Hofmeister series salts have an unusual effect on the stability of Toxoplasma gondii Ferredoxin NADP+ Reductase

The ionic interactions play an important role in the stabilization of native conformation of proteins. Toxoplasma gondii Ferredoxin NADP+ Reductase (TgFNR) remains stable at pH 4.0 however the modulation of ionic interactions leads to the compaction and non-cooperativity in the folding of domains. To gain further insights into the role of ionic interactions in the stability of TgFNR, stabilized at neutral and acidic pH, salt dependent changes in the structure and thermodynamic stability was investigated. The kosmotropic salts (sodium fluoride and sodium sulphate) appears to induce the biphasic response on the structure and stability TgFNR. At pH about 4.0, the addition of low concentrations of kosmotropic salts significantly perturb 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 that 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. Increased salt concentrations, on the other hand, reversed the destabilizing effect, and the salts now behave according to their Hofmeister series ranking. Our findings imply that electrostatic interactions are exceptionally significant in the structure and stability of TgFNR, and the abnormal behaviour of the salts may be mediated by the existence of salt bridges, which stabilizes the native conformation of enzyme.