Fibrillation in human serum albumin is enhanced in the presence of copper(II)

Invitro anti-biofilm activity and the artificial chaperone activity of quinoline-based ionic liquids

The maintenance of protein conformation under stressful conditions is one of the prevailing challenges. This has led to a rapid growth in the ingenious protein therapies, in the past few decades, prioritizing the investigation of the structure and function of proteins in novel environments. Ionic Liquids (ILs) are currently dominating the biomedical industry, by endowing great solubility and stability to bio-molecules, especially proteins. Recently, researchers have devoted their attention towards the artificial chaperone activity of several classes of ILs. Thus, comprehending the long-term as well as momentary stability of protein conformation in IL formulations is an absolute necessity. In this context, we present the activity of quinoline-based ionic liquids (ILs) as artificial cheperones against time-dependent, self induced fibril formation in Bovine Serum Albumin (BSA). Herein, a series of quinoline-based ILs were synthesized and characterized. The structural and morphological changes induced in BSA in the presence and absence of these ILs are corroborated using several spectroscopic measurements and in-silico studies. The anti-microbial and antibiofilm activity of these compounds demonstrating their medicinal properties is substantiated in this study. Furthermore, the present research also gives an account of the toxicity of these compounds under in vivo conditions, using C. elegans as the model organism.

Anti-Aggregative and Protective Effects of Vicenin-2 on Heat and Oxidative Stress-Induced Damage on Protein Structures

Vicenin-2, a flavonoid categorized as a flavones subclass, exhibits a distinctive and uncommon C-glycosidic linkage. Emerging evidence challenges the notion that deglycosylation is not a prerequisite for the absorption of C-glycosyl flavonoid in the small intestine. Capitalizing on this experimental insight and considering its biological attributes, we conducted different assays to test the anti-aggregative and antioxidant capabilities of vicenin-2 on human serum albumin under stressful conditions. Within the concentration range of 0.1-25.0 μM, vicenin-2 effectively thwarted the heat-induced HSA fibrillation and aggregation of HSA. Furthermore, in this study, we have observed that vicenin-2 demonstrated protective effects against superoxide anion and hydroxyl radicals, but it did not provide defense against active chlorine. To elucidate the underlying mechanisms, behind this biological activity, various spectroscopy techniques were employed. UV-visible spectroscopy revealed an interaction between HSA and vicenin-2. This interaction involves the cinnamoyl system found in vicenin-2, with a peak of absorbance observed at around 338 nm. Further evidence of the interaction comes from circular dichroism spectrum, which shows that the formation of bimolecular complex causes a reduction in α-helix structures. Fluorescence and displacement investigations indicated modifications near Trp214, identifying Sudlow's site I, similarly to the primary binding site. Molecular modeling revealed that vicenin-2, in nonplanar conformation, generated hydrophobic interactions, Pi-pi stacking, and hydrogen bonds inside Sudlow's site I. These findings expand our understanding of how flavonoids bind to HSA, demonstrating the potential of the complex to counteract fibrillation and oxidative stress.

Biophysical insight into the anti-fibrillation potential of Glyburide for its possible implication in therapeutic intervention of amyloid associated diseases

Protein aggregation is an underlying cause of many neurodegenerative diseases. Also, the overlapping pathological disturbances between neurodegenerative diseases and type-2 diabetes mellitus have urged the scientific community to explore potential of already available anti-diabetic medications in impeding amyloid formation too. Recent study brief out promising potential of an anti-diabetic drug Glyburide(GLY) as an inhibitor of amyloid fibrillation utilizing several biophysical techniques, computational methods and imaging tools. The mechanism of interaction was elucidated and the structural alterations in human serum albumin(HSA) as well as the microenvironment changes of its fluorophores(tryptophan, tyrosine) upon interacting with GLY were studied by spectroscopic techniques like Circular dichroism and synchronous fluorescence. Binding studies detailing about the GLY-HSA complex distance and the energy transfer efficiency was obtained by Fluorescence resonance energy transfer. For aggregation inhibition studies, the existence and size of aggregates formed in HSA and their inhibition by GLY was determined by Turbidity assay, Dynamic light scattering and Rayleigh light scattering along with dye binding assays. The ThT kinetics measurements analysis suggested that GLY deaccelerates fibrillation by decrement of apparent rate(K_app) constant. The inhibitory effect of GLY might be attributed to native structure stabilization of HSA by obstruction into β-sheet conversion as confirmed by CD spectroscopy results. Amyloid inhibition and suppression of amyloid-induced hemolysis by GLY was further delineated by TEM and SEM analysis respectively. All these findings for the first time report the new facet of the anti-amyloidogenic potential of GLY, making it a promising candidate to treat neurodegenerative diseases too in the near future.

Micro-Raman spectroscopic analysis of liquid-liquid phase separation

Liquid-liquid phase separation (LLPS) plays a significant role in various biological processes, including the formation of membraneless organelles and pathological protein aggregation. Although many studies have found various factors that modulate the LLPS process or the liquid-to-solid phase transition (LSPT) using microscopy or fluorescence-based methods, the molecular mechanistic details underlying LLPS and protein aggregation within liquid droplets remain uncharacterized. Therefore, structural information on proteins inside liquid droplets is required to understand the mechanistic link to amyloid formation. In the present study, we monitored droplet formation related to protein fibrillation using micro-Raman spectroscopy in combination with differential interference contrast (DIC) microscopy to study the conformational change in proteins and the hydrogen-bonding (H-bonding) structure of water during LLPS. Interestingly, we found that the O-D stretching band for water (HOD in H₂O) inside the droplets exhibited a distinct Raman spectrum from that of the bulk water, suggesting that the time-dependent change in the hydration environment in the protein droplets during the process of LLPS can be studied. These results demonstrate that the superior spatial resolution of micro-Raman spectroscopy offers significant advantages in investigating the molecular mechanisms of LLPS and following LSPT processes.

Hen lysozyme fibrillogenesis, molten globule intermediate and effect of copper salts

The amyloid fibres have been related to many diseases. The molten globule intermediate has been proposed to form part of the folding pathway of many proteins. In the present study, we investigated the mechanism of amyloid-fibres formation of hen egg-white lysozyme (HEWL) incubated in a potassium phosphate buffer, pH 11.8, 100 mM, at 37 °C for 30 h, and evaluated the influence of Cu(II) present in two salts (CuSO₄ and CuCl₂) during fibrillogenesis. Co-incubation and post-incubation of lysozyme with copper salts reduced the fluorescence signal of thioflavin T with an increment in the intrinsic fluorescence of the protein. The ANS fluorescence test showed that incubation of HEWL for 6 h generated a molten globule intermediate state that formed amyloid fibres when incubation was carried out for a 30-h timespan. Dynamic light scattering showed a heterogeneous population of states in samples incubated in the absence or the presence of salts during the fibrillation process. The existence of a reducing potential was verified during the formation of HEWL amyloid fibres with the bathocuproine disulphonate test. Transmission electron microscopy confirmed the presence and absence of fibres in solutions incubated with and without Cu(II). This work demonstrated that lysozyme formed amyloid fibres at 37 °C and copper inhibited its formation.Communicated by Ramaswamy H. Sarma.

Human Serum Albumin Misfolding in Aging and Disease

Age-dependent conformational stability of human serum albumin was determined by the method of fluorescent bilayer liposome assay. After pre-heating at 80 °C, albumin in the sera of 74-year-old healthy subjects exhibited hydrophobic effects on liposomes and made liposomal membrane phospholipids more susceptible to hydrolysis by the lipolytic enzyme phospholipase A2. In contrast, albumin in the sera of 24-year-old individuals was stable at 80 °C and displayed no increased hydrophobic effects on liposomes. The results suggest that albumin in the sera of 74-year-old subjects is more easily converted to a misfolded form in which its protein structure is altered when compared to albumin in the sera of 24-year-old individuals. Misfolded albumin can lose its ability to carry out its normal homeostatic functions and may promote alterations in membrane integrity under inflammatory conditions. However, our investigation has limitations that include the lack of testing sera from large numbers of individuals across a broad range of age to validate our preliminary observations of age-dependent differences in albumin stability and its interactions with liposomes.

The albumin-based nanoparticle formation in relation to protein aggregation

Albumin is an attractive protein for the preparation of nanoparticle with possible therapeutic applications, due to its biodegradable, nontoxic, non-immunogenic, and metabolizable properties. Many studies have investigated the formation of albumin nanoparticles, generally by the desolvation or coacervation approaches. One of the most important parameters that should be considered in the formation of nanoparticles is their morphology (size and shape). There are many proposals to control the nanoparticle size, but it remains a challenge for researchers yet. In this study, we showed that control of BSA-based nanoparticles/microparticles size could be achieved by varying the temperature and pH and therefore controlling the rate of aggregation. The aggregation behavior was monitored by UV-Vis spectroscopy, SEM, and dye-binding assay. Our results provide more options for the size and shape control of BSA-based nanoparticle in natural buffer systems. The aggregation of BSA at different temperatures within the range of 50-80 °C were studied under the effect of different pHs in the range of 4.7-6.2. In this research, we found that protein aggregation under extreme conditions of pH and temperature, or at the pH near to pI appears to be amorphous, and at the pH above the pI seems to be the amyloid fibril structure. In some instances where the aggregation is neither too fast nor too slow, in the initial phase of the aggregation process, nanoparticle structures can be identified and separated by mechanistic approaches. This observation suggests that the best condition for monitoring the formation of albumin-based nanoparticles could be pH 5.7, 70 °C. Satisfactory rationalization of all aspects of our experimental observation requires further and more detailed study.

Cationic gemini surfactant stimulates amyloid fibril formation in bovine liver catalase at physiological pH. A biophysical study

Surfactant molecules stimulate amyloid fibrillation and conformational switching in proteins but the mechanisms by which they accomplish these effects are unclear. A cationic gemini surfactant, C₁₆C₄C₁₆Br₂, with two positively charged heads and two-16C hydrophobic tails induces the amyloid fibrillation of bovine liver catalase (BLC) in vitro at physiological pH. The BLC transformed into amyloid aggregates in the presence of low concentrations (2–150 μM) of C₁₆C₄C₁₆Br₂ at pH 7.4, as confirmed by the use of several biophysical techniques (Rayleigh light scattering (RLS), intrinsic fluorescence, thioflavin T fluorescence (ThT), far-UV circular dichroism, and transmission electron microscopy). The secondary structure of BLC also changed according to the concentration of C₁₆C₄C₁₆Br₂: the α-helical structure of BLC decreased in the presence of 2–100 μM of C₁₆C₄C₁₆Br₂ but at concentrations above 200 μM BLC regained a α-helical structure very similar to the native BLC. In silico molecular docking between BLC and C₁₆C₄C₁₆Br₂ suggest that the positively charged heads of the surfactant interact with Asp127 through attractive electrostatic interactions. Moreover, a Pi-cation electrostatic interaction and hydrophobic interactions also take place between the tails of the surfactant and BLC. The stability of the BLC–C₁₆C₄C₁₆Br₂ complex was confirmed by performing a molecular dynamics simulation and evaluating parameters such as root mean square deviation (RMSD), root mean square fluctuation (RMSF), radius of gyration (R_g), and solvent accessible surface area (SASA). Apart from its aggregation inducing properties, the gemini surfactant itself causes toxicity to the cancerous cell (A549): which is confirmed by MTT assay. This work delivers new insight into the effect of cationic gemini surfactants in amyloid aggregation and paves the way to the rational design of new anti-amyloidogenic agents.

Surfactant molecules stimulate amyloid fibrillation and conformational switching in proteins but the mechanisms by which they accomplish these effects are unclear.

Al (III) metal augment thermal aggregation and fibrillation in protein: Role of metal toxicity in neurological diseases

Protein fibrillation is a leading cause of innumerable neurodegenerative diseases. The exact underlying mechanism associated with the formation of fibrils is yet to be known. Recently, the role of metal ions resulting into fibrillation of proteins has gained attention of the scientific community. In this piece of work, we have investigated the effect of the aluminum (Al) metal ion on the kinetics of aggregation of bovine serum albumin (BSA) protein under physiological conditions by employing several biophysical and microscopic techniques. Quenching of tryptophan fluorescence was observed along with 9 nm blue shift, demonstrating BSA becomes more hydrophobic during unfolding pathway of thermal denaturation. Moreover, ANS (8-Anilino-1-naphthalene sulfonic acid) binding shows quenching in fluorescence intensity with increasing time of incubation at 65 °C, suggesting unfolding leading to the disruption of hydrophobic patches in BSA. Besides, Thioflavin T intensity indicated a significant acceleration in BSA fibrillation at a ratio of 1:1 and 1:2 of BSA and Al (III) metal ion respectively. In addition, circular dichroism (CD) spectroscopy study revealed the transition of BSA from α-helical conformation to the β-sheet rich structure. Molecular docking analysis demonstrated significant binding affinity (-1.2 kcal/mol) of Al (III) with BSA involving Phe501, Phe506, Val575, Thr578, Gln579, Leu531 residues. Transmission electron microscopy (TEM) reaffirm augmentation of thermal-induced BSA fibril formation in the presence of Al (III) metal ions. This study highlights the metal chelating potency as the possible therapeutic target for neurological diseases.

Studies on the effect of a Fupenzi glycoprotein on the fibrillation of bovine serum albumin and its antioxidant activity

In this study, the effect of a glycoprotein obtained from Fupenzi (FPZ) (Rubus chingii Hu.) on the fibrillation of bovine serum album (BSA) was investigated by multi-spectroscopic methods and transmission electron microscopy. Moreover, the cytotoxicity of the glycoprotein and the effect of it on H₂O₂-induced cell viability were investigated by cell counting kit and β-galactosidase kit, respectively. The experimental results indicated that the glycoprotein showed very low toxicity to NRK-52E cells and could obviously delay cell senescence and improve cell viability. Moreover, the glycoprotein could effectively inhibit the formation of BSA fibrils and destroy the stability of preformed BSA fibrils in a concentration-dependent manner. Generally, antioxidant capacities are thought to be related to the anti-amyloidogenic activity of inhibitors; therefore, to reveal the inhibitory mechanism, the anti-oxidative property of the glycoprotein was examined by DPPH and ABTS assays. The results demonstrated that FPZ glycoprotein had a remarkable antioxidant activity and the IC₅₀ values of DPPH and ABTS were 0.249 mg mL^-1 and 0.092 mg mL^-1, respectively. This work suggested that the FPZ glycoprotein had the potential to be designed a new therapeutic agent for attenuating aging and preventing the age-related diseases.

Aging-Related and Gender Specific Albumin Misfolding in Alzheimer's Disease

Aging-related protein misfolding and aggregation may play critical roles in the pathogenesis of numerous diseases. In the brain, extracellular aggregated amyloid-β (Aβ) is closely related to the death of neurons in individuals with Alzheimer’s disease (AD). Albumin-Aβ binding is important in preventing Aβ fibril aggregation. However, because albumin is the most abundant and important antioxidant in the circulation, aging-related oxidative stress could have a significant effect on the molecular conformation and binding capacities of albumin. To investigate the link between misfolded albumin and AD, we developed fluorescent assays to determine the effects of misfolded albumin on membrane integrity in the presence of a lipolytic, inflammatory response-like enzyme, secretory phospholipase A2 (sPLA2). We found that misfolded albumin increased degradation of phospholipids in highly fluid bilayer membranes in the presence of sPLA2 due to hydrophobic effects of misfolded albumin. High amounts of misfolded albumin were present in sera of elderly (average 74 years) versus young (average 24 years) subjects (p < 0.0001). Albumin in cerebrospinal fluid (CSF) of elderly subjects, though present in small concentrations, had a 2- to 3-fold increased capacity to promote sPLA2-catalyzed membrane phospholipid degradation as compared with the same amount of albumin in serum (p < 0.0001). In addition, the fatty acid binding capacity of albumin in CSF from female subjects was considerably lower than values obtained for men, especially for individuals diagnosed with AD (p = 0.0006). This study suggests that inflammation, misfolded albumin and/or other dysfunctional proteins, and changes in membrane fluidity could alter cell membrane integrity and homeostasis and contribute to the pathogenesis of aging-related dementia and AD.

Study of Nanofibrils Formation of Fibroin Protein in Specific Thermal and Acidity Conditions

Background:

Amyloid fibrils are insoluble arranged aggregates of proteins that are fibrillar in structure and related to many diseases (at least 20 types of illnesses) and also create many pathologic conditions. Therefore understanding the circumstance of fibril formation is very important

Objectives:

This study aims to work on fibrillar structure formation of fibroin (as a model protein)

Material and Methods:

In this experimental study, fibroin was extracted from bombyx mori silk cocoon, and the concentration was obtained by Bradford method. The protein was incubated in a wide range of times (0 min to 7 days) in specific acidity and thermal conditions (pH=1.6, T=70 °C). The assays of UV-vis spectroscopy with congo red, field emission scanning electron microscopy, transmission electron microscopy, atomic force microscopy and circular dichroism spectroscopy were employed to monitor the fibrillation process.

Results:

Fibroin assemblies were formed upon the process of aggregation and fibril formation with a variety of morphology ranging from nanoparticles to elongated fibrils.

Conclusion:

The results showed progressive pathway of fibril formation

Probing the Nongeneralized Amyloid Inhibitory Mechanism of Hydrophobic Chaperone

Increasing prevalence of protein misfolding disorders urges the search for effective therapies. Although several antiaggregation molecules have been identified, their molecular process of aggregation and clinical trials are underway. The present study is focused on the mechanism through which phenyl butyrate (PB), a chemical chaperone, triggers inhibition of human serum albumin (HSA) fibrillation. Turbidity and Rayleigh light scattering (RLS) measurements reveal the marked presence of aggregates in HSA that were confirmed as amyloid fibrils by thioflavin T (ThT) and Congo red (CR) and were subsequently inhibited by PB in a dose dependent manner. ThT fluorescence kinetics reveals a decrease in the apparent rate constant, K_app, in the presence of PB without triggering a lag phase in HSA suggesting PB's interference with the elongation phase. Dynamic light scattering (DLS) results display a reduction in the aggregate size in the presence of PB. Isothermal titration calorimetry (ITC) data reveals strong binding of PB at site II both at 25 °C (K_b ≈ 1.94 × 10⁵ M^-1) and 65 °C (K_b ≈ 2.90 × 10⁴ M^-1), mediated by hydrogen bonding. Overall, our finding establishes that PB stabilizes partially unfolded HSA molecules through hydrogen bonding, thereby preventing establishment of hydrogen bonds between them and hindering their progression into amyloid fibrils. This is in contrast to its chaperone effect manifested with other proteins.

Human Serum Albumin Aggregation/Fibrillation and its Abilities to Drugs Binding

Human serum albumin (HSA) is a protein that transports neutral and acid ligands in the organism. Depending on the environment's pH conditions, HSA can take one of the five isomeric forms that change its conformation. HSA can form aggregates resembling those in vitro formed from amyloid at physiological pH (neutral and acidic). Not surprisingly, the main goal of the research was aggregation/fibrillation of HSA, the study of the physicochemical properties of formed amyloid fibrils using thioflavin T (ThT) and the analysis of ligand binding to aggregated/fibrillated albumin in the presence of dansyl-l-glutamine (dGlu), dansyl-l-proline (dPro), phenylbutazone (Phb) and ketoprofen (Ket). Solutions of human serum albumin, both non-modified and modified, were examined with the use of fluorescence, absorption and circular dichroism (CD) spectroscopy. The experiments conducted allowed observation of changes in the structure of incubated HSA (HSA_INC) in relation to nonmodified HSA (HSA_FR). The formed aggregates/fibrillation differed in structure from HSA monomers and dimers. Based on CD spectroscopy, previously absent βstructural constructs have been registered. Whereas, using fluorescence spectroscopy, the association constants differing for fresh and incubated HSA solutions in the presence of dansyl-amino acids and markers for binding sites were calculated and allowed observation of the conformational changes in HSA molecule.

Assembly of Gold Nanorods on HSA Amyloid Fibrils to Develop a Conductive Nanoscaffold for Potential Biomedical and Biosensing Applications

Today, Gold Nanorods have promised variety of applications in conjugation with biomolecules of interest. Discovery of functional amyloids has also been highlighted with possible use in designing high performance materials. To exploit dual properties of both Nano and Bio counterparts in new functional materials, this effort has focused on synthesis of a potential hybrid system of Gold nanorods (GNRs) and HSA amyloid fibrils to develop a conductive nanoscaffold. UV-Vis spectroscopy, Thioflavin T (ThT) assay, Far-UV Circular Dichroism (CD) spectropolarimetry, fluorescence and Transmission Electron microscopy were used to characterize formation of the nanostructures and amyloid fibrils. Surface plasmon resonance of GNRs was also monitored upon interaction with HSA amyloid fibrils, showing that the plasmonic component of the hybrid system has maintained its characteristic rod morphology without any perturbations. Analysis of Nyquist plots for the hybrid nanoscaffold showed that the electronic behavior of the hybrid system has been enhanced due to the presence of the assembled GNRs. Results of this investigation highlight the possibility of fabricating hybrid nano-bioscaffolds as promising candidates in versatile biomedical and biosensing applications.

Morphological Effects of CuO Nanostructures on Fibrillation of Human Serum Albumin

The influence of different morphologies of nanostructures on amyloid fibrillation has been investigated by monitoring the fibrillation of human serum albumin (HSA) in the presence of rod-, sphere-, flower-, and star-shaped copper oxide (CuO) nanostructures. The different morphologies of CuO have been synthesized from an aqueous solution-based precipitation method using various organic acids, viz., acetic acid, citric acid, and tartaric acid. The fibrillation process of HSA has been examined using various biophysical techniques, e.g., Thioflavin T fluorescence, Congo red binding studies through UV spectroscopy, circular dichroism spectroscopy, and fluorescence microscopy. The monolayer protein coverage on the CuO nanostructures has been established through DLS studies, and the well-fitted Langmuir isotherm model has been used to interpret the differential adsorption behavior of HSA molecules on the CuO nanostructures. The nanostar-shaped CuO, by virtue of their higher specific surface area (94.45 m² g^-1), presence of high indexed facets {211} and high positive surface charge potential (+16.2 mV at pH 7.0) was found to show the highest adsorption of the HSA monomers and thus was more competent to inhibit the formation of HSA fibrils compared to the other nanostructures of CuO.

Al cation induces aggregation of serum proteins

Al cation is known to induce protein fibrillation and causes several neurodegenerative disorders. We report the spectroscopic, thermodynamic analysis and AFM imaging for the Al cation binding process with human serum albumin (HSA), bovine serum albumin (BSA) and milk beta-lactoglobulin (b-LG) in aqueous solution at physiological pH. Hydrophobicity played a major role in Al-protein interactions with more hydrophobic b-LG forming stronger Al-protein complexes. Thermodynamic parameters ΔS, ΔH and ΔG showed Al-protein bindings occur via hydrophobic and H-bonding contacts for b-LG, while van der Waals and H-bonding interactions prevail in HSA and BSA adducts. AFM clearly indicated that aluminum cations are able to force BSA and b-LG into larger or more robust aggregates than HSA, with HSA 4±0.2 (SE, n=801) proteins per aggregate, for BSA 17±2 (SE, n=148), and for b-LG 12±3 (SE, n=151). Thioflavin T test showed no major protein fibrillation in the presence of Al cation. Al complexation induced major alterations of protein conformations with the order of perturbations b-LG>BSA>HSA.

Aggregation of trypsin and trypsin inhibitor by Al cation

Al cation may trigger protein structural changes such as aggregation and fibrillation, causing neurodegenerative diseases. We report the effect of Al cation on the solution structures of trypsin (try) and trypsin inhibitor (tryi), using thermodynamic analysis, UV-Visible, Fourier transform infrared (FTIR) spectroscopic methods and atomic force microscopy (AFM). Thermodynamic parameters showed Al-protein bindings occur via H-bonding and van der Waals contacts for trypsin and trypsin inhibitor. AFM showed that Al cations are able to force trypsin into larger or more robust aggregates than trypsin inhibitor, with trypsin 5±1 SE (n=52) proteins per aggregate and for trypsin inhibitor 8.3±0.7 SE (n=118). Thioflavin T test showed no major protein fibrillation in the presence of Al cation. Al complexation induced more alterations of trypsin inhibitor conformation than trypsin.

The interaction and binding of flavonoids to human serum albumin modify its conformation, stability and resistance against aggregation and oxidative injuries

BACKGROUND

Interactions of ligands with proteins imply changes in the properties of the macromolecules that may deeply modify their biological activities and conformations and allow them to acquire new and, sometimes, unexpected abilities. The flavonoid phloretin has several pharmacological properties that are starting to be elucidated, one of which is the well-known inhibition of glucose transport.

METHODS

The interactions of phloretin to human serum albumin have been investigated by fluorescence, UV-visible, FTIR spectroscopy, native electrophoresis, protein ligand docking studies, fluorescence and scanning electron microscopy.

RESULTS

Spectroscopic investigations suggest that the flavonoid binds to human serum albumin inducing a decrease in α-helix structures as shown by deconvolution of FTIR Amide I' band. Fluorescence and displacement studies highlight modifications of environment around Trp214 with the primary binding site located in the Sudlow's site I. In the hydrophobic cavity of subdomain IIA, molecular modeling studies suggest that phloretin is in non-planar conformation and hydrogen-bonded with Ser202 and Ser454. These changes make HSA able to withstand protein degradation due to HCLO and fibrillation.

GENERAL SIGNIFICANCE

Our work aims to open new perspectives as far as the binding of flavonoids to HSA are concern and shows as the properties of both compounds can be remarkable modified after the complex formation, resulting, for instance, in a protein structure much more resistant to oxidation and fibrillation. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Effect of sodium salicylate and sodium deoxycholate on fibrillation of bovine serum albumin: comparison of fluorescence, SANS and DLS techniques

Adsorbed sodium salicylate (NaSal) and sodium deoxycholate (NaDC) retard the thermal denaturation of bovine serum albumin.

Protein Misfolding in Lipid-Mimetic Environments

Among various cellular factors contributing to protein misfolding and subsequent aggregation, membranes occupy a special position due to the two-way relations between the aggregating proteins and cell membranes. On one hand, the unstable, toxic pre-fibrillar aggregates may interact with cell membranes, impairing their functions, altering ion distribution across the membranes, and possibly forming non-specific membrane pores. On the other hand, membranes, too, can modify structures of many proteins and affect the misfolding and aggregation of amyloidogenic proteins. The effects of membranes on protein structure and aggregation can be described in terms of the "membrane field" that takes into account both the negative electrostatic potential of the membrane surface and the local decrease in the dielectric constant. Water-alcohol (or other organic solvent) mixtures at moderately low pH are used as model systems to study the joint action of the local decrease of pH and dielectric constant near the membrane surface on the structure and aggregation of proteins. This chapter describes general mechanisms of structural changes of proteins in such model environments and provides examples of various proteins aggregating in the "membrane field" or in lipid-mimetic environments.

Modulation of bovine serum albumin fibrillation by ester bonded and conventional gemini surfactants

Modulation of bovine serum albumin fibrillation by gemini surfactants.

Sodium dodecyl sulphate modulates the fibrillation of human serum albumin in a dose-dependent manner and impacts the PC12 cells retraction

Protein aggregation is impacted by many factors including temperature, pH, and the presence of surfactants, electrolytes, and metal ions. The addition of sodium dodecyl sulphate (SDS) at different concentrations may play a significant role in the human serum albumin (HSA) fibrillation pathway. Here the heat induction of HSA fibrillation incubated with different concentrations of SDS was evaluated using a variety of techniques. These included ThT fluorescence, Congo red absorbance, circular dichroism, dynamic light scattering, and atomic force microscopy (AFM). To explore HSA surface properties, the surface tension of solutions was measured using Du Noüy Ring method tensiometry. In addition, the criteria of neurite outgrowth and complexity were monitored by exposing PC12 cells to different forms of HSA amyloid intermediates. ThT fluorescence kinetic studies indicated that SDS at low concentrations induced more fibrillation of HSA, while SDS at high concentrations inhibited the fibrillation of HSA. At higher SDS concentrations hydrophobic forces had a significant role whereas at lower SDS concentrations electrostatic forces were dominant. The cell culture studies demonstrated the significant impact of SDS concentration on HSA fibrillation and subsequent neuronal cell morphology. The HSA incubated with low concentrations of SDS inhibited neurite outgrowth and complexity of the PC12 cells, whereas high concentrations of SDS had lesser effect. Thus, SDS acts as a salt at lower concentrations, while at higher concentrations acts as a chaperon, with significant impact on fibrillation of HSA.

Effect of (-)-epigallocatechin gallate on the fibrillation of human serum albumin

Human serum albumin (HSA), the most abundant plasma protein in the human body is known to form fibrils under partial denaturing conditions. Natural polyphenols are known to interact with HSA and some polyphenols have been shown to be potent inhibitors of amyloid fibrillation. (-)-Epigallocatechin gallate (EGCG), the major component of green tea is known to inhibit amyloid fibrillation. In this report, we have investigated the effect of EGCG on native HSA as well as on the fibrillation process of HSA from amide III band analysis of their respective visible Raman spectra. The differential role of the tryptophan (Trp214) residue present in domain II of HSA in the absence and presence of EGCG has been pointed out using fluorescence anisotropy and visible Raman spectroscopy.

Copper(II) directs formation of toxic amorphous aggregates resulting in inhibition of hen egg white lysozyme fibrillation under alkaline salt-mediated conditions

Hen egg white lysozyme (HEWL) adopts a molten globule-like state at high pH (~12.75) and is found to form amyloid fibrils at alkaline pH. Here, we report that Cu(II) inhibits self-association of HEWL at pH 12.75 both at 37 and 65 °C. A significant reduction in Thioflavin T fluorescence intensity, attenuation in β-sheet content and reduction in hydrophobic exposure were observed with increasing Cu(II) stoichiometry. Electron paramagnetic resonance spectroscopy suggests a 4N type of coordination pattern around Cu(II) during fibrillation. Cu(II) is also capable of altering the cytotoxicity of the proteinaceous aggregates. Fibrillar species of diverse morphology were found in the absence of Cu(II) with the generation of amorphous aggregates in the presence of Cu(II), which are more toxic compared to the fibrils alone.

New insight into the binding interaction of hydroxylated carbon nanotubes with bovine serum albumin

In order to understand the effects of carbon nanotubes on the structural stability of proteins, the ligand-binding ability, fibrillation, and chemical denaturation of bovine serum albumin in the presence of a multi-walled hydroxylated carbon nanotubes (HO-MWCNTs) was characterized by UV-vis, circular dichroism, fluorescence spectroscopy and molecule modeling methods at the molecular level. The experiment results indicated that the fluorescence intensity of BSA was decreased obviously in presence of HO-MWCNTs. The binding interaction of HO-MWCNTs with BSA led to the secondary structure changes of BSA. This interaction could not only affect the ligand-binding ability of BSA, but also change the rate of fibrillation and denaturation of BSA. This work gave us some important information about the structures and properties of protein induced by carbon nanotubes.

Distribution of protein Ramachandran psi (ψ) angle using non-resonance visible raman scattering measurements

Knowing the distribution of Ramachandran angles helps in understanding peptide and protein backbone conformation. Empirical relations are proposed to correlate the spectral profile of the amide III3 band, obtained from ultraviolet resonance Raman measurements (UVRR), with the Ramachandran dihedral psi angle distribution in small peptide and protein molecules, in different environmental conditions (Mikhonin et al. J. Phys. Chem. B 2006, 110, 1928-1943). It has also been used for more complicated structures, like large globular proteins and protein fibrils. In our work here, we use visible Raman spectra and available empirical relations to obtain similar correlations for human serum albumin, hen egg white lysozyme, and human gamma crystallin. We also report the dihedral angle distribution in fibrils and a denatured protein in an ethanol environment using the same spectroscopic technique.

Physicochemical characteristics of protein-NP bioconjugates: the role of particle curvature and solution conditions on human serum albumin conformation and fibrillogenesis inhibition

Gold nanoparticles (Au NPs) from 5 to 100 nm in size synthesized with HAuCl(4) and sodium citrate were complexed with the plasma protein human serum albumin (HSA). Size, surface charge, and surface plasmon bands of the Au NPs are largely modified by the formation of a protein corona via electrostatic interactions and hydrogen bonding as revealed by thermodynamic data. Negative values of the entropy of binding suggested a restriction in the biomolecule mobility upon adsorption. The structure of the adsorbed protein molecules is slightly affected by the interaction with the metal surface, but this effect is enhanced as the NP curvature decreases. Also, it is observed that the protein molecules adsorbed onto the NP surface are more resistant to complete thermal denaturation than free protein ones as deduced from the increases in the melting temperature of the adsorbed protein. Differences in the conformations of the adsorbed protein molecules onto small (<40 nm) and large NPs were observed on the basis of ζ-potential data and FTIR spectroscopy, also suggesting a better resistance of adsorbed protein molecules to thermal denaturing conditions. We think this enhanced protein stability is responsible for a reduced formation of HSA amyloid-like fibrils in the presence of small Au NPs under HSA fibrillation conditions.

Modifications of ribonuclease A induced by p-benzoquinone

The nature of ribonuclease A (RNase) modifications induced by p-benzoquinone (pBQ) was investigated using several analysis methods. SDS-PAGE experiments revealed that pBQ was efficient in producing oligomers and polymeric aggregates when RNase was incubated with pBQ. The fluorescence behavior and anisotropy changes of the modified RNase were monitored for a series of incubation reactions where RNase (0.050 mM) was incubated with pBQ (0.050, 0.25, 0.50, 1.50 mM) at 37 °C in phosphate buffer (pH 7.0, 50 mM). The modified RNase exhibited less intense fluorescence and slightly higher anisotropy than the unmodified RNase. UV-Vis spectroscopy indicated that pBQ formed covalent bonds to the modified RNase. Confocal imaging analysis confirmed the formation of the polymeric RNase aggregates with different sizes upon exposure of RNase to high concentrations of pBQ. The interaction between the modified RNase and salts affecting biomineralization of salts was also investigated by scanning electron microscopy. Overall, our results show that pBQ can induce formation of both RNase adducts and aggregates thus providing a better understanding of its biological activity.