Mellitate: A multivalent anion with extreme charge density causes rapid aggregation and misfolding of wild type lysozyme at neutral pH

Deltamethrin modulates the native structure of Hen Egg White Lysozyme and induces its aggregation at physiological pH

Deltamethrin, a type II pyrethroid pesticide was initially considered as safe for human use. Recent studies have reported several pathophysiological effects of deltamethrin on human and non-human species. However, its effect on structure and function of protein leading to progressive neurodegeneration is poorly understood. In present study, we investigated the interaction of deltamethrin with Hen Egg White Lysozyme (HEWL) at physiological pH and tried to understand the effect of pesticide on structure and function of protein. Employing different biophysical techniques, we shown that deltamethrin induces in vitro aggregation of HEWL in concentration dependent manner. Interaction of pesticide with different amino acids, followed by exposure of hydrophobic regions was driving force of aggregation process. Apart from modulating the hydrophobic domain, deltamethrin is observed to reduce α-helical and promote β-sheet content of lysozyme, eventually converting the globular protein into ThT sensitive amyloid fibrils and amorphous aggregates. Our study also indicate that deltamethrin induced aggregation reduces the catalytic activity of lysozyme.

Model for Counterion Binding and Charge Reversal on Protein Surfaces

The structural stability and solubility of proteins in liquid therapeutic formulations is important, especially since new generations of therapeutics are designed for efficacy before consideration of stability. We introduce an electrostatic binding model to measure the net charge of proteins with bound ions in solution. The electrostatic potential on a protein surface is used to separately group together acidic and basic amino acids into patches, which are then iteratively bound with oppositely charged counterions. This model is aimed toward formulation chemists for initial screening of a range of conditions prior to lab-work. Computed results compare well with experimental zeta potential measurements from the literature covering a range of solution conditions. Importantly, the binding model reproduces the charge reversal phenomenon that is observed with polyvalent ion binding to proteins and its dependence on ion charge and concentration. Intriguingly, protein sequence can be used to give similarly good agreement with experiment as protein structure, interpreted as resulting from the close proximity of charged side chains on a protein surface. Further, application of the model to human proteins suggests that polyanion binding and overcharging, including charge reversal for cationic proteins, is a general feature. These results add to evidence that addition of polyanions to protein formulations could be a general mechanism for modulating solution stability.

Controlling Phase Separation of Lysozyme with Polyvalent Anions

The ability of polyvalent anions to influence protein-protein interactions and protein net charge was investigated through solubility and turbidity experiments, determination of osmotic second virial coefficients ( B₂₂), and ζ-potential values for lysozyme solutions.  B₂₂ values showed that all anions reduce protein-protein repulsion between positively charged lysozyme molecules, and those anions with higher net valencies are more effective. The polyvalent anions pyrophosphate and tripolyphosphate were observed to induce protein reentrant condensation, which has been previously observed with negatively charged proteins in the presence of trivalent cations. Reentrant condensation is a phenomenon in which low concentrations of polyvalent ions induce protein precipitation, but further increasing polyvalent ion concentration causes the protein precipitate to resolubilize. Interestingly, citrate does not induce lysozyme reentrant condensation despite having a similar charge, size, and shape to pyrophosphate. We observe qualitative differences in protein behavior when compared against negatively charged proteins in solutions of trivalent cations. The polyphosphate ions induce a much stronger protein-protein attraction, which correlates with the occurrence of a liquid-gel transition that replaces the liquid-liquid transition observed with trivalent cations. The results indicate that solutions of polyphosphate ions provide a model system for exploring the link between the protein-phase diagram and model interaction potentials and also highlight the importance that ion-specific effects can have on protein solubility.