Kinetics of salt-dependent unfolding of [2Fe-2S] ferredoxin of Halobacterium salinarum

The [2Fe-2S] ferredoxin from the extreme haloarchaeon Halobacterium salinarum is stable in high (>1.5 M) salt concentration. At low salt concentration the protein exhibits partial unfolding. The kinetics of unfolding was studied in low salt and in presence of urea in order to investigate the role of salt ions on the stability of the protein. The urea dependent unfolding, monitored by fluorescence of the tryptophan residues and circular dichroism, suggests that the native protein is stable at neutral pH, is destabilized in both acidic and alkaline environment, and involves the formation of kinetic intermediate(s). In contrast, the unfolding kinetics in low salt exhibits enhanced rate of unfolding with increase in pH value and is a two state process without the formation of intermediate. The unfolding at neutral pH is salt concentration dependent and occurs in two stages. The first stage, involves an initial fast phase (indicative of the formation of a hydrophobic collapsed state) followed by a relatively slow phase, and is dependent on the type of cation and anion. The second stage is considerably slower, proceeds with an increase in fluorescence intensity and is largely independent of the nature of salt. Our results thus show that the native form of the haloarchaeal ferredoxin (in high salt concentration) unfolds in low salt concentration through an apparently hydrophobic collapsed form, which leads to a kinetic intermediate. This intermediate then unfolds further to the low salt form of the protein.