Engineering amyloid and amyloid-like morphologies of β-lactoglobulin

Complexes of soybean protein fibrils and chlorogenic acid: Interaction mechanism and antibacterial activity

This study explored the mechanism of interaction between chlorogenic acid (CA) and protein fibrils (PF) as well as the effects of varying the CA/PF concentration ratio on antibacterial activity. Analysis of various parameters, such as ζ-potential, thioflavin T fluorescence intensity, surface hydrophobicity, and free sulfhydryl groups, revealed that the interaction between PF and CA altered the structure of PF. Fluorescence analysis revealed that hydrogen bonding and hydrophobic interactions were the primary interaction forces causing conformational rearrangement, resulting in a shorter, more flexible, and thicker fibril structure, as observed through transmission electron microscopy. Fourier-transform infrared spectroscopy, small-angle X-ray scattering, and X-ray diffraction analyses revealed that the characteristic fibril structure was destroyed when the CA/PF ratio exceeded 0.05. Notably, the CA-PF complexes inhibited the growth of Escherichia coli and Staphylococcus aureus and also exhibited antioxidant activity. Overall, this study expands the application prospects of CA and PF in the food industry.

Fibrillation of soy protein isolate in the presence of metal ions: Structure and gelation behavior

The structure and functional properties of protein fibrils are closely related to environmental factors in fibrillation. Herein, soy protein isolate fibrils (SPIFs, 22 mg/mL) were prepared under acid-heating conditions in the presence of 100 mM metal ions (K+, Na+, Ca2+, Mg2+, and Fe3+). Except for Fe3+, fibrillation and subsequent larger fibril aggregates were promoted, ultimately leading to gel formation. Compared with K+ or Na+, the addition of Ca2+ or Mg2+ resulted in more organized SPIF structures with increased β-sheet contents and higher ThT fluorescence intensities. Furthermore, both of them resulted in longer fibrils with an average contour length of 700-800 nm, which significantly enhanced the storage modulus. However, the presence of Fe3+ accelerated protein hydrolysis and inhibited SPIF formation, resulting in samples consistently exhibited liquid behavior. These findings provide a foundation for understanding the influence of metal ions on regulating the fibrillation and gelling properties of SPIFs.

Preventing thermal aggregation of ovalbumin through dielectric-barrier discharge plasma treatment and enhancing its emulsification properties

The impact of Dielectric-Barrier Discharge (DBD) plasma treatment on the prevention of heat-induced aggregation of Ovalbumin (OVA) and improvement in emulsification properties was investigated. Results highlighted the effective inhibition of thermal aggregation of OVA following exposure to plasma. Structural analysis revealed that the plasma-induced oxidation of sulfhydryl and intermolecular disulfide bonds played a pivotal role in inhibiting the thermal aggregation, considered by Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis (SDS-PAGE), multiplies spectroscopy, and analysis of dynamic exchange of sulfhydryl-disulfide bonds. Meanwhile, the oxidation of exposed hydrophobic sites due to plasma treatment resulted in the transformation of the OVA molecule's surface from hydrophobic to hydrophilic, contributing significantly to the aggregation inhibition. Additionally, compared to an untreated sample of OVA, almost one-fold increase in emulsifying ability (EAI) and 1.5-fold in emulsifying stability (ESI) was observed after 4 min of plasma treatment. These findings demonstrated that plasma treatment not only enhanced the thermal stability of OVA, but also improved its emulsification properties.

Molecular Dynamics Insights into the Aggregation Behavior of N-Terminal β-Lactoglobulin Peptides

β-lactoglobulin (BLG) forms amyloid-like aggregates at high temperatures, low pH, and low ionic strengths. At a pH below 2, BLG undergoes hydrolysis into peptides, with N-terminal peptides 1-33 and 1-52 being prone to fibrillization, forming amyloid-like fibrils. Due to their good mechanical properties, BLG amyloids demonstrate great potential for diverse applications, including biosensors, nanocomposites, and catalysts. Consequently, further studies are essential to comprehensively understand the factors governing the formation of BLG amyloid-like morphologies. In this study, all-atom molecular dynamics simulations were employed to explore the aggregation of N-terminal 1-33 and 1-52 BLG peptides under conditions of pH 2 and at 10 mM NaCl concentration. The simulations revealed that the peptides spontaneously assembled into aggregates of varying sizes. The aggregation process was enabled by the low charge of peptides and the presence of hydrophobic residues within them. As the peptides associated into aggregates, there was a concurrent increase in β-sheet structures and the establishment of hydrogen bonds, enhancing the stability of the aggregates. Notably, on average, 1-33 peptides formed larger aggregates compared to their 1-52 counterparts, while the latter exhibited a slightly higher content of β-sheets and higher cluster orderliness. The applied approach facilitated insights into the early stages of amyloid-like aggregation and molecular-level insight into the formation of β-sheets, which serve as nucleation points for further fibril growth.

UVB pretreatment of β-lactoglobulin affects the temperature-induced formation of functional amyloid-like aggregates and promotes oxidative degradation

Unfolding in combination with or without acid hydrolysis is crucial for the formation of functional amyloid (fibrillar) or amyloid-like (worm-like) β-lactoglobulin (BLG) aggregates, which can be induced through temperature treatment for several hours at pH 2-4. A preceding conformational destabilization of BLG might affect its aggregation. We investigated ultraviolet (UV) B radiation as conformational perturbing treatment to facilitate temperature-induced protein aggregation. 2-h UVB pretreated BLG (UV-BLG) exhibited an accelerated worm-like aggregation at pH 3.5, while at pH 2 the formation of fibrils was decelerated. The UV-induced conformational destabilization lowered the thermal stability and thus facilitates unfolding during thermal treatment. Thereby, the formation of covalent and non-covalent intermolecular interactions was favored, which promoted assembly of intact proteins resulting in worm-like aggregates. The oxidative degradation of UV-BLG was suggested to alter fibrillation-prone protein regions and thereby impede peptide assembly.

Flow-induced alignment of protein nanofibril dispersions

HYPOTHESIS

Protein nanofibrils (PNF) resulting from the self-assembly of proteins or peptides can present structural ordering triggered by numerous factors, including the shear flow. We hypothesize that i) depending on the contour length of the PNF and the magnitude of the shear rate applied to the PNF dispersion, they exhibit specific orientation, and ii) it is possible to predict the alignment of PNF by establishing a flow-alignment relationship. Understanding such a relationship is pivotal to improving the fundamental knowledge and application of fibril systems.

EXPERIMENTS

We use β-lactoglobulin PNF aqueous dispersions with different average contour lengths but equal persistence lengths. We employ simple shear-dominated microfluidic devices with state-of-the-art imaging techniques: flow-induced birefringence (FIB) and micro-particle image velocimetry (μ-PIV), to probe the effect of shear flow on PNF alignment.

FINDINGS

We provide an empirical relationship connecting the birefringence Δn (quantifying the extent of PNF alignment), and the Péclet number Pe (correlating the shear rate of the flow relative to the rotational diffusion of PNF) to understand the flow-alignment behavior of PNF under shear-dominated flows. Furthermore, we assess the alignment and flow profile of PNF at both high and low flow rates. The length of PNF emerges as a controlling parameter capable of modulating PNF alignment at specific shear rates. Our results shed new insights into the hydrodynamic behavior of PNF, which is highly relevant to various industrial processes involving the fibril systems.

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.

Effect of Thermal Treatment on the Self-Assembly of Wheat Gluten Polypeptide

Self-assembled fibrillation of wheat gluten is a common phenomenon in the daily production and processing of wheat flour products. The driving forces for its formation and the factors that influence the morphology of fibrils have not been thoroughly investigated. In this study, the effect of three bonding changes (breaking hydrogen bonds, strengthening hydrophobic interactions, and SH-SS exchange reactions) on gluten polypeptide (GP) fibrillation was simulated by adjusting the heating temperature (room temperature (RT), 45 °C, 65 °C, and 95 °C). The results showed that the breakage of hydrogen bonds could induce conformational transitions in GPs and help to excite fibrillation in GPs. Strengthened hydrophobic interactions significantly contributed to the fibrillation of GPs. Covalent crosslinks generated by SH-SS exchange reactions might also promote the fibrillation of GPs. GPs with different degrees of hydrolysis (4.0%, 6.0%, and 10.0%, represented by DH 4, DH 6, and DH 10, respectively) presented different extents of fibrillation, with DH 10 GPs having a higher propensity to fibrillation than DH 4 and DH 6 GPs. The results of Fourier's transform infrared spectroscopy indicated that hydrophobic interactions drive the transition from a random coil and α-helix to a β-sheet. In addition, hydrophobic interactions also drive the intermolecular polymerization of GPs, resulting in larger molecular weight aggregates. The morphology presented by transmission electron microscopy showed that the greater the DH, the stronger the tendency for the worm-like aggregation of GPs.

Influence of Centrifugation and Shaking on the Self-Assembly of Lysozyme Fibrils

Protein self-assembly into fibrils and oligomers plays a key role in the etiology of degenerative diseases. Several pathways for this self-assembly process have been described and shown to result in different types and ratios of final assemblies, therewith defining the effective physiological response. Known factors that influence assembly pathways are chemical conditions and the presence or lack of agitation. However, in natural and industrial systems, proteins are exposed to a sequence of different and often complex mass transfers. In this paper, we compare the effect of two fundamentally different mass transfer processes on the fibrilization process. Aggregation-prone solutions of hen egg white lysozyme were subjected to predominantly non-advective mass transfer by employing centrifugation and to advective mass transport represented by orbital shaking. In both cases, fibrilization was triggered, while in quiescent only oligomers were formed. The fibrils obtained by shaking compared to fibrils obtained through centrifugation were shorter, thicker, and more rigid. They had rod-like protofibrils as building blocks and a significantly higher β-sheet content was observed. In contrast, fibrils from centrifugation were more flexible and braided. They consisted of intertwined filaments and had low β-sheet content at the expense of random coil. To the best of our knowledge, this is the first evidence of a fibrilization pathway selectivity, with the fibrilization route determined by the mass transfer and mixing configuration (shaking versus centrifugation). This selectivity can be potentially employed for directed protein fibrilization.

A new fibrillization mechanism of β-lactoglobulin in glycine solutions

Even though amyloid aggregates were discovered many years ago the mechanism of their formation is still a mystery. Because of their connection to many of untreatable neurodegenerative diseases the motivation for finding a common aggregation path is high. We report a new high heat induced fibrillization path of a model protein β-lactoglobulin (BLG) when incubated in glycine instead of water at pH 2. By combining atomic force microscopy (AFM), transmission emission microscopy (TEM), dynamic light scattering (DLS) and circular dichroism (CD) we predict that the basic building blocks of fibrils made in glycine are not peptides, but rather spheroid oligomers of different height that form by stacking of ring-like structures. Spheroid oligomers linearly align to form fibrils by opening up and combining. We suspect that glycine acts as an hydrolysation inhibitor which consequently promotes a different fibrillization path. By combining the known data on fibrillization in water with our experimental conclusions we come up with a new fibrillization scheme for BLG. We show that by changing the fibrillization conditions just by small changes in buffer composition can dramatically change the aggregation pathway and the effect of buffer shouldn't be neglected. Fibrils seen in our study are also gaining more and more attention because of their pore-like structure and a possible cytotoxic mechanism by forming pernicious ion-channels. By preparing them in a simple model system as BLG we opened a new way to study their formation.

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.