Thermal aggregation of β-lactoglobulin in presence of metal ions

Carbohydrate-Derived Polytriazole Nanoparticles Enhance the Anti-Inflammatory Activity of Cilostazol

Poly(amide-triazole) and poly(ester-triazole) synthesized from d-galactose as a renewable resource were applied for the synthesis of nanoparticles (NPs) by the emulsification/solvent evaporation method. The NPs were characterized as stable, spherical particles, and none of their components, including the stabilizer poly(vinyl alcohol), were cytotoxic for normal rat kidney cells. These NPs proved to be useful for the efficient encapsulation of cilostazol (CLZ), an antiplatelet and vasodilator drug currently used for the treatment of intermittent claudication, which is associated with undesired side-effects. In this context, the nanoencapsulation of CLZ was expected to improve its therapeutic administration. The carbohydrate-derived polymeric NPs were designed taking into account that the triazole rings of the polymer backbone could have attractive interactions with the tetrazole ring of CLZ. The activity of the nanoencapsulated CLZ was measured using a matrix metalloproteinase model in a lipopolysaccharide-induced inflammation system. Interestingly, the encapsulated drug exhibited enhanced anti-inflammatory activity in comparison with the free drug. The results are very promising since the stable, noncytotoxic NP systems efficiently reduced the inflammation response at low CLZ doses. In summary, the NPs were obtained through an innovative methodology that combines a carbohydrate-derived synthetic polymer, designed to interact with the drug, ease of preparation, adequate biological performance, and environmentally friendly production.

Investigation of bovine serum albumin aggregation upon exposure to silver(i) and copper(ii) metal ions using Zetasizer

Depending upon the metal coordination capacity and the binding sites of proteins, interaction between metal and proteins leads to a number of changes in the protein molecule which may include the change in conformation, unfolding, overall charge, and aggregation in some cases. In this study, Cu(ii) and Ag(i) metal ions were selected to investigate aggregation of bovine serum albumin (BSA) molecule upon interaction by measuring the size and charge of the aggregates using nano-Zetasizer instrument. Two concentrations of metal ions were made to interact with a specific concentration of BSA and the size and zeta potential of BSA aggregates were measured from 0 min upto 18 h. The Cu(ii) and Ag(i) metal ions showed almost similar behavior in inducing the BSA aggregation and the intensity of peak corresponding to the normal-sized protein decreased with time, whereas the peak corresponding to the protein aggregate increased. However, the effect on zeta potential of the aggregates was observed to be different with both metal ions. The aggregation of protein due to interaction of different metal ions is important to study as it gives insight to the pathogenesis of many neurological disorders and would result in developing effective ways to limit their exposure.

Change in conformational, digestive and immunological characteristics of bovine allergen β-lactoglobulin induced by metal ions in combination with heating

Aggregation of bovine β-lactoglobulin is affected easily by external factors. In this study, effects of metal ions combining with temperature on aggregation of β-lactoglobulin were explored. The conformational characteristics of aggregates were detected by environment scanning electron microscope, CD spectrum and free sulfhydryl group, respectively. Digestive and immunological characteristics were assessed by simulated digestion in vitro and ELISA respectively. The results showed that the morphology of β-lactoglobulin aggregates became more amorphous in Cu²⁺ and Mg²⁺ treated samples and more constricted in Zu²⁺-induced protein. Among them, Cu²⁺ altered the secondary structure of β-lactoglobulin aggregates and free sulfhydryl content most as well as that in gastric digestion. However, all ion-treated groups had similar digestive stability in intestinal digestion. Specially, Ca²⁺ and Mg²⁺ made the antigenicity and potential allergenicity of β-lactoglobulin aggregates decrease, which helps us understand the role of metal ions in immunological characteristics.

Effects of free soluble iron on thermal aggregation of hemoglobin

Aggregation of hemoglobin is implicated in the presentation of diseases like sickle cell disease and thalassemia. Hallmark of the disease being imbalance in the production of globin chains leading to aggregation of excess globin chains and aberrant hemoglobins associated with the disease, broadly categorized as hemoglobinopathy. We have studied thermal aggregation of hemoglobin at 70 °C and pH 6.5 using light scattering, flow cytometry and optical microscopy and tried to investigate effects of few abundant soluble metal ions on such aggregation. Our study indicate that only iron, both in Fe2+ and Fe3+ forms, could inhibit hemoglobin aggregation and the extent of inhibition was 60% in presence of 100 mgL-1 FeCl3. Similar effect was not seen in lysozyme aggregation. Metal ions such as, Cu2+, Zn2+ and Ni2+ also did not have any significant effects on hemoglobin aggregation. Results show this important chaperone like behavior of free iron affecting the kinetics and yield of the aggregation process which could have important consequence in the extent of severity of such hematological diseases.

pH titration of β-lactoglobulin monitored by laser-based Mid-IR transmission spectroscopy coupled to chemometric analysis

A novel external cavity-quantum cascade laser (EC-QCL)-based setup for mid-IR transmission spectroscopy in the amide I and amide II region was employed for monitoring pH-induced changes of protein secondary structure. pH titration of β-lactoglobulin revealed unfolding of the native β-sheet secondary structure occurring at basic pH. Chemometric analysis of the dynamic IR spectra was performed by multivariate curve resolution-alternating least squares (MCR-ALS). Using this approach, spectral and abundance distribution profiles of the conformational transition were obtained. A proper post-processing procedure was implemented allowing to extract information about pure protein spectra and spurious signals that may interfere in the interpretation of the system. This work demonstrates the potential and versatility of the EC-QCL-based IR transmission setup for flow-through applications, benefitting from the high available optical path length.

In Situ Characterization of Hfq Bacterial Amyloid: A Fourier-Transform Infrared Spectroscopy Study

Hfq is a bacterial protein that regulates gene expression at the post-transcriptional level in Gram-negative bacteria. We have previously shown that Escherichia coli Hfq protein, and more precisely its C-terminal region (CTR), self-assembles into an amyloid-like structure in vitro. In the present work, we present evidence that Hfq unambiguously forms amyloid structures also in vivo. Taking into account the role of this protein in bacterial adaptation and virulence, our work opens possibilities to target Hfq amyloid self-assembly and cell location, with important potential to block bacterial adaptation and treat infections.

Peroxidase-like Catalytic Activity of Copper-Mediated Protein-Inorganic Hybrid Nanoflowers and Nanofibers of β-Lactoglobulin and α-Lactalbumin: Synthesis, Spectral Characterization, Microscopic Features, and Catalytic Activity

A free Cys-SH-containing protein, β-lactoglobulin (β-LG), and another protein not possessing the same, viz., apo-α-lactoglobulin (apo-α-LA), were used in studies to demonstrate the role of this amino acid, along with its secondary structure, in the formation of a protein dimer and a protein-inorganic hybrid nanoflower and in the creation of the peroxidase-like activity of the nanomaterials produced when the proteins were treated with varying Cu(2+) concentration under different pH conditions. An increase in the pH as well as the Cu(2+) mole ratio results in increasing dimer formation in case of β-LG due to the presence of free Cys121-SH, while the dimer is not formed in case of apo-α-LA under the same conditions. The role of Cys in the dimer formation has been demonstrated both by MALDI and sodium dodecyl sulfate-polyacrylamide gel electrophoresis studies. Both of the proteins exhibited changes in their secondary structures to different extents as a function of pH, and the structures were stabilized by Cu(2+) interactions, as studied by CD and fluorescence spectroscopy. The small and spherical nanoparticles formed at pH 7 with lower equivalents of Cu(2+) join together to form larger aggregates at higher equivalents of Cu(2+). For the same concentration at pH 9, both the aggregates and the nanoflowers were noticed. However, at pH 12, the Cu(2+) binding induces the formation of fibers along with the flowers. Both the nanoflowers and nanofibers exhibited peroxidase-like activity in a catalytic manner. Nanoflowers were also shown to detect phenol in the concentration range from 10 to 200 μM. The copper-induced nanobiomaterial obtained in the case of apo-α-LA also exhibited peroxidase-like activity. Thus, this paper deals with the green synthesis of copper-induced protein (β-LG/apo-α-LA)-inorganic hybrid nanomaterials that are important due to their applications as nanobiomaterials.

Curcumin inhibits the Al(iii) and Zn(ii) induced amyloid fibrillation of β-lactoglobulin in vitro

Accumulation of ordered protein aggregates (or amyloids) is responsible for several neurodegenerative diseases.

A brief overview of amyloids and Alzheimer's disease

Amyloid fibrils are self-assembled fibrous protein aggregates that are associated with a number of presently incurable diseases such as Alzheimer's and Parkinson's disease. Millions of people worldwide suffer from amyloid diseases. This review summarizes the unique cross-β structure of amyloid fibrils, morphological variations, the kinetics of amyloid fibril formation, and the cytotoxic effects of these fibrils and oligomers. Alzheimer's disease is also explored as an example of an amyloid disease to show the various approaches to treat these amyloid diseases. Finally, this review investigates the nanotechnological and biological applications of amyloid fibrils; as well as a summary of the typical biological pathways involved in the disposal of amyloid fibrils and their precursors.

Multifunctional 8-hydroxyquinoline-appended cyclodextrins as new inhibitors of metal-induced protein aggregation

Mounting evidence suggests a pivotal role of metal imbalances in protein misfolding and amyloid diseases. As such, metal ions represent a promising therapeutic target. In this context, the synthesis of chelators that also contain complementary functionalities to combat the multifactorial nature of neurodegenerative diseases is a highly topical issue. We report two new 8-hydroxyquinoline-appended cyclodextrins and highlight their multifunctional properties, including their Cu(II) and Zn(II) binding abilities, and capacity to act as antioxidants and metal-induced antiaggregants. In particular, the latter property has been applied in the development of an effective assay that exploits the formation of amyloid fibrils when β-lactoglobulin A is heated in the presence of metal ions.

Characterization of the nucleation process of lysozyme at physiological pH: primary but not sole process

We report on a kinetic study of the heat-induced aggregation process of lysozyme at physiological pH. The time evolution of the aggregation extent and the conformational changes of the protein were followed by dynamic light scattering (DLS) and FTIR spectroscopy, respectively, whereas the morphology of the aggregates was observed by Atomic Force Microscopy (AFM). The conformational changes of the secondary and tertiary structures were simultaneous and distinct in time with respect to the formation of aggregates. Oligomer formation occurred through at least two different aggregation processes: a nucleation process and a homogeneous non-nucleative diffusion-controlled process. FTIR measurements showed that supramolecular aggregation proceeded without the formation of β-aggregated structures and AFM images revealed the presence of oligomers and amorphous aggregates; no fibrillar structures were observed.

Calorimetric and Spectroscopic investigations οf β-lactoglobulin upon interaction with copper ion

The effect of copper(II) ions (Cu+2) on the structure of β-lactoglobulin (β-lg) was investigated spectroscopically using UV-visible, fluorescence and circular dichroism (CD) and calorimetrically using isothermal titration calorimetry (ITC), at different temperatures. Results of the UV-visible studies showed that adding Cu+2 to β-lg solution caused increasing turbidity, indicative of protein aggregation. It was noticeable that the rate of increasing turbidity was directly proportional to increasing temperature. The far-UV CD studies displayed that the Cu+2 cannot induce any significant changes in the secondary structures of β-lg at different temperatures. Also, the ITC data indicated that the binding process of Cu+2 to β-lg is mainly entropically driven. The results highlight that copper ions cause the tertiary structure of β-lg to change and induce a slightly open structure leading to the formation of supramolecular aggregates in β-lg which may result in the reduced allergenicity of β-lg and its increased use in industrial applications.

Isoelectric focusing of small non-covalent metal species from plants

IEF is known as a powerful electrophoretic separation technique for amphoteric molecules, in particular for proteins. The objective of the present work is to prove the suitability of IEF also for the separation of small, non-covalent metal species. Investigations are performed with copper-glutathione complexes, with the synthetic ligand ethylenediamine-N,N'-bis(o-hydroxyphenyl)acetic acid (EDDHA) and respective metal complexes (Fe, Ga, Al, Ni, Zn), and with the phytosiderophore 2'-deoxymugineic acid (DMA) and its ferric complex. It is shown that ethylenediamine-N,N'-bis(o-hydroxyphenyl)acetic acid and DMA species are stable during preparative scale IEF, whereas copper-glutathione dissociates considerably. It is also shown that preparative scale IEF can be applied successfully to isolate ferric DMA from real plant samples, and that multidimensional separations are possible by combining preparative scale IEF with subsequent HPLC-MS analysis. Focusing of free ligands and respective metal complexes with di- and trivalent metals results in different pIs, but CIEF is usually needed for a reliable estimation of pI values. Limitations of the proposed methods (preparative IEF and CIEF) and consequences of the results with respect to metal speciation in plants are discussed.

Multiscale characterization of individualized beta-lactoglobulin microgels formed upon heat treatment under narrow pH range conditions

Aqueous dispersions of demineralized beta-lactoglobulin (beta-lg) were held at 85 degrees C for 15 min at a constant protein concentration of 1 wt % in the pH range of 3.0-7.0. This led to denatured protein content ranging from 20% (pH 3.0) to 90% (pH 5.0). The protein aggregates formed were characterized as to their stability to sedimentation (turbidity), morphology, size, surface charge, ANS surface hydrophobicity, and content in accessible thiol groups. Additionally, the changes in secondary structures of the protein upon heating were followed by Fourier transform infrared spectroscopy (FTIR). Stable dispersions (no sedimentation for 10 min) of individualized beta-lg microgels were obtained at specific pH 4.6 and 5.8, corresponding to an aggregation yield of about 80%. The width of the pH region leading to these microgels was 0.3 pH unit below or above the two specific pH values. Microgels were characterized by a spherical shape and remarkably low polydispersity in size (<0.2). Their z-average hydrodynamic diameter determined by dynamic light scattering (DLS) was between 160 and 220 nm, and their zeta-potential was +30 or -40 mV, depending on the initial pH before heating. Microgels obtained at pH 4.6 displayed a lower binding capacity for ANS and a lower content of accessible thiol groups as compared to those obtained at pH 5.8. Both types of microgels might therefore differ in their internal and interfacial structures. Between pH 4.6 and 5.8, large sedimenting protein particulates were obtained, whereas soluble aggregates were formed at pH <4.6 or >5.8. Interestingly, DLS experiments showed that before heating, beta-lg was mainly present in an oligomeric state at pH 4.6 and 5.8. This result was confirmed by FTIR measurements indicating the stronger contribution of the 1616-1624 cm(-1) spectral band corresponding to intermolecular beta-sheets in the pH range of 4.0-6.0. Upon heating, FTIR spectroscopy revealed that individualized microgels were obtained under pH conditions where a balance between attractive forces arising from protein unfolding leading to the formation of intermolecular beta-sheets (1616-1624 cm(-1 )band) and the repulsive electrostatic forces due to the initial protein net charge was achieved.

In vivo aggregation of bovine beta-lactoglobulin is affected by Cys at position 121

Bovine beta-lactoglobulin (BLG) has been widely used as a model system to study protein folding and aggregation and for biotechnology applications. Native BLG contains two disulfide bonds and one free cysteine at position 121. This free thiol group has been shown to be responsible for the irreversibility of BLG denaturation in vitro, but nothing is known about its relevance during protein folding inside the cell. Here, we report the expression of soluble wild type recombinant BGL in Escherichia coli cells at about 109 mg rBLG/g wet weight cells and a comparison between the aggregation of wt BLG and its variant C121S upon intracellular expression. We show that in E. coli C121SBLG is more prone to aggregation than the wild type protein and that their different behavior depends on the oxidation of disulfide bonds. Our results underline the key contribution of the unpaired cysteine residue during the oxidative folding pathway and indicate BLG as a useful tool for the study of protein aggregation in vivo.