Salt dependent stability and unfolding of [Fe2-S2] ferredoxin of Halobacterium salinarum: Spectroscopic investigations

Ferredoxin from the haloarchaeon Halobacterium salinarum is a 14.6-kDa prot ein with a [Fe2-S2] center and is involved in the oxidative decarboxylation of 2-oxoacids. It possesses a high molar excess of acidic amino acid resid ues and is stable at high salt concentration. We have purified the protein from this extreme haloarchaeon and investigated its salt-dependent stabilit y by circular dichroism, fluorescence, and absorption techniques. The predo minantly beta-sheeted protein is stable in salt concentrations of greater t han or equal to 1.5 M NaCl. At lower concentrations a time-dependent increa se in fluorescence intensity ratio (I-360:/(330)), a decrease in the absorp tion at 420 nm, and a decrease in ellipticity values are observed. The rate of fluorescence intensity change at any low salt concentration is the high est, followed by absorption and ellipticity. This suggests that at low salt the unfolding of ferredoxin starts with the loss of tertiary structure, wh ich leads to the disruption of the [Fe2-S2] center, resulting in the loss o f secondary structural elements.