An Unusual Effect of Hofmeister Series Salts on the Stability of Toxoplasma gondii Ferredoxin NADP⁺ Reductase

Background: The ionic interactions play an important role in the stabilization of the native conformation of proteins. Toxoplasma gondii Ferredoxin NADP+ Reductase (TgFNR) remains stable at pH 4.0. However, such modulation of ionic interactions leads to compaction and non-cooperativity in its folding. Objective: To gain insights into the role of ionic interactions in the modulation of structure and thermodynamic stability of TgFNR. Methods: Protein preparations, circular dichroism and fluorescence spectroscopy were used to determine salt-induced changes in the structure and stability of TgFNR. Results: The kosmotropic salts (sodium fluoride and sodium sulphate) appear to induce the biphasic response on the structure and stability of TgFNR. At pH about 4.0, the addition of low concentrations of kosmotropic salts significantly perturbs the existing native-like secondary structure of TgFNR, whereas higher quantities of salt reversed the denaturing impact. This is a one-of-a-kind situation we are unaware of in any other protein. The urea-induced unfolding of TgFNR in the presence of a low dose of salt (100 mM) drastically affected the protein's thermodynamic stability at neutral pH. The increased salt concentrations, on the other hand, reversed the destabilizing effect. Conclusion: Our findings imply that electrostatic interactions are exceptionally significant for the TgFNR stability, however, render highly unusual behavior of Hofmeister series salts, indicating a possible crucial role of salt bridges in the stabilization of different conformations of the protein.