The role of electrostatics in protein-protein interactions of a monoclonal antibody

Arresting amyloid with coulomb's law: acetylation of ALS-linked SOD1 by aspirin impedes aggregation

Although the magnitude of a protein's net charge (Z) can control its rate of self-assembly into amyloid, and its interactions with cellular membranes, the net charge of a protein is not viewed as a druggable parameter. This article demonstrates that aspirin (the quintessential acylating pharmacon) can inhibit the amyloidogenesis of superoxide dismutase (SOD1) by increasing the intrinsic net negative charge of the polypeptide, i.e., by acetylation (neutralization) of multiple lysines. The protective effects of acetylation were diminished (but not abolished) in 100 mM NaCl and were statistically significant: a total of 432 thioflavin-T amyloid assays were performed for all studied proteins. The acetylation of as few as three lysines by aspirin in A4V apo-SOD1-a variant that causes familial amyotrophic lateral sclerosis (ALS)-delayed amyloid nucleation by 38% and slowed amyloid propagation by twofold. Lysines in wild-type- and ALS-variant apo-SOD1 could also be peracetylated with aspirin after fibrillization, resulting in supercharged fibrils, with increases in formal net charge of ∼2 million units. Peracetylated SOD1 amyloid defibrillized at temperatures below unacetylated fibrils, and below the melting temperature of native Cu2,Zn2-SOD1 (e.g., fibril Tm = 84.49°C for acetylated D90A apo-SOD1 fibrils). Targeting the net charge of native or misfolded proteins with small molecules-analogous to how an enzyme's Km or Vmax are medicinally targeted-holds promise as a strategy in the design of therapies for diseases linked to protein self-assembly.

Critical examination of the colloidal particle model of globular proteins

Recent studies of globular protein solutions have uniformly adopted a colloidal view of proteins as particles, a perspective that neglects the polymeric primary structure of these biological macromolecules, their intrinsic flexibility, and their ability to sample a large configurational space. While the colloidal perspective often serves as a useful idealization in many cases, the macromolecular identity of proteins must reveal itself under thermodynamic conditions in which the native state is no longer stable, such as denaturing solvents and high protein concentrations where macromolecules tend to have screened excluded volume, charge, and hydrodynamic interactions. Under extreme pH conditions, charge repulsion interactions within the protein chain can overcome the attractive hydrogen-bonding interactions, holding it in its native globular state. Conformational changes can therefore be expected to have great significance on the shear viscosity and other rheological properties of protein solutions. These changes are not envisioned in conventional colloidal protein models and we have initiated an investigation of the scattering and rheological properties of model proteins. We initiate this effort by considering bovine serum albumin because it is a globular protein whose solution properties have also been extensively investigated as a function of pH, temperature, ionic strength, and concentration. As we anticipated, near-ultraviolet circular dichroism measurements and intrinsic viscosity measurements clearly indicate that the bovine serum albumin tertiary structure changes as protein concentration and pH are varied. Our findings point to limited validity of the colloidal protein model and to the need for further consideration and quantification of the effects of conformational changes on protein solution viscosity, protein association, and the phase behavior. Small-angle Neutron Scattering measurements have allowed us to assess how these conformational changes influence protein size, shape, and interprotein interaction strength.

Weak protein interactions and pH- and temperature-dependent aggregation of human Fc1

The Fc (fragment crystallizable) is a common structural region in immunoglobulin gamma (IgG) proteins, IgG-based multi-specific platforms, and Fc-fusion platform technologies. Changes in conformational stability, protein-protein interactions, and aggregation of NS0-produced human Fc1 were quantified experimentally as a function of pH (4 to 6) and temperature (30 to 77 °C), using a combination of differential scanning calorimetry, laser light scattering, size-exclusion chromatography, and capillary electrophoresis. The Fc1 was O-glycosylated at position 3 (threonine), and confirmed to correspond to the intact IgG1 by comparison with Fc1 produced by cleavage of the parent IgG1. Changing the pH caused large effects for thermal unfolding transitions, but it caused surprisingly smaller effects for electrostatic protein-protein interactions. The aggregation behavior was qualitatively similar across different solution conditions, with soluble dimers and larger oligomers formed in most cases. Aggregation rates spanned approximately 5 orders of magnitude and could be divided into 2 regimes: (i) Arrhenius, unfolding-limited aggregation at temperatures near or above the midpoint-unfolding temperature of the CH2 domain; (ii) a non-Arrhenius regime at lower temperatures, presumably as a result of the temperature dependence of the unfolding enthalpy for the CH2 domain. The non-Arrhenius regime was most pronounced for lower temperatures. Together with the weak protein-protein repulsions, these highlight challenges that are expected for maintaining long-term stability of biotechnology products that are based on human Fc constructs.

Colloidal Stability & Conformational Changes in β-Lactoglobulin: Unfolding to Self-Assembly

A detailed understanding of the mechanism of unfolding, aggregation, and associated rheological changes is developed in this study for β-Lactoglobulin at different pH values through concomitant measurements utilizing dynamic light scattering (DLS), optical microrheology, Raman spectroscopy, and differential scanning calorimetry (DSC). The diffusion interaction parameter kD emerges as an accurate predictor of colloidal stability for this protein consistent with observed aggregation trends and rheology. Drastic aggregation and gelation were observed at pH 5.5. Under this condition, the protein's secondary and tertiary structures changed simultaneously. At higher pH (7.0 and 8.5), oligomerizaton with no gel formation occurred. For these solutions, tertiary structure and secondary structure transitions were sequential. The low frequency Raman data, which is a good indicator of hydrogen bonding and structuring in water, has been shown to exhibit a strong correlation with the rheological evolution with temperature. This study has, for the first time, demonstrated that this low frequency Raman data, in conjunction with the DSC endotherm, can be been utilized to deconvolve protein unfolding and aggregation/gelation. These findings can have important implications for the development of protein-based biotherapeutics, where the formulation viscosity, aggregation, and stability strongly affects efficacy or in foods where protein structuring is critical for functional and sensory performance.

TS, Setlow P, Donskey CJ. Unlocking the Sporicidal Potential of Ethanol: Induced Sporicidal Activity of Ethanol against Clostridium difficile and Bacillus Spores under Altered Physical and Chemical Conditions

Background

Due to their efficacy and convenience, alcohol-based hand sanitizers have been widely adopted as the primary method of hand hygiene in healthcare settings. However, alcohols lack activity against bacterial spores produced by pathogens such as Clostridium difficile and Bacillus anthracis. We hypothesized that sporicidal activity could be induced in alcohols through alteration of physical or chemical conditions that have been shown to degrade or allow penetration of spore coats.

Principal Findings

Acidification, alkalinization, and heating of ethanol induced rapid sporicidal activity against C. difficile, and to a lesser extent Bacillus thuringiensis and Bacillus subtilis. The sporicidal activity of acidified ethanol was enhanced by increasing ionic strength and mild elevations in temperature. On skin, sporicidal ethanol formulations were as effective as soap and water hand washing in reducing levels of C. difficile spores.

Conclusions

These findings demonstrate that novel ethanol-based sporicidal hand hygiene formulations can be developed through alteration of physical and chemical conditions.

A Colloidal Description of Intermolecular Interactions Driving Fibril-Fibril Aggregation of a Model Amphiphilic Peptide

We apply a kinetic analysis platform to study the intermolecular interactions underlying the colloidal stability of dispersions of charged amyloid fibrils consisting of a model amphiphilic peptide (RADA 16-I). In contrast to the aggregation mechanisms observed in the large majority of proteins and peptides, where several elementary reactions involving both monomers and fibrils are present simultaneously, the system selected in this work allows the specific investigation of the fibril-fibril aggregation process. We examine the intermolecular interactions driving the aggregation reaction at pH 2.0 by changing the buffer composition in terms of salt concentration, type of ion as well as type and concentration of organic solvent. The aggregation kinetics are followed by dynamic light scattering, and the experimental data are simulated by Smoluchowski population balance equations, which allow to estimate the energy barrier between two colliding fibrils in terms of the Fuchs stability ratio (W). When normalized on a dimensionless time weighted on the Fuchs stability ratio, the aggregation profiles under a broad range of conditions collapse on a single master curve, indicating that the buffer composition modifies the aggregation kinetics without affecting the aggregation mechanism. Our results show that the aggregation process does not occur under diffusion-limited conditions. Rather, the reaction rate is limited by the presence of an activation energy barrier that is largely dominated by electrostatic repulsive interactions. Such interactions could be reduced by increasing the concentration of salt, which induces charge screening, or the concentration of organic solvent, which affects the dielectric constant. It is remarkable that the dependence of the activation energy on the ionic strength can be described quantitatively in terms of charge screening effects in the frame of the DLVO theory, although specific anion and cation effects are also observed. While anion effects are mainly related to the binding to the positive groups of the fibril surface and to the resulting decrease of the surface charge, cation effects are more complex and involve additional solvation forces.

Anisotropic protein-protein interactions due to ion binding

Self-association of proteins is strongly affected by long-range electrostatic interactions caused by equilibrium adsorption of small ions such as protons and multivalent metals. By affecting the molecular net charge, solution pH is thus a widely used parameter to tune stability and phase behavior of proteins. We here review recent studies where the charge distribution is perturbed not only by protons, but also by other binding ions, leading to a rich and inherently anisotropic charge distribution. Focus is on coarse grained simulation techniques, coupled to experiments of protein-protein interaction at varying salt and pH conditions. Finally, and with future bio-colloidal models in mind, we discuss the validity of coarse graining charge anisotropy using electric multipoles.

Charge-induced patchy attractions between proteins

Static light scattering (SLS) combined with structure-based Monte Carlo (MC) simulations provide new insights into mechanisms behind anisotropic, attractive protein interactions. A nonmonotonic behavior of the osmotic second virial coefficient as a function of ionic strength is here shown to originate from a few charged amino acids forming an electrostatic attractive patch, highly directional and complementary. Together with Coulombic repulsion, this attractive patch results in two counteracting electrostatic contributions to the interaction free energy which, by operating over different length scales, is manifested in a subtle, salt-induced minimum in the second virial coefficient as observed in both experiment and simulations.

Determination of the second virial coefficient of bovine serum albumin under varying pH and ionic strength by composition-gradient multi-angle static light scattering

Composition-gradient multi-angle static light scattering (CG-MALS) is an emerging technique for the determination of intermolecular interactions via the second virial coefficient B22. With CG-MALS, detailed studies of the second virial coefficient can be carried out more accurately and effectively than with traditional methods. In addition, automated mixing, delivery and measurement enable high speed, continuous, fluctuation-free sample delivery and accurate results. Using CG-MALS we measure the second virial coefficient of bovine serum albumin (BSA) in aqueous solutions at various values of pH and ionic strength of a univalent salt (NaCl). The systematic variation of the second virial coefficient as a function of pH and NaCl strength reveals the net charge change and the isoelectric point of BSA under different solution conditions. The magnitude of the second virial coefficient decreases to 1.13 x 10(-5) ml*mol/g(2) near the isoelectric point of pH 4.6 and 25 mM NaCl. These results illuminate the role of fundamental long-range electrostatic and van der Waals forces in protein-protein interactions, specifically their dependence on pH and ionic strength.