Curtiss P. Schneider

Investigation of cosolute-protein preferential interaction coefficients: new insight into the mechanism by which arginine inhibits aggregation.

The relatively new technique of vapor pressure osmometry was utilized to determine the preferential interaction of five common solution additives (arginine HCl, guanidine HCl, glycerol, glucose, and urea) using three different model proteins (BSA, lysozyme, and alpha-chymotrypsinogen). Results for guanidine, glycerol, glucose, and urea are comparable to literature values, which utilized the dialysis/densimetry technique. However, values for arginine differ greatly from literature values, making it unclear what is the nature of arginine-protein interactions. A repeat of the dialysis/densimetry measurements reported in the literature supports the vapor pressure osmometry measurements and reveals a never before seen trend in the interaction of arginine with proteins as a function of concentration. This trend is dependent on the protein size and shows arginine to be unique among solution additives. For concentrations below 0.5 M, arginine has a preferential interaction coefficient near zero (slightly greater than zero for small proteins but decreases as the size of the protein increases), which indicates that arginine is neither strongly bound nor excluded from the protein surface. This trend differs greatly from cosolutes that influence the protein folding equilibrium. However, as the concentration of arginine increases beyond 0.5 M, arginine becomes increasingly excluded. Such behavior might be indicative of the protein surface becoming saturated with arginine, thus causing any additional arginine added to the solution to be excluded from interacting with the surface. All of this behavior is most likely the result of a balance between the affinity arginine has for the peptide backbone and certain amino acids and the repulsion generated by surface tension increment and volume exclusion effects [Arakawa et al. Biophysical Chemistry 2007, 127, 1]. In addition, such behavior may explain why arginine has little effect on protein folding equilibrium but is an effective aggregation suppressor.

Arginine and the Hofmeister Series: The Role of Ion-Ion Interactions in Protein Aggregation Suppression

L-Arginine hydrochloride is a very important aggregation suppressor for which there has been much attention given regarding elucidating its mechanism of action. Little consideration, however, has been given toward other salt forms besides chloride, even though the counterion likely imparts a large influence per the Hofmeister Series. Here, we report an in depth analysis of the role the counterion plays in the aggregation suppression behavior of arginine. Consistent with the empirical Hofmeister series, we found that the aggregation suppression ability of other arginine salt forms on a model protein (α-chymotrypsinogen) follows the order: H(2)PO(4)(-) > SO(4)(2-) > citrate(2-) > acetate(-) ≈ F(-) ≈ Cl(-) > Br(-) > I(-) ≈ SCN(-). Mechanistically, preferential interaction and osmotic virial coefficient measurements, in addition to molecular dynamics simulations, indicate that attractive ion-ion interactions, particularly attractive interactions between arginine molecules, play a dominate role in the observed behavior. Furthermore, it appears that dihydrogen phosphate, sulfate, and citrate have strong attractive interactions with the guanidinium group of arginine, which seems to contribute to the superior aggregation suppression ability of those salt forms by bridging together multiple arginine molecules into clusters. These results not only further our understanding of how arginine influences protein stability, they also help to elucidate the mechanism behind the Hofmeister Series. This should help to improve biopharmaceutical stabilization through the use of other arginine salts and possibly, the development of novel excipients.

Molecular level insight into intra-solvent interaction effects on protein stability and aggregation

Protein based therapeutics hold great promise in the treatment of human diseases and disorders and subsequently, they have become the fastest growing sector of new drugs being developed. Proteins are, however, inherently unstable and the degraded form can be quite harmful if administered to a patient. Of the various degradation pathways, aggregation is one of the most common and a cause for great concern. Aggregation suppressing additives have long been used to stabilize proteins, and they still remain the most viable option for combating this problem. Much work has been devoted toward investigating the behavior of commonly used additives and the resulting models give valuable insight toward explaining aggregation suppression. In a few cases, an explanation for unique behavior is lacking or new insight provides an alternate explanation. Additive selection and the development of better performing additives may benefit from a more refined understanding of how commonly used additives inhibit or enhance aggregation. In this review, we focus on recent molecular-level studies into how a select group of commonly used additives interact with proteins and subsequently influence aggregation. The intent of the review is not meant to be comprehensive for each additive but rather to provide new insights into additive-additive interactions, which may be contributing to protein-additive interactions. This is something that is often overlooked but yet essential to understanding the effect of additives on aggregation. The importance of understanding such interactions is clear when one considers that most formulations contain a mixture of cosolutes and that ideal stability might be better achieved through tuning intra-solvent interactions. We give an example of this when we describe how novel aggregation suppressing additives were developed from the knowledge gained from the reviewed studies.

Complex Interactions between Molecular Ions in Solution and Their Effect on Protein Stability

Protein stability in ionic solutions depends on the delicate balance between protein-ion and ion-ion interactions. For molecular ions containing multiple charged groups, the role of ion-ion interactions is particularly important. In this study, we show how the interplay between homo- and heteroion pairing influences protein stability using polyarginine salts as a model system. For the chloride salts, protein thermostability decreases as the size of the peptide increases, indicating enhanced binding to the protein. Moreover, it indicates reduced homoion pairing between Gdm(+) and carboxylate groups that is largely responsible for aggregation suppression, rather than denaturation, in monomeric arginine solutions. However, for the sulfate salts, strong heteroion pairing between the Gdm(+) groups and the sulfate counterions compensates for the loss of homoion pairing and, in return, leads to enhanced thermostability and a dramatically reduced (up to 10-30 times) rate of protein aggregation. Molecular dynamics simulations reveal how this ion pairing enhances conformational stability and, at the same time, reduces protein association. This study provides insight into complex ion effects on protein stability and serves as an example of how these intrasolvent interactions can be leveraged to enhance protein stability.

Rational Design of Biobetters with Enhanced Stability

Biotherapeutics are the fastest growing class of pharmaceutical with a rapidly evolving market facing the rise of biosimilar and biobetter products. In contrast to a biosimilar, which is derived from the same gene sequence as the innovator product, a biobetter has enhanced properties, such as enhanced efficacy or reduced immunogenicity. Little work has been carried out so far to increase the intrinsic stability of biotherapeutics via sequence changes, even though, aggregation, the primary degradation pathway of proteins, leads to issues ranging from manufacturing failure to immunological response and to loss of therapeutic activity. Using our spatial aggregation propensity tool as a first step to a rational design approach to identify aggregation-prone regions, biobetters of rituximab have been produced with enhanced stability by introducing site-specific mutations. Significant stabilization against aggregation was achieved for rituximab with no decrease in its binding affinity to the antigen.

Effects of solute-solute interactions on protein stability studied using various counterions and dendrimers.

Much work has been performed on understanding the effects of additives on protein thermodynamics and degradation kinetics, in particular addressing the Hofmeister series and other broad empirical phenomena. Little attention, however, has been paid to the effect of additive-additive interactions on proteins. Our group and others have recently shown that such interactions can actually govern protein events, such as aggregation. Here we use dendrimers, which have the advantage that both size and surface chemical groups can be changed and therein studied independently. Dendrimers are a relatively new and broad class of materials which have been demonstrated useful in biological and therapeutic applications, such as drug delivery, perturbing amyloid formation, etc. Guanidinium modified dendrimers pose an interesting case given that guanidinium can form multiple attractive hydrogen bonds with either a protein surface or other components in solution, such as hydrogen bond accepting counterions. Here we present a study which shows that the behavior of such macromolecule species (modified PAMAM dendrimers) is governed by intra-solvent interactions. Attractive guanidinium-anion interactions seem to cause clustering in solution, which inhibits cooperative binding to the protein surface but at the same time, significantly suppresses nonnative aggregation.