Thermally induced fibrillar aggregation of hen egg white lysozyme

Rutin attenuates negatively charged surfactant (SDS)-induced lysozyme aggregation/amyloid formation and its cytotoxicity

Amyloid fibrils are highly ordered protein assemblies known to contribute to the pathology of a variety of genetic and aging-associated diseases. Here, we have investigated the aggregation propensity of lysozyme in the presence of a negatively charged surfactant (SDS) and evaluated the anti-aggregation activity of rutin. Multiple approaches such as turbidity measurements, dye binding assays, intrinsic fluorescence, circular dichroism (CD), transmission electron microscopy (TEM), MTT and comet assays have been used for this purpose. We inferred that SDS induces aggregation of lysozyme in 0.2-0.6 mM concentration range while at higher concentration range (0.8-1.0 mM), it leads to solubilization/stabilization of protein. Intrinsic/extrinsic fluorescence and CD analysis confirmed significant conformational changes in lysozyme at 0.2 mM SDS. Thioflavin T (ThT), congo red binding and TEM analysis further reaffirmed the formation of lysozyme fibrils. Moreover, MTT assay demonstrated cytotoxicity of these fibrils towards neuroblastoma cell lines (SH-SY5Y) and their attenuation by rutin. Comet assay supported the cytotoxicity mechanism via DNA damage. Molecular docking results also advocate a strong interaction between lysozyme and rutin. The current study indicates a mechanistic approach assuming structural constraints and specific aromatic interactions of rutin with HEWL aggregates.

Self-Assembled Nanofibers for Strong Underwater Adhesion: The Trick of Barnacles

Developing adhesives that can function underwater remains a major challenge for bioengineering, yet many marine creatures, exemplified as mussels and barnacles, have evolved their unique proteinaceous adhesives for strong wet adhesion. The mechanisms underlying the strong adhesion of these natural adhesive proteins provide rich information for biomimetic efforts. Here, combining atomic force microscopy (AFM) imaging and force spectroscopy, we examine the effects of pH on the self-assembly and adhesive properties of cp19k, a key barnacle underwater adhesive protein. For the first time, we confirm that the bacterial recombinant Balanus albicostatus cp19k (rBalcp19k), which contains no 3,4-dihydroxyphenylalanine (DOPA) or any other amino acids with post-translational modifications, can self-assemble into aggregated nanofibers at acidic pHs. Under moderately acidic conditions, the adhesion strength of unassembled monomeric rBalcp19k on mica is only slightly lower than that of a commercially available mussel adhesive protein mixture, yet the adhesion ability of rBalcp19k monomers decreases significantly at increased pH. In contrast, upon preassembly at acidic and low-salinity conditions, rBalcp19k nanofibers keep stable in basic and high-salinity seawater and display much stronger adhesion and thus show resistance to its adverse impacts. Besides, we find that the adhesion ability of Balcp19k is not impaired when it is combined with an N-terminal Thioredoxin (Trx) tag, yet whether the self-assembly property will be disrupted is not determined. Collectively, the self-assembly-enhanced adhesion presents a previously unexplored mechanism for the strong wet adhesion of barnacle cement proteins and may lead to the design of barnacle-inspired adhesive materials.

Specific Ion Effects on Protein Thermal Aggregation from Dilute Solutions to Crowded Environments

We have investigated specific ion effects on protein thermal aggregation from dilute solutions to crowded environments. Ovalbumin and poly(ethylene glycol) have been employed as the model protein and crowding agent, respectively. Our studies demonstrate that the rate-limiting step of ovalbumin thermal aggregation is changed from the aggregation of unfolded protein molecules to the unfolding of the protein molecules, when the solution conditions are varied from a dilute solution to a crowded environment. The specific ion effects acting on the thermal aggregation of ovalbumin generated by kosmotropic and chaotropic ions are different. The thermal aggregation of ovalbumin molecules is promoted by kosmotropic anions in dilute solutions via an increase in protein hydrophobic interactions. In contrast, ovalbumin thermal aggregation is facilitated by chaotropic ions in crowded environments through accelerated unfolding of protein molecules. Therefore, there are distinct mechanisms causing the ion specificities of protein thermal aggregation between dilute solutions and crowded environments. The ion specificities are dominated by ion-specific hydrophobic interactions between protein molecules and ion-specific unfolding of protein molecules in dilute solutions and crowded environments, respectively.

Synthesis of water-soluble curcumin derivatives and their inhibition on lysozyme amyloid fibrillation

The potential application of curcumin was heavily limited in biomedicine because of its poor solubility in pure water. To circumvent the detracting feature, two novel water-soluble amino acid modified curcumin derivatives (MLC and DLC) have been synthesized through the condensation reaction between curcumin and N^α-Fmoc-N^ε-Boc-l-lysine. Benefiting from the enhanced solubility of 3.32×10^-2g/mL for MLC and 4.66×10^-2g/mL for DLC, the inhibition effects of the as-prepared derivatives on the amyloid fibrillation of lysozyme (HEWL) were investigated detaily in water solution. The obtained results showed that the amyloid fibrillation of HEWL was inhibited to a great extent when the concentrations of MLC and DLC reach to 20.139mM and 49.622mM, respectively. The fluorescence quenching upon the addition of curcumin to HEWL provide a support for static and dynamic recombination quenching process. The binding driving force was assigned to classical hydrophobic interaction between curcumin derivatives and HEWL. In addition, UV-Vis absorption and circular dichroism (CD) spectra confirmed the change of the conformation of HEWL.

Protein charge transfer absorption spectra: an intrinsic probe to monitor structural and oligomeric transitions in proteins

The utility of ProCharTS as an intrinsic spectral probe to track protein aggregation and monitor conformational changes is reported.

Reversible association of proteins into sub-visible amorphous aggregates using short solubility controlling peptide tags

Careful analysis of sub-visible amorphous aggregates, where proteins associate non-covalently in either native or denatured states without forming a specific quaternary structure, may shed insight into the mechanisms of protein aggregation and solubility. Here we report a biophysical and biochemical analysis of our model protein, a bovine pancreatic trypsin inhibitor variant (BPTI-19A), whose oligomerization were controlled by attaching solubility controlling peptide tags (SCP tags) to its C terminus, which are short peptides composed of a single type of amino acid that modulate protein solubility. The dynamic light scattering and static light scattering at 25°C indicated that 11 out of 15 SCP tags merely affected the hydrodynamic radius and light scattering intensity of our reference variants BPTI-19A and BPTI-C2G. On the other hand, hydrophobic SCP tags composed of 5 Ile (C5I) or 5 Leu (C5L) were associated into sub-visible aggregates. Circular dichroism indicated that all tagged BPTI variants had the same secondary structure contents as the reference BPTI-19A at 25°C, suggesting that BPTI-C5I and C5L kept their native structure upon association. Furthermore, the thermal denaturation of all of the BPTI variants was fully reversible and typical of natively folded small globular proteins, as monitored by CD at 222 nm. However, the thermal stability of BPTI-19A tagged with hydrophobic residues decreased with increasing protein concentration and tag's hydrophobicity, and BPTI-C5I and C5L were partially denatured at 37°C. Biochemical stability assessed by limited proteolysis with pepsin correlated with the extent of the variants' aggregation, and the large sub-visible aggregates formed by BPTI-C5I and C5L significantly increased their resistance to pepsin proteolysis. Altogether, these observations indicated that hydrophobic SCP tags led to the reversible association of native-like proteins into sub-visible soluble amorphous aggregates resistant to pepsin digestion.

Envisaging the Structural Elevation in the Early Event of Oligomerization of Disordered Amyloid β Peptide

In Alzheimer's disease (AD), amyloid β (Aβ) protein plays a detrimental role in neuronal injury and death. Recent in vitro and in vivo studies suggest that soluble oligomers of the Aβ peptide are neurotoxic. Structural properties of the oligomeric assembly, however, are largely unknown. Our present investigation established that the 40-residue-long Aβ peptide (Aβ40) became more helical, ordered, and compact in the oligomeric state, and both the helical and β-sheet components were found to increase significantly in the early event of oligomerization. The band-selective two-dimensional NMR analysis suggested that majority of the residues from sequence 12 to 22 gained a higher-ordered secondary structure in the oligomeric condition. The presence of a significant amount of helical conformation was confirmed by Raman bands at 1650 and 1336 cm^-1. Other residues remained mostly in the extended polyproline II (PPII) and less compact β-conformation space. In the event of maturation of the oligomers into an amyloid fiber, both the helical content and the PPII-like structural components declined and ∼72% residues attained a compact β-sheet structure. Interestingly, however, some residues remained in the collagen triple helix/extended 2.5₁-helix conformation as evidenced by the amide III Raman signature band at 1272 cm^-1. Molecular dynamics analysis using an optimized potential for liquid simulation force field with the peptide monomer indicated that some of the residues may have preferences for helical conformation and this possibly contributed in the event of oligomer formation, which eventually became a β-sheet-rich amyloid fiber.

Ultrasonic Characterization of Amyloid-Like Ovalbumin Aggregation

Thermal processing conditions, pH, and salt content affect the formation of egg white ovalbumin amyloid, which was investigated using high-precision measurements of ultrasonic velocity and attenuation. These were related to fluorescence and particle size measurements. Fluorescence changes indicated the formation of amyloid-like aggregates that was enhanced by increasing time-temperature treatments. The ultrasonic velocity of ovalbumin after heating at neutral pH (60 min at 70 or 80 °C) was lower than that of unheated ovalbumin, whereas the attenuation was higher. The decrease in the velocity represents increased compressibility associated with a change in the compactness of the protein, whereas changes in attenuation are due to protein conformational changes. Heating ramp studies revealed transitions at approximately 58 and 73 °C. During heating at a constant temperature, the ultrasonic velocity decreased slowly with increasing heating time, indicating an increase in ovalbumin compressibility. It is suggested that the obtained amyloid-like ovalbumin aggregates contain a compact core surrounded by loosely packed protein segments.

Kinetic Mechanism of Thioflavin T Binding onto the Amyloid Fibril of Hen Egg White Lysozyme

Thioflavin T (ThT) is widely used as a fluorescent probe for amyloid fibril detection. Yet the exact kinetic mechanism of ThT binding onto amyloid fibril remains elusive. Previously reported kinetic studies using ThT-fluorescence-detected kinetic design suggested two completely different ThT-binding mechanisms. In one study, a multistep sequential binding mechanism onto a single ThT-binding site was suggested. In another study, a one-step parallel binding mechanism onto multiple ThT-binding sites was suggested. The discrepancy is likely due to the incapability of ThT-fluorescence-detected kinetic design to differentiate the two above-mentioned mechanisms. Considering the weakness of the ThT-fluorescence-detected approach, we investigated the ThT-binding mechanism onto the amyloid fibril of hen egg white lysozyme (HEWL) using a new approach, ThT-absorbance-detected kinetic design. Our new results suggest that ThT binds to HEWL fibril through the one-step parallel binding mechanism. We hope our work can offer some new insights into the interactions between dye molecules and amyloid fibrils.

Chitosan-coated amyloid fibrils increase adipogenesis of mesenchymal stem cells

Mesenchymal stem cells (MSCs) have the potential to revolutionize medicine due to their ability to differentiate into specific lineages for targeted tissue repair. Development of materials and cell culture platforms that improve differentiation of either autologous or allogenic stem cell sources into specific lineages would enhance clinical utilization of MCSs. In this study, nanoscale amyloid fibrils were evaluated as substrate materials to encourage viability, proliferation, multipotency, and differentiation of MSCs. Fibrils assembled from the proteins lysozyme or β-lactoglobulin, with and without chitosan coatings, were deposited on planar mica surfaces. MSCs were cultured and differentiated on fibril-covered surfaces, as well as on unstructured controls and tissue culture plastic. Expression of CD44 and CD90 proteins indicated that multipotency was maintained for all fibrils, and osteogenic differentiation was similarly comparable among all tested materials. MSCs grown for 7days on fibril-covered surfaces favored multicellular spheroid formation and demonstrated a >75% increase in adipogenesis compared to tissue culture plastic controls, although this benefit could only be achieved if MSCs were transferred to TCP for the final differentiation step. The largest spheroids and greatest tendency to undergo adipogenesis was evidenced among MSCs grown on fibrils coated with the positively-charged polysaccharide chitosan, suggesting that spheroid formation is prompted by both topography and cell-surface interactivity and that there is a connection between multicellular spheroid formation and adipogenesis.

Catechol-Containing Hydroxylated Biomimetic 4-Thiaflavanes as Inhibitors of Amyloid Aggregation

The study of compounds able to interfere in various ways with amyloid aggregation is of paramount importance in amyloid research. Molecules characterized by a 4-thiaflavane skeleton have received great attention in chemical, medicinal, and pharmaceutical research. Such molecules, especially polyhydroxylated 4-thiaflavanes, can be considered as structural mimickers of several natural polyphenols that have been previously demonstrated to bind and impair amyloid fibril formation. In this work, we tested five different 4-thiaflavanes on the hen egg-white lysozyme (HEWL) amyloid model for their potential anti-amyloid properties. By combining a thioflavin T assay, atomic force microscopy, and a cell toxicity assay, we demonstrated that such compounds can impair the formation of high-order amyloid aggregates and mature fibrils. Despite this, the tested 4-thiaflavanes, although non-toxic per se, are not able to prevent amyloid toxicity on human neuroblastoma cells. Rather, they proved to block early aggregates in a stable, toxic conformation. Accordingly, 4-thiaflavanes can be proposed for further studies aimed at identifying blocking agents for the study of toxicity mechanisms of amyloid aggregation.

Interaction of nonionic detergents with the specific sites of lysozyme amyloidogenic region - inhibition of amyloid fibrillization

Two nonionic detergents, Triton X-100 (TX-100) and n-dodecyl-β-d-maltoside (DDM) were tested for their ability to affect lysozyme amyloid aggregation. We have demonstrated that fibrillization of lysozyme is completely inhibited by low sub-micellar concentrations of both of these detergents. The apparent IC₅₀ values were calculated to be 22μM and 26μM for TX-100 and DDM, respectively. The detergent/protein ratio is not the only parameter controlling inhibition. The precise timing of the detergent addition was found to be also crucial. It appears that the primary inhibitory activity of detergents resulted from inhibition of nuclei formation, in addition to inhibition of fibril polymerization at the early stage of protofibrils growth. The docking study revealed that Asn-59, Trp-63 and Ala-107, all present within the lysozyme amyloidogenic region, were involved in the interaction with both detergents. In addition, TX-100 also interacted with Gln-57 and Asp-103 within lysozyme. Moreover, based on our computational results, TX-100 bridges the Gln-57 and Ala-107 amino acids of the amyloidogenic segment of lysozyme and therefore inhibits more effectively the amyloid fibril formation. Along these lines, the knowledge gained from our study indicates that the detergents or their derivatives may be applicable as a promising strategy for the modulation of lysozyme protein aggregation.

Fabrication of fibrillosomes from droplets stabilized by protein nanofibrils at all-aqueous interfaces

All-aqueous emulsions exploit spontaneous liquid-liquid separation and due to their water-based nature are particular advantageous for the biocompatible storage and processing of biomacromolecules. However, the ultralow interfacial tensions characteristic of all-aqueous interfaces represent an inherent limitation to the use of thermally adsorbed particles to achieve emulsion stability. Here, we use protein nanofibrils to generate colloidosome-like two-dimensional crosslinked networks of nanostructures templated by all-aqueous emulsions, which we term fibrillosomes. We show that this approach not only allows us to operate below the thermal limit at ultra-low surface tensions but also yields structures that are stable even in the complete absence of an interface. Moreover, we show that the growth and multilayer deposition of fibrils allows us to control the thickness of the capsule shells. These results open up the possibility of stabilizing aqueous two-phase systems using natural proteins, and creating self-standing protein capsules without the requirement for three-phase emulsions or water/oil interfaces.

Mechanisms for the inhibition of amyloid aggregation by small ligands

This work investigates by biochemical, biophysical and MD techniques the opposite anti-amyloid properties of resveratrol and rosmarinic acid on the aggregation of hen egg white lysozyme (HEWL). Differences in association energy and contact maps were found that explain the different behaviours.

The formation of amyloid aggregates is the hallmark of systemic and neurodegenerative disorders, also known as amyloidoses. Many proteins have been found to aggregate into amyloid-like fibrils and this process is recognized as a general tendency of polypeptides. Lysozyme, an antibacterial protein, is a well-studied model since it is associated in human with systemic amyloidosis and that is widely available from chicken eggs (HEWL, hen egg white lysozyme). In the present study we investigated the mechanism of interaction of aggregating HEWL with rosmarinic acid and resveratrol, that we verified to be effective and ineffective, respectively, in inhibiting aggregate formation. We used a multidisciplinary strategy to characterize such effects, combining biochemical and biophysical methods with molecular dynamics (MD) simulations on the HEWL peptide 49–64 to gain insights into the mechanisms and energy variations associated to amyloid formation and inhibition. MD revealed that neither resveratrol nor rosmarinic acid were able to compete with the initial formation of the β-sheet structure. We then tested the association of two β-sheets, representing the model of an amyloid core structure. MD showed that rosmarinic acid displayed an interaction energy and a contact map comparable to that of sheet pairings. On the contrary, resveratrol association energy was found to be much lower and its contact map largely different than that of sheet pairings. The overall characterization elucidated a possible mechanism explaining why, in this model, resveratrol is inactive in blocking fibril formation, whereas rosmarinic acid is instead a powerful inhibitor.

Fluorescence monitoring of the effect of oxidized lipids on the process of protein fibrillization

The kinetics of lysozyme and insulin amyloid formation in the presence of the oxidized phospholipids (oxPLs) was investigated using Thioflavin T fluorescence assay. The kinetic parameters of fibrillization process (lag time and apparent rate constant) have been determined upon varying the following experimental parameters: the type of lipid assemblies (premicellar aggregates and lipid bilayer vesicles), pH, temperature and lipid-to-protein molar ratio. It was found that oxPLs premicellar aggregates induced the more pronounced increase of the maximum Thioflavin T fluorescence, which is proportional to the extent of fibril formation, compared to the vesicles composed of the oxidized and unoxidized lipids. In contrast, the oxPLs, used as dispersions or included into vesicles, inhibited fibril nucleation and elongation under near-physiological conditions in vitro compared to liposomes containing unoxidized lipids. The results obtained provide deeper insight into the molecular mechanisms of the oxidative stress-modulated conformational diseases, and could be employed for the anti-amyloid drug development.

Novel benzanthrone probes for membrane and protein studies

The applicability of a series of novel benzanthrone dyes to monitoring the changes in physicochemical properties of lipid bilayer and to differentiating between the native and aggregated protein states has been evaluated. Based on the quantitative parameters of the dye-membrane and dye-protein binding derived from the fluorimetric titration data, the most prospective membrane probes and amyloid tracers have been selected from the group of examined compounds. Analysis of the red edge excitation shifts of the membrane- and amyloid-bound dyes provided information on the properties of benzanthrone binding sites within the lipid and protein matrixes. To understand how amyloid specificity of benzanthrones correlates with their structure, quantitative structure activity relationship (QSAR) analysis was performed involving a range of quantum chemical molecular descriptors. A statistically significant model was obtained for predicting the sensitivity of novel benzanthrone dyes to amyloid fibrils.

Production of lysozyme nanofibers using deep eutectic solvent aqueous solutions

Amyloid fibrils have recently gained a lot of attention due to their morphology, functionality and mechanical strength, allowing for their application in nanofiber-based materials, biosensors, bioactive membranes and tissue engineering scaffolds. The in vitro production of amyloid fibrils is still a slow process, thus hampering the massive production of nanofibers and its consequent use. This work presents a new and faster (2-3h) fibrillation method for hen egg white lysozyme (HEWL) using a deep eutectic solvent based on cholinium chloride and acetic acid. Nanofibers with dimensions of 0.5-1μm in length and 0.02-0.1μm in thickness were obtained. Experimental variables such as temperature and pH were also studied, unveiling their influence in fibrillation time and nanofibers morphology. These results open a new scope for protein fibrillation into nanofibers with applications ranging from medicine to soft matter and nanotechnology.

Unidirectional Living Growth of Self-Assembled Protein Nanofibrils Revealed by Super-resolution Microscopy

Protein-based nanofibrils are emerging as a promising class of materials that provide unique properties for applications such as biomedical and food engineering. Here, we use atomic force microscopy and stochastic optical reconstruction microscopy imaging to elucidate the growth dynamics, exchange kinetics, and polymerization mechanism for fibrils composed of a de novo designed recombinant triblock protein polymer. This macromolecule features a silk-inspired self-assembling central block composed of GAGAGAGH repeats, which are known to fold into a β roll with turns at each histidine and, once folded, to stack, forming a long, ribbon-like structure. We find several properties that allow the growth of patterned protein nanofibrils: the self-assembly takes place on only one side of the growing fibrils by the essentially irreversible addition of protein polymer subunits, and these fibril ends remain reactive indefinitely in the absence of monomer ("living ends"). Exploiting these characteristics, we can grow stable diblock protein nanofibrils by the sequential addition of differently labeled proteins. We establish control over the block length ratio by simply varying monomer feed conditions. Our results demonstrate the use of engineered protein polymers in creating precisely patterned protein nanofibrils and open perspectives for the hierarchical self-assembly of functional biomaterials.

Protein Fibrils Induce Emulsion Stabilization

The behavior of an oil-in-water emulsion was studied in the presence of protein fibrils for a wide range of fibril concentrations by using rheology, diffusing wave spectroscopy, and confocal laser scanning microscopy. Results showed that above a minimum fibril concentration depletion flocculation occurred, leading to oil droplet aggregation and enhanced creaming of the emulsion. Upon further increasing the concentration of the protein fibrils, the emulsions were stabilized. In this stable regime both aggregates of droplets and single droplets are present, and these aggregates are smaller than the aggregates in the flocculated emulsion samples at the lower fibril concentrations. The size of the droplet aggregates in the stabilized emulsions is independent of fibril concentration. In addition, the droplet aggregation was reversible upon dilution both by a pH 2 HCl solution and by a fibril solution at the same concentration. The viscosity of the emulsions containing fibrils was comparable to that of the pure fibril solution. Neither fibril networks nor droplet gel networks were observed in our study. The stabilization mechanism of emulsions containing long protein fibrils at high protein fibril concentrations points toward the mechanism of a kinetic stabilization.

Characterization of heat induced spherulites of lysozyme reveals new insight on amyloid initiation

Here, we report results obtained during our experiments to visualize how heat transforms globular protein, lysozyme into building block of β-amyloids. Light scattering experiments showed formation of lower order associated species around 50-70 °C followed by rapid cooperativity to β-amyloid fibrils. Interestingly, crystallization drops set at higher temperatures either led to aggregates or spherulites. The latter possess an amorphous β-fibril rich core with thin crystalline needles projecting outwards. Diffraction of the crystalline outgrowths revealed novel dimers and trimers of lysozyme where individual chains were similar to monomer with marginal gain in β-sheet content. Importantly, analysis of Amide I stretching frequencies showed that protein loses its secondary structure at temperatures higher than where we obtained crystals followed by rapid gain in β-sheet content. Interestingly, attempts to use the needles as seeds for more crystals led to "broom-like" fibril formations at the ends. Further, aggregation inhibitors like arginine and benzyl alcohol completely obliterated spherulites formation during crystallization. Refinement of crystals of lysozyme in presence of these molecules showed these small molecules bind to the interfaces of heat associated dimers and trimers. Overall our work concludes that heat induced weakly associated structures of lysozyme are the first step towards its amyloid formation.

Bottom-Up Synthesis and Sensor Applications of Biomimetic Nanostructures

The combination of nanotechnology, biology, and bioengineering greatly improved the developments of nanomaterials with unique functions and properties. Biomolecules as the nanoscale building blocks play very important roles for the final formation of functional nanostructures. Many kinds of novel nanostructures have been created by using the bioinspired self-assembly and subsequent binding with various nanoparticles. In this review, we summarized the studies on the fabrications and sensor applications of biomimetic nanostructures. The strategies for creating different bottom-up nanostructures by using biomolecules like DNA, protein, peptide, and virus, as well as microorganisms like bacteria and plant leaf are introduced. In addition, the potential applications of the synthesized biomimetic nanostructures for colorimetry, fluorescence, surface plasmon resonance, surface-enhanced Raman scattering, electrical resistance, electrochemistry, and quartz crystal microbalance sensors are presented. This review will promote the understanding of relationships between biomolecules/microorganisms and functional nanomaterials in one way, and in another way it will guide the design and synthesis of biomimetic nanomaterials with unique properties in the future.

Micro- and nanoscale hierarchical structure of core–shell protein microgels

In this work, we fabricate core–shell protein microgels stabilized by protein fibrillation with hierarchical structuring on scales ranging from a few nanometers to tens of microns.

Insights into Kinetics of Agitation-Induced Aggregation of Hen Lysozyme under Heat and Acidic Conditions from Various Spectroscopic Methods

Protein misfolding and amyloid formation are an underlying pathological hallmark in a number of prevalent diseases of protein aggregation ranging from Alzheimer’s and Parkinson’s diseases to systemic lysozyme amyloidosis. In this context, we have used complementary spectroscopic methods to undertake a systematic study of the self-assembly of hen egg-white lysozyme under agitation during a prolonged heating in acidic pH. The kinetics of lysozyme aggregation, monitored by Thioflavin T fluorescence, dynamic light scattering and the quenching of tryptophan fluorescence by acrylamide, is described by a sigmoid curve typical of a nucleation-dependent polymerization process. Nevertheless, we observe significant differences between the values deduced for the kinetic parameters (lag time and aggregation rate). The fibrillation process of lysozyme, as assessed by the attenuated total reflection-Fourier transform infrared spectroscopy, is accompanied by an increase in the β-sheet conformation at the expense of the α-helical conformation but the time-dependent variation of the content of these secondary structures does not evolve as a gradual transition. Moreover, the tryptophan fluorescence-monitored kinetics of lysozyme aggregation is described by three phases in which the temporal decrease of the tryptophan fluorescence quantum yield is of quasilinear nature. Finally, the generated lysozyme fibrils exhibit a typical amyloid morphology with various lengths (observed by atomic force microscopy) and contain exclusively the full-length protein (analyzed by highly performance liquid chromatography). Compared to the data obtained by other groups for the formation of lysozyme fibrils in acidic pH without agitation, this work provides new insights into the structural changes (local, secondary, oligomeric/fibrillar structures) undergone by the lysozyme during the agitation-induced formation of fibrils.