Concentration dependent switch in the kinetic pathway of lysozyme fibrillation: Spectroscopic and microscopic analysis

Frequency dependence of ultrasonic effects on the kinetics of hen egg white lysozyme fibrillation

Our study aimed to investigate the effects of ultrasound on the fibrillation kinetics of HEWL (hen egg white lysozyme) and its physicochemical properties. Ultrasound, a mechanical wave, can induce conformational changes in proteins. To achieve this, we developed an ultrasound exposure system and used various biophysical techniques, including ThT fluorescence spectroscopy, ATR-FTIR, Far-UV CD spectrophotometry, Fluorescence microscopy, UV-spectroscopy, and seeding experiments. Our results revealed that higher frequencies significantly accelerated the fibrillation of lysozyme by unfolding the native protein and promoting the fibrillation process, thereby reducing the lag time. We observed a change in the secondary structure of the sonicated protein change to the β-structure, but there was no difference in the Tm of native and sonicated proteins. Furthermore, we found that higher ultrasound frequencies had a greater seeding effect. We propose that the effect of frequency can be explained by the impact of the Reynolds number, and for the Megahertz frequency range, we are almost at the transition regime of turbulence. Our results suggest that laminar flows may not induce any significant change in the fibrillation kinetics, while turbulent flows may affect the process.

Aggregation studies of alpha-lactalbumin induced by edible azo dyes with different sulfonyl group numbers: A comparative study

As an opaque and complex colloidal mixture, milk is usually present as a positively charged colloid under acidic conditions. Adding negatively charged colloids can lead to protein aggregation in milk. Alpha-lactalbumin (α-La) is an essential component of whey protein and has good physicochemical properties for functional food development. We combined spectroscopy, computer simulations, and other techniques to comparative analyze the mechanisms and characteristics of isolated α-La aggregation induced by CI Acid Red 27 (C27)/CI Acid Red 14 (FB) containing different sulfonyl groups in vitro. The results showed that C27/FB (5.25 × 10-5 mol·L-1 to 3.15 × 10-4 mol·L-1) induced the formation of fibril-like aggregates under acidic conditions (pH 2.0 and 4.0) mainly benefit from hydrophobic and electrostatic forces. Weakening and redshift of α-La's characteristics negative peak were observed (208 nm to 218 nm) on circular dichroism. β-Crosslinks self-assembly and reorganization of disulfide bonds occurred during protein fibrillation. Moreover, the different redshift intensity of Congo red binding to amyloid fibrils was observed to be induced by C27 (>551 nm) and FB (>536 nm), and the direct observation by TEM demonstrated the ability to induce protein fibrillation is C27 > FB. Edible azo dyes with more sulfonyl groups would possess a stronger ability to induce protein fibrillation.

Ambient Temperature Affects Protein Self-Assembly by Interfering with the Interfacial Aggregation Behavior

Amyloid fibrillation is known to be associated with degenerative diseases, and mature fibrils are also considered as valuable biomedical materials. Thus, the mechanism and influencing factors of fibrillation have always been the focus of research. However, in vitro studies are always plagued by low reproducibility of kinetics and the molecular mechanism of amyloid fibrillation is under debate until now. Here, we identified the ambient temperature (AT) as a non-negligible interfering factor in in vitro self-assembly of globular protein hen egg-white lysozyme for the first time. By multimodal molecular spectroscopy methods, not only the effect of ATs on the kinetics of protein aggregation was described but also the conformational changes of the molecular structure with different ATs were captured. Through investigating the dependence of interfacial area and catalysis, the reason for this influence was construed by the various aggregation behaviors of protein molecules in the two-phase interface. The results suggest that in vitro mechanism research on protein fibrillation needs to first clarify the AT for a more accurate comparative analysis. The proposal of this concept will provide a new clue for a deeper understanding of the mechanism of protein self-assembly and may have an impact on evaluating the efficiency of amyloid accelerators or inhibitors based on the comparative analysis of protein self-assembly.

A guide to studying protein aggregation

Disrupted protein folding or decreased protein stability can lead to the accumulation of (partially) un- or misfolded proteins, which ultimately cause the formation of protein aggregates. Much of the interest in protein aggregation is associated with its involvement in a wide range of human diseases and the challenges it poses for large-scale biopharmaceutical manufacturing and formulation of therapeutic proteins and peptides. On the other hand, protein aggregates can also be functional, as observed in nature, which triggered its use in the development of biomaterials or therapeutics as well as for the improvement of food characteristics. Thus, unmasking the various steps involved in protein aggregation is critical to obtain a better understanding of the underlying mechanism of amyloid formation. This knowledge will allow a more tailored development of diagnostic methods and treatments for amyloid-associated diseases, as well as applications in the fields of new (bio)materials, food technology and therapeutics. However, the complex and dynamic nature of the aggregation process makes the study of protein aggregation challenging. To provide guidance on how to analyse protein aggregation, in this review we summarize the most commonly investigated aspects of protein aggregation with some popular corresponding methods.

Effect of ligands with different affinity on albumin fibril formation

The effect of binding of several ligands to bovine serum albumin on the kinetics of fibril formation at denaturing conditions is studied. The considered ligands are clinical drugs with different binding constants to albumin: relatively strong binders (naproxen, ibuprofen, warfarin with 10⁵ to 10⁷ binding constant values) and weak binders (isoniazid, ranitidine with 10³ to 10⁴ binding constant values). The data of thioflavin fluorescence binding assay, Congo red binding assay, and circular dichroism spectroscopy indicate ligand concentration-dependent suppression of fibril formation in the presence of strong binders and no effects in the presence of weak binders. Analysis of kinetic curves shows no induction lag associated with fibril nucleation and the first-order kinetics of fibril formation with respect to albumin concentration for all the studied systems. Using DSC method, the fractions of unfolded albumin at incubation temperature were determined for each albumin-ligand system and ligand concentration. Their magnitudes ranging from 0 to 1 correlate with the initial rates of fibril formation and with equilibrium concentrations of fibrils formed in the system after incubation for at least 120 min. The results indicate that fibrils are formed from partially or completely denatured albumin form with the rate proportional to the fraction of this form. Strong albumin binders act as thermodynamic inhibitors of fibrillation shifting the unfolding equilibrium to the side of the native ligand-bound protein.

Spectroscopic studies on the stability and nucleation-independent fibrillation of partially-unfolded proteins in crowded environment

Fibril formation of globular proteins is driven by attaining an appropriate partially-unfolded conformation. Excluded volume effect exerted by the presence of other macromolecules in the solution, as found in the cellular interior, might affect the conformational state of proteins and alter their fibril formation process. The change in structure, stability and rate of fibril formation of aggregation-prone partially-unfolded states of lysozyme (Lyz) and α-lactalbumin (ALA) in the presence of different sizes of polyethylene glycol (PEG) is examined using spectroscopic methods. Thermal denaturation and far-UV CD studies suggest that Lyz is stabilized by PEGs and the stability increases with increasing concentration of PEGs. However, the stability of ALA depends on the size and concentration of PEG. The change in enthalpy of unfolding indicates the existence of soft-interactions between the proteins and PEG along with excluded volume effect. Fibrillation rate of Lyz is not significantly altered in the presence of lower concentrations of PEGs suggesting that the crowding effect dominates the viscosity-induced retardation of protein association whereas at higher concentrations the rates are reduced. In case of ALA, the rate of fibrillation is drastically reduced; however, there is a marginal increase with the increasing concentration of PEG. The results suggest that the fibril formation is influenced by change in initial conformation of the partially-unfolded states of the proteins and their stability in the presence of the crowding agent. Further, the size and concentration of the crowding agent, and the soft-interaction between the proteins and PEG also affects the fibrillation.

The dual function of impurity in protein crystallization

Protein crystallization could be promoted with a low concentration of impurities and inhibited with a high concentration of impurities, and this inhibition can be weakened by an audible sound.

Imidazolium-based ionic liquids with increasing alkyl chain length of cations decrease the stability and fibrillation propensity of lysozyme

Imidazolium ionic liquids with longer alkyl side chains show a larger destabilization effect on lysozyme. Increased hydrophobicity of the IL increases its binding affinity and inhibits the fibril formation of lysozyme.

Using amyloid autofluorescence as a biomarker for lysozyme aggregation inhibition

The assembly of proteins into amyloidogenic aggregates underlies the onset and symptoms of several pathologies, including Alzheimer's disease, Parkinson's disease and type II diabetes. Among the efforts for fighting these diseases, there is a great demand for developing novel, fast and reliable methods for in vitro screening of new drugs that may suppress or reverse amyloidogenesis. Recent studies unravelled a progressive increase in a blue autofluorescence upon amyloid formation originated from many different proteins, including the peptide amyloid-β, lysozyme or insulin. Herein, we propose a drug screening method using this property, avoiding the use of external probe dyes. We demonstrate that the inhibition of lysozyme amyloid formation by means of two known inhibitors, tartrazine and amaranth, can be monitored based on the autofluorescence of lysozyme amyloid aggregates. Our results show that amyloid luminescence is an intrinsic property that can be potentially applied in a screening assay, allowing the ranking of drug efficiency. The assays demonstrated here are fast to perform and suitable for scaling using microplate assays, configuring a new sensitive and economically feasible method.

Interactions of the cis and trans states of an azobenzene photoswitch with lysozyme induced by red and blue light

Exploring the interaction between an azobenzene-based photoswitch and natural protein can help elucidate how the photo-control of an optical molecule participates in the transmission and delivery of proteins, as well as the effects of azo-switch trans and cis states on protein configurations. In this study, fluorescence analysis, circular dichroism spectroscopy, molecular docking, and molecular dynamics simulations were used to study the interaction among different configurations of tetra-ortho-methoxy substituted azobenzene di-maleimide (toM-ABDM), a red light-induced optical azo-switch, and lysozyme (LYZ). Results showed that toM-ABDM caused the static quenching of LYZ. The cis toM-ABDM had stronger binding affinity than trans toM-ABDM. The noncovalent interaction, hydrogen bonds and van der Waals forces, could not regulate the conformation of LYZ in photo-control. A binding model of toM-ABDM and LYZ in different forms induced by red and blue light was further established by computer simulation.

What is new in lysozyme research and its application in food industry? A review

Lysozyme, an important bacteriostatic protein, is widely distributed in nature. It is generally believed that the high efficiency of lysozyme in inhibiting gram-positive bacteria is caused by its ability to cleave the β-(1,4)-glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine. In recent years, there has been growing interest in modifying lysozyme via physical or chemical interactions in order to improve its sensitivity against gram-negative bacterial strains. This review addresses some significant techniques, including sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), infrared (IR) spectra, fluorescence spectroscopy, nuclear magnetic resonance (NMR), UV-vis spectroscopy, circular dichroism (CD) spectra and differential scanning calorimetry (DSC), which can be used to characterize lysozymes and methods that modify lysozymes with carbohydrates to enhance their various physicochemical characteristics. The applications of biomaterials based on lysozymes in different food matrices are also discussed.

Unveiling the stimulatory effects of tartrazine on human and bovine serum albumin fibrillogenesis: Spectroscopic and microscopic study

Amyloid fibrils are playing key role in the pathogenesis of various neurodegenerative diseases. Generally anionic molecules are known to induce amyloid fibril in several proteins. In this work, we have studied the effect of anionic food additive dye i.e., tartrazine (TZ) on the amyloid fibril formation of human serum albumins (HSA) and bovine serum albumin (BSA) at pHs7.4 and 3.5. We have employed various biophysical methods like, turbidity measurements, Rayleigh Light Scattering (RLS), Dynamic Light Scattering (DLS), intrinsic fluorescence, Congo red assay, far-UV CD, transmission electron microscopy (TEM) and atomic force microscopy (AFM) to decipher the mechanism of TZ-induce amyloid fibril formation in both the serum albumins at pHs7.4 and 3.5. The obtained results suggest that both the albumins forms amyloid-like aggregates in the presence of 1.0 to 15.0mM of TZ at pH3.5, but no amyloid fibril were seen at pH7.4. The possible cause of TZ-induced amyloid fibril formation is electrostatic and hydrophobic interaction because sulfate group of TZ may have interacted electrostatically with positively charged amino acids of the albumins at pH3.5 and increased protein-protein and protein-TZ interactions leading to amyloid fibril formation. The TEM, RLS and DLS results are suggesting that BSA forms bigger size amyloids compared to HSA, may be due to high surface hydrophobicity of BSA.

Aggregation and conformational stability evaluation of myoglobin in the presence of ionic surfactant

Sodium lauroyl sarcosinate (SLS) is frequently used for the solubilization of inclusion bodies in vitro due to its structural similarity to lipid plasma membrane. There are many factors that could influence protein aggregation propensity, including overall protein surface charge and hydrophobicity. Here, the aggregation pathway of myoglobin protein was studied under different conditions (pH 3.5 and 7.4) in the presence of varying concentrations of SLS to evaluate the underlying forces dictating protein aggregation. Data obtained from Rayleigh light scattering, ThT binding assay, and far-UV CD indicated that SLS have different effects on the protein depending on its concentration and environmental conditions. In the presence of low concentrations of SLS (0.05-0.1 mM), no aggregation was detected at both pH conditions tested. Whereas, as we reach higher SLS concentrations (0.5-10.0 mM), myoglobin started forming larger-sized aggregates at pH 3.5 and not pH 7.4. These results suggest that electrostatics interactions as well as hydrophobic forces play an important role in SLS-induced myoglobin aggregation.

Gold nanocolloid–protein interactions and their impact on β-sheet amyloid fibril formation

The influence of the presence of small molecules and nanoparticles on the mechanism of amyloid fibril formation has attracted attention because amyloid protein fibrils are associated with degenerative diseases. Here, we studied the interaction between gold nanoparticles (AuNPs) and a model protein (lysozyme). Both the formation of amyloid fibrils in the presence of gold nanoparticles, as well as the interaction between lysozyme and the amyloid fibrils with AuNPs, were investigated to gain an understanding of the distinct behaviour of lysozyme in its fibrillar and globular form. It was observed that the presence of AuNPs delayed the unfolding of α-helixes present in the globular lysozyme and the formation of the amyloid fibrils. However, the addition of AuNPs was also associated with a larger amount of β-sheet structures in the system once equilibrium was reached. Furthermore, the results showed that the driving force of the interaction between AuNPs and lysozyme in its fibrillar and globular forms was significantly different, and that the interaction of AuNPs with the preformed lysozyme amyloid fibrils led to a structural change in the protein.

Formation of amyloid protein fibrils is associated with degenerative diseases. Here, the interaction mechanism between globular and fibrillar proteins with AuNPs were investigated in order to potentially control and reverse the fibrillation process.