used to elucidate the approach and adsorption mechanism of β-lactoglobulin (β-LG) at a decane-water interface. Through multiple independent simulations starting from three representative initial orientations of β-LG relative to the decane surface the rate at which β-LG approaches the oil/water interface is found to be independent of its initial orientation, and largely stochastic in nature. While the residues that first make contact with the decane and the final orientation of β-LG upon adsorption are similar in all cases, the adsorption process is driven predominantly by structural rearrangements that preserve the secondary structure but expose hydrophobic residues to the decane surface. This detailed characterization of the adsorption of β-LG at an oil/water interface should inform the design and development of novel encapsulation and delivery systems in the food and pharmaceutical sciences.

Fibrillation of β-lactoglobulin at low pH in the presence of a complexing anionic polysaccharide

The influence of electrostatic complexation with κ-carrageenan was tested on the fibrillation process of β-lactoglobulin at pH 2.0. Morphology and structural development were monitored through cross correlation dynamic light scattering, transmission electron microscopy, and atomic force microscopy. Scattering indicated that noncomplexed β-lactoglobulin monomers aggregated to form fibrils after 15-90 min of heating at 90 °C. However, electrostatic protein-carrageenan complexes found in the unheated system were unchanged by the thermal process. Images and scattering results showed that carrageenan complexes slowed fibrillation kinetics, possibly through reduction in available monomer concentration. Complexes adhered to fibrils at ends and junctions in TEM images, indicating interactive affinity with the fibers, presumably as heterogeneous nucleation sites.

Surface hydrophobicity of physicochemically and enzymatically treated milk proteins in relation to techno-functional properties

Surface hydrophobicity (SH) of milk proteins treated physicochemically (by heating and Maillard reaction) or modified enzymatically (by transglutaminase, lactoperoxidase, laccase, and glucose oxidase) was assessed in relation to their techno-functional properties. Heat-treatment increased SH of whey protein isolate and decreased SH of sodium caseinate and bovine serum albumin. Maillard reaction of milk proteins caused time-depended decreases of SH. Only for total milk protein reacting with glucose and lactose elevated SH-values were detected. Protein modification with transglutaminase, laccase, and lactoperoxidase strongly increased the SH of whey protein isolate and total milk protein. Incubation with glucose oxidase elevated SH values of sodium caseinate, whey protein isolate, and total milk protein. When correlating SH with techno-functional properties, a positive correlation was observed between SH and foam formation, and a negative correlation was observed between SH and foam stability as well as emulsion stability. No clear correlation was detected between SH and emulsifying activity, surface tension, viscosity, and heat stability of enzymatically and physicochemically treated milk proteins.