Calorimetric and structural investigation of the interaction of lysozyme and bovine serum albumin with poly(ethylene oxide) and its copolymers

Hydrophobicity Enhances the Formation of Protein-Stabilized Foams

Screening proteins for their potential use in foam applications is very laborious and time consuming. It would be beneficial if the foam properties could be predicted based on their molecular properties, but this is currently not possible. For protein-stabilized emulsions, a model was recently introduced to predict the emulsion properties from the protein molecular properties. Since the fundamental mechanisms for foam and emulsion formation are very similar, it is of interest to determine whether the link to molecular properties defined in that model is also applicable to foams. This study aims to link the exposed hydrophobicity with the foam ability and foam stability, using lysozyme variants with altered hydrophobicity, obtained from controlled heat treatment (77 °C for 0-120 min). To establish this link, the molecular characteristics, interfacial properties, and foam ability and stability (at different concentrations) were analysed. The increasing hydrophobicity resulted in an increased adsorption rate constant, and for concentrations in the protein-poor regime, the increasing hydrophobicity enhanced foam ability (i.e., interfacial area created). At higher relative exposed hydrophobicity (i.e., ~2-5 times higher than native lysozyme), the adsorption rate constant and foam ability became independent of hydrophobicity. The foam stability (i.e., foam collapse) was affected by the initial foam structure. In the protein-rich regime-with nearly identical foam structure-the hydrophobicity did not affect the foam stability. The link between exposed hydrophobicity and foam ability confirms the similarity between protein-stabilized foams and emulsions, and thereby indicates that the model proposed for emulsions can be used to predict foam properties in the future.

Application of aqueous biphasic systems as strategy to purify tannase from Aspergillus tamarii URM 7115

The aims of the current study are to assess the influence of polyethylene glycol (PEG) concentration, molar mass, pH, and citrate concentrations on aqueous biphasic systems based on 2⁴ factorial designs, as well as to check their capacity to purify tannase secreted by Aspergillus tamarii URM 7115. Tannase was produced through submerged fermentation at 26°C for 67 h in Czapeck-Dox modified broth and added with yeast extract and tannic acid. The factorial design was followed to assess the influence of PEG molar mass (M_PEG 600; 4,000 and 8,000 g/ mol), and PEG (C_PEG 20.0; 22.0 and 24.0% w/w) and citrate concentrations (C_CIT 15.0, 17.5, and 20.0%, w/w), as well as of pH (6.0, 7.0, and 8.0) on the response variables; moreover, partition coefficient (K), yield (Y), and purification factor (PF) were analyzed. The most suitable parameters to purify tannase secreted by A. tamarii URM 7115 through a biphasic system were 600 (g/mol) M_PEG, 24% (w/w) C_PEG, 15% (w/w) C_CIT at pH 6.0 and they resulted in 6.33 enzyme partition, 131.25% yield, 19.80 purification factor and 195.08 selectivity. Tannase secreted by A. tamarii URM 7115 purified through aqueous biphasic systems composed of PEG/citrate can be used for industrial purposes, since it presents suitable purification factor and yield.

Effects of gene carrier polyethyleneimines on the structure and binding capability of bovine serum albumin

Polyethyleneimine (PEI), one of the most effective non-viral gene carriers, is also cytotoxic, however the molecular basis is poorly understood. Little is known about the effects of PEI on the structure and functions of the biomacromolecules. In this work, fluorescence, UV-vis absorption, circular dichroism (CD) spectroscopy and zeta-potential measurement were conducted to reveal the interaction between PEIs (average molecular weight 25, 10 and 1.8kDa) and bovine serum albumin (BSA), and to evaluate the effects on the conformation of BSA as long as its binding capability to the model compounds, 8-anilino-1-naphthalenesulfonic acid (ANS) and quercetin. PEIs were found to complex with BSA and induced a conformational change of the protein by a major reduction of α-helix at PEI concentration <0.2mg·mL^-1 and an increase at higher PEI concentration. The binding efficacy of ANS and quercetin to BSA was greatly reduced by the competitive binding by PEI and influenced by the conformational change of BSA, which was found to display a similar trend to the change of the α-helix content of the protein. The polymer size played an important role in PEI-BSA interaction. PEI of higher molecular weight was more favorable to interact with BSA and more efficient to perturb the conformation and binding capability of the protein.

Production of alpha-amylase from Aspergillus oryzae for several industrial applications in a single step

A one-step method as a strategy of alpha-amylase concentration and purification was developed in this work. This methodology requires the use of a very low concentration of biodegradable polyelectrolyte (Eudragit(®) E-PO) and represents a low cost, fast, easy to scale up and non-polluting technology. Besides, this methodology allows recycling the polymer after precipitation. The formation of reversible soluble/insoluble complexes between alpha-amylase and the polymer Eudragit(®) E-PO was studied, and their precipitation in selected conditions was applied with bioseparation purposes. Turbidimetric assays allowed to determine the pH range where the complexes are insoluble (4.50-7.00); pH 5.50 yielded the highest turbidity of the system. The presence of NaCl (0.05M) in the medium totally dissociates the protein-polymer complexes. When the adequate concentration of polymer was added under these conditions to a liquid culture of Aspergillus oryzae, purification factors of alpha-amylase up to 7.43 and recoveries of 88% were obtained in a simple step without previous clarification. These results demonstrate that this methodology is suitable for the concentration and production of alpha-amylase from this source and could be applied at the beginning of downstream processing.

Cationic poly(amidoamine) promotes cytosolic delivery of bovine RNase A in melanoma cells, while maintaining its cellular toxicity

Ribonucleases are known to cleave ribonucleic acids, inducing cell death. RNase A, a member of the ribonuclease family, generally displayed poor in vitro activity. This has been attributed to factors such as low intracellular delivery. Poly(amidoamine)s have been used to promote the translocation of non-permeant proteins to the cytosol. Our objective was to demonstrate that poly(amidoamine)s could potentially promote the delivery of RNase A to selected cell line. Interactions of three cationic poly(amidoamine)s (P1, P2 and ISA1) with wild-type bovine RNase A were investigated using gel retardation assays, DLS and microcalorimetry. Although the polymers and the protein are essentially cationic at physiological pH, complexation between the PAAs and RNase A was observed. The high sensitivity differential scanning calorimetry (HSDSC) thermograms demonstrated that the thermal stability of the protein was reduced when complexed with ISA1 (T_max decreased by 6.5 °C) but was not affected by P1 and P2. All the polymers displayed low cytotoxicity towards non-cancerous cells (IC₅₀ > 3.5 mg mL^-1). While RNase A alone was not toxic to mouse melanoma cells (B16F1), P1 was able to promote cytosolic delivery of biologically active RNase A, increasing cell death (IC₅₀ = 0.09 mg mL^-1).

Tuning hemoglobin-poly(acrylic acid) interactions by controlled chemical modification with triethylenetetramine

Protein-polymer interactions play a very important role in a number of applications, but details of these interactions are not fully understood. Chemical modification was introduced here to tune protein-polymer interactions in a systematic manner, where methemoglobin (Hb) and poly(acrylic acid) (PAA) served as a model system. Under similar conditions of pH and ionic strength, the influence of protein charge on Hb/PAA interaction was studied using chemically modified Hb by isothermal titration calorimetry (ITC). A small fraction of COOH groups of Hb were amidated with triethylenetetramine (TETA) or ammonium chloride to produce the corresponding charge ladders of Hb-TETA and Hb-ammonia derivatives, respectively. All the Hb/PAA complexes produced here are bioactive, entirely soluble in water, and indicated the retention of Hb structure to a significant extent. Binding of Hb to PAA was exothermic (ΔH < 0). The binding of Hb-TETA charge ladder to PAA indicated decrease of ΔH from -8 ± 0.2 to -89 ± 4 kcal/mol, at a rate of -3.8 kcal/mol per unit charge introduced via modification. The Hb-ammonia charge ladder, in contrast, showed a decrease of ΔH from -8 ± 0.2 to -17 ± 1.5 kcal/mol, at much slower rate of -1.0 kcal/mol per unit charge. Thus, the amine used for the modification played a strong role in tuning Hb/PAA interactions, even after correcting for the charge, synergistically. Charge clustering may be responsible for this synergy, and this interesting observation may be exploited to construct protein/polymer platforms for advanced biomacromolecular applications.

Interaction of a Rhodococcus sp. trehalose lipid biosurfactant with model proteins: thermodynamic and structural changes

One major application of surfactants is to prevent aggregation during various processes of protein manipulation. In this work, a bacterial trehalose lipid (TL) with biosurfactant activity, secreted by Rhodococcus sp., has been identified and purified. The interactions of this glycolipid with selected model proteins have been studied by using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, isothermal titration calorimetry (ITC), and fluorescence spectroscopy. Bovine serum albumin (BSA) and cytochrome c (Cyt-c) have been chosen because of their quite different secondary structures: BSA contains essentially no β-sheets and an average 66% α-helix, whereas Cyt-c possesses up to 25% β-sheets and up to 45% α-helical structure. Differential scanning calorimetry shows that addition of TL to BSA at concentrations below the critical micelle concentration (cmc) shifts the thermal unfolding temperature to higher values. FTIR indicates that TL does not alter the secondary structure of native BSA, but the presence of TL protects the protein toward thermal denaturation, mainly by avoiding formation of β-aggregates. Studies on the intrinsic Trp fluorescence of BSA show that addition of TL to the native protein results in conformational changes. BSA unfolding upon thermal denaturation in the absence of TL makes the Trp residues less accessible to the quencher, as shown by a decrease in the value of Stern-Volmer dynamic quenching constant, whereas denaturation in the presence of the biosurfactant prevents unfolding, in agreement with FTIR results. In the case of Cyt-c, interaction with TL gives rise to a new thermal denaturation transition, as observed by DSC, at temperatures below that of the native protein, therefore facilitating thermal unfolding. Binding of TL to native BSA and Cyt-c, as determined by ITC, suggests a rather nonspecific interaction of the biosurfactant with both proteins. FTIR indicates that TL slightly modifies the secondary structure of native Cyt-c, but protein denaturation in the presence of TL results in a higher proportion of β-aggregates than in its absence (20% vs 3.9%). The study of Trp fluorescence upon TL addition to Cyt-c results in a completely opposite scenario to that described above for BSA. In this case, addition of TL considerably increases the value of the dynamic quenching constant, both in native and denatured protein; that is, the interaction with the glycolipid induces conformational changes which facilitate the exposure of Trp residues to the quencher. Considering the structures of both proteins, it could be derived that the characteristics of TL interactions, either promoting or avoiding thermal unfolding, are highly dependent on the protein secondary structure. Our results also suggest the rather unspecific nature of these interactions. These might well involve protein hydrophobic domains which, being buried into the protein native structures, become exposed upon thermal unfolding.

Complex formation between protein and poly vinyl sulfonate as a strategy of proteins isolation

The complex formation between the basic protein trypsin and the strong anionic polyelectrolyte poly vinyl sulfonic acid was studied by using turbidimetric and isothermal calorimetric titrations. The trypsin-polymer complex was insoluble at pH lower than 5, with a stoichiometric ratio polymer mol per protein mol of 1:136. NaCl, 0.5M inhibited the complex precipitation in agreement with the proposed coulombic mechanism of complex formation. The protein structure and its thermodynamic stability were not significantly affected by the presence of the polyelectrolyte. The enzymatic activity of trypsin increases throughout time, even in the presence of the polymer.

Thermodynamic and structural changes associated with the interaction of a dirhamnolipid biosurfactant with bovine serum albumin

The interaction of a dirhamnolipid biosurfactant secreted by Pseudomonas aeruginosa with bovine serum albumin was studied by means of various physical techniques. Binding of the biosurfactant to bovine serum albumin was first characterized by isothermal titration calorimetry, showing that one or two molecules of dirhamnolipid, in the monomer state, bound to one molecule of the protein with high affinity. These results were confirmed by surface tension measurements in the absence and presence of bovine serum albumin. As seen by differential scanning calorimetry, dirhamnolipid shifted the temperature of the thermal unfolding of bovine serum albumin toward higher values, thus increasing the stability of the protein on heating. The impact of dirhamnolipid on the structure of the native protein was low, since most of the secondary structure remained unaffected upon interaction with the biosurfactant, as shown by FTIR spectroscopy. However, 2D correlation infrared spectroscopy indicated that the sequence of temperature-induced structural changes in native bovine serum albumin was modified by the presence of the biosurfactant. The consequences of these results in relation to possible applications of these dirhamnolipid biosurfactants for protein studies are discussed.

Macroscopic alignment of nanoparticle arrays in soft crystals of cubic and cylindrical polymer micelles

We describe a method to organize nanometer-sized hydrophilic particles into ordered arrays by templating them in the soft, micelle-crystal phases (spherical and cylindrical) of a thermoreversible block copolymer. Small-angle neutron scattering (SANS) with contrast variation is used to show that the dispersed particles (in this case, proteins or silica) form structured arrays by being constrained in the interstitial cavities between the polymer micelles in the ordered micelle crystal. Simple shear is used to macroscopically align both phases of the nanocomposites (micelles and particles) into macro-domains. The temperature-induced order-order transition between templates of spherical and cylindrical micelles is demonstrated as a reversible technique to modify the structure of the templated nanoparticle arrays.

Calorimetric investigation of the protein-flexible chain polymer interactions and its relationship with protein partition in aqueous two-phase systems

The binding of polyethyleneglycol of molecular mass 1000, 3300 and 6000 and polyethylene-propylene oxide (molecular mass 8400) to lysozyme and ovoalbumin was measured by isothermal calorimetric titration. A binding process was found to be associated with a saturation effect, which suggests a protein-polymer interaction. The proteins showed an affinity for the polymers in the order of 10(2)M(-1) and it decreased with the increase in the polymer molecular mass. The number of polymer molecules bound per protein molecule varied from 0.01 to 0.2 for polyethyleneglycol 1000, 3300 and polyethylene-polypolypropylene oxide 8400, while for polyethyleneglycol 6000 such number got closer to the unity. The enthalpic change associated with the binding was positive in the order of 1 kcal/mol for lysozyme, while ovoalbumin showed values around 2-3 kcal/mol. Entropic changes were also positive with values around 17-20 e.u. for ovoalbumin and 1-7 e.u. for lysozyme. The heat associated with the protein transfer from a buffer to a medium containing the polymer or the salt (a process similar to protein partitioning in aqueous two-phase systems) was obtained. These results allow the direct calculation of the enthalpic change associated with a protein partition process in aqueous two-phase systems without applying the van'tHoff equation. In this way, it is possible to calculate the associated true heat when the protein is transferred from the bottom to the top phase.

Thermodynamic Insights into the Binding of Triton X-100 to Globular Proteins: A Calorimetric and Spectroscopic Investigation

The interaction of the nonionic surfactant Triton X-100 (TX-100) with two proteins (bovine serum albumin (BSA) and alpha-lactalbumin (alpha-LA)) has been investigated by using a combination of differential scanning calorimetry, isothermal titration calorimetry, and fluorescence and circular dichroism spectroscopies. All of the calorimetric transitions in BSA were partially reversible, while being two-state and reversible in the case of alpha-LA. TX-100 molecules do not reduce the thermal stability of the protein in the monomeric form. However, in the micellar form the protein might become thermally destabilized by the micelles depending upon the nature of the protein. Isothermal titration calorimetry has been used to demonstrate that TX-100 binds to BSA at two sets of sites with 4:1 stoichiometry in each case. The van't Hoff enthalpy calculated from the temperature dependence of the binding constant did not match with the calorimetric enthalpy indicating conformational change in the protein upon surfactant binding. The surfactant binds to alpha-LA with one class of binding site, and the thermal unfolding results indicate it to be a stronger destabilizer than BSA. The fluorescence, circular dichroism, and differential scanning calorimetric results corroborate well with each other. The effect of ionic strength on the binding parameters suggests that TX-100 can bind to the protein surface via both hydrophobic and polar interactions depending upon the nature of the protein. The physical chemistry underlying the interactions between TX-100 and proteins has been presented. The mode of interaction of TX-100 with proteins is via direct binding, which has been discussed quantitatively in this work.

Calorimetric and Structural Investigation of the Interaction between Bovine Serum Albumin and High Molecular Weight Dextran in Water

This work studies specific interactions between a small globular protein and a highly flexible, branched polysaccharide using differential scanning calorimetry (DSC), circular dichroism (CD), fluorescence, and turbidimetry measurements. It uses the system water/bovine serum albumin (BSA)/dextran (D 2000) as a model. Dextran molecules are able to form interpolymeric complexes with BSA in water at both low and high temperatures if the polysaccharide is in excess and if the protein exists in its associated state. It leads to a partial destabilization of the secondary and tertiary structures of the protein and an additional exposure of the hydrophobic tryptophan residues to the surface of globule. If the total concentration of biopolymers in the mixture is high enough, the stability of the protein molecules with respect to unfolding and thermoaggregation is significantly decreased as a result of an increase in the protein hydrophobicity.

Survey of the year 2004: literature on applications of isothermal titration calorimetry

The market for commercially available isothermal titration calorimeters continues to grow as new applications and methodologies are developed. Concomitantly the number of users (and abusers) increases dramatically, resulting in a steady increase in the number of publications in which isothermal titration calorimetry (ITC) plays a role. In the present review, we will focus on areas where ITC is making a significant contribution and will highlight some interesting applications of the technique. This overview of papers published in 2004 also discusses current issues of interest in the development of ITC as a tool of choice in the determination of the thermodynamics of molecular recognition and interaction.