The effect of charged residue substitutions on the thermodynamics of protein-surface interactions

Electrostatics introduce a trade-off between mesophilic stability and adaptation in halophilic proteins

Extremophile organisms have adapted to extreme physicochemical conditions. Halophilic organisms, in particular, survive at very high salt concentrations. To achieve this, they have engineered the surface of their proteins to increase the number of short, polar and acidic amino acids, while decreasing large, hydrophobic and basic residues. While these adaptations initially decrease protein stability in the absence of salt, they grant halophilic proteins remarkable stability in environments with extremely high salt concentrations, where non-adapted proteins unfold and aggregate. The molecular mechanisms by which halophilic proteins achieve this, however, are not yet clear. Here, we test the hypothesis that the halophilic amino acid composition destabilizes the surface of the protein, but in exchange improves the stability in the presence of salts. To do that, we have measured the folding thermodynamics of various protein variants with different degrees of halophilicity in the absence and presence of different salts, and at different pH values to tune the ionization state of the acidic amino acids. Our results show that halophilic amino acids decrease the stability of halophilic proteins under mesophilic conditions, but in exchange improve salt-induced stabilization and solubility. We also find that, in contrast to traditional assumptions, contributions arising from hydrophobic effect and preferential ion exclusion are more relevant for haloadaptation than electrostatics. Overall, our findings suggest a trade-off between folding thermodynamics and halophilic adaptation to optimize proteins for hypersaline environments.

The effect of charged residue substitutions on the thermodynamics of protein-surface interactions

The interactions of proteins with surfaces are important in both biological processes and biotechnologies. In contrast to decades of study regarding the biophysics of proteins in bulk solution, however, our mechanistic understanding of the biophysics of proteins interacting with surfaces remains largely qualitative. In response, we have set to explore quantitatively the thermodynamics of protein-surface interactions. In this work, we explore systematically the role of electrostatics in modulating the interaction between proteins and charged surfaces. In particular, we use electrochemistry to explore the extent to which a macroscopic, hydroxyl-coated surface held at a slightly negative potential affects the folding thermodynamics of surface-attached protein variants with different composition of charged amino acids. Doing so, we find that attachment to the surface generally leads to a net stabilization, presumably due to excluded volume effects that reduce the entropy of the unfolded state. The magnitude of this stabilization, however, is strongly correlated with the charged-residue content of the protein. In particular, we find statistically significant correlations with both the net charge of the protein, with greater negative charge leading to less stabilization by the surface, and with the number of arginines, with more arginines leading to greater stabilization. Such findings refine our understanding of protein-surface interactions, providing in turn a guiding rationale to achieve the functional deposition of proteins on artificial surfaces for implementation in, for example, protein-based biotechnologies.