Additional supra-self-assembly of human serum albumin under amyloid-like-forming solution conditions

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.

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.

Thermally induced conformational changes and protein–protein interactions of bovine serum albumin in aqueous solution under different pH and ionic strengths as revealed by SAXS measurements

Temperature-induced oligomerization of albumin before and after protein melting was studied using SAXS and interpreted in terms of interaction potential.

Prion like behavior of HSA-hydroxylated MWCNT interface

Carbon nanotubes (CNTs) with unique and outstanding properties were expected to revolutionize various aspects of the biomedical sector. Interaction studies of proteins with functionalized CNTs would shed light on their toxicological aspects upon entering the human body. Hyperchromicity of the UV-Visible spectra and declining fluorescence potential of HSA on interaction with CNTs suggested ground state complex to exist between them. Synchronous and 3D spectral features of CNT-HSA system proposed their possible binding site to occur nearby Trp and Tyr residues. FTIR and FT-Raman spectra showed a shift in the amide band region that proportionate the possible alteration to occur in the alpha-helical structures. CD far and near spectra showed loss of alpha-helical structures and shift in the Trp position of the polypeptide backbone. CNT's UV and FTIR band showed shift on interaction with HSA, which conveys the possible aggregation of CNTs in the presence of protein. The promoting role of CNTs against HSA fibril formation has been confirmed by spectroscopic and microscopic evaluations. Secondary conformational changes, besides the existence of increased beta-sheet structures of HSA amyloid fibrils, remain similar to the amyloid behavior of Prion protein. Hence, HSA fibril-CNT interface predominates the possible mechanism for several amyloid-related disorders concerning their toxic accumulations in the body.

Appearance of annular ring-like intermediates during amyloid fibril formation from human serum albumin

The self-assembly of proteins triggered by a conformational switch into highly ordered β-sheet rich amyloid fibrils has captivated burgeoning interest in recent years due to the involvement of amyloids in a variety of human diseases and a diverse range of biological functions. Here, we have investigated the mechanism of fibrillogenesis of human serum albumin (HSA), an all-α-helical protein, using an array of biophysical tools that include steady-state as well as time-resolved fluorescence, circular dichroism and Raman spectroscopy in conjunction with atomic force microscopy (AFM). Investigations into the temporal evolution of nanoscale morphology using AFM revealed the presence of ring-like intermediates that subsequently transformed into worm-like fibrils presumably by a ring-opening mechanism. Additionally, a multitude of morphologically-diverse oligomers were observed on the pathway to amyloid formation. Kinetic analysis using multiple structural probes in-tandem indicated that HSA amyloid assembly is a concerted process encompassing a major structural change that is primarily mediated by hydrophobic interactions between thermally-induced disordered segments originating in various domains. A slower growth kinetics of aggregates suggested that the protein structural reorganization is a prerequisite for fibril formation. Moreover, time-dependent Raman spectroscopic studies of HSA aggregation provided key molecular insights into the conformational transitions occurring within the protein amide backbone and at the residue-specific level. Our data revealed the emergence of conformationally-diverse disulfides as a consequence of structural reorganization and sequestration of tyrosines into the hydrophobic amyloid core comprising antiparallel cross β-sheets.

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.

Oxidation enhances human serum albumin thermal stability and changes the routes of amyloid fibril formation

Oxidative damages are linked to several aging-related diseases and are among the chemical pathways determining protein degradation. Specifically, interplay of oxidative stress and protein aggregation is recognized to have a link to the loss of cellular function in pathologies like Alzheimer's and Parkinson's diseases. Interaction between protein and reactive oxygen species may indeed induce small changes in protein structure and lead to the inhibition/modification of protein aggregation process, potentially determining the formation of species with different inherent toxicity. Understanding the temperate relationship between these events can be of utmost importance in unraveling the molecular basis of neurodegeneration. In this work, we investigated the effect of hydrogen peroxide oxidation on Human Serum Albumin (HSA) structure, thermal stability and aggregation properties. In the selected conditions, HSA forms fibrillar aggregates, while the oxidized protein undergoes aggregation via new routes involving, in different extents, specific domains of the molecule. Minute variations due to oxidation of single residues affect HSA tertiary structure leading to protein compaction, increased thermal stability, and reduced association propensity.

Tracking the heterogeneous distribution of amyloid spherulites and their population balance with free fibrils

The analysis of amyloidogenic systems reveals the appearance of distinct states of aggregation for amyloid fibrils. For different proteins and under specific experimental conditions, amyloid spherulites are recognized as a significant component occurring in several protein model systems used for in vitro fibrillation studies. In this work we have developed an approach to characterize solutions containing a mixture of amyloid spherulites and individual fibrils. Using bovine insulin as the model system, sedimentation kinetics for the amyloid aggregates were followed using a combination of UV-Vis spectroscopy and cross-polarized optical microscopy. Spherulites were identified as the species undergoing sedimentation. A simple mathematical approach allows the description of the kinetics in terms of decay time/rate distribution. Moreover, based on the sedimentation kinetics, a rough estimate of the balance between amyloid spherulites and individual fibrils can be provided. Fitting the experimental data with the proposed physico-chemical approach shows self-consistent results in reasonable agreement with quantitative imaging analysis previously reported. Our results provide new physical insights into the analysis of amyloidogenic systems, providing a method to characterize the heterogeneous distribution of amyloid spherulites and simultaneously distinguish spherulites and free fibril populations. Importantly, the method can be generally applied to the characterization of polydisperse solutions containing optically traceable spherical particles in the micrometric range.

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

The aggregation process in proteins is governed by several factors such as temperature, pH, presence of electrolytes, denaturants, and metal ions. Here, we report the role of Cu(II) in inducing rapid fibrillation in human serum albumin. We have monitored this process via UV-vis spectroscopy, fluorescence spectroscopy, circular dichroism, zeta-potential measurements, electron paramagnetic resonance studies, fluorescence microscopy, and field emission scanning electron microscopy. Images show a fibrillar network of human serum albumin in the presence of Cu(II) in 60% ethanol incubated at 65 degrees C at physiological pH. All other studies also support the enhanced fibrillation in presence of Cu(II).

Spherulites of amyloid-beta42 in vitro and in Alzheimer's disease

Several amyloidogenic proteins including insulin, beta-lactoglobulin, and albumin form spherulites in vitro under non-physiological conditions. These micrometer-sized, roughly spherical structures are composed of ordered arrays of amyloid fibrils in radial arrangements which, characteristically, show a typical Maltese cross pattern of light extinction under the polarizing microscope. The physiological significance of amyloid spherulites is unknown though in Alzheimer's disease, senile plaques composed primarily of beta sheets of amyloid-beta (Abeta)42 have, very occasionally, been shown to give a Maltese cross pattern of light extinction under crossed polarizers. Herein we describe the first observation of the formation in vitro of spherulites of Abeta42. They were formed under near-physiological conditions in which the beta sheet conformation of pre-formed aggregates of Abeta42 had been abolished following the addition of an excess of copper. Incubation of these preparations at 37 degrees C for up to 9 months resulted in the formation of globular structures, 5-20 microm in diameter, which exhibited a Maltese cross pattern of light extinction typical of spherulites. Near-identical spherulitic structures were also observed in abundance in 30 microm thick sections of Alzheimer's disease brain tissue. Synchrotron x-ray fluorescence showed that the location of these spherulites in AD tissue coincided with locally elevated concentrations of tissue copper. The formation in vitro of spherulites of Abeta42 which morphologically appeared analogous to spherulitic structures observed in vivo strongly supports the hypothesis that spherulites and senile plaques in AD tissue are one and the same structures and that their ultimate formation may involve copper.