title: Exceptional stability of a [2Fe-2S] ferredoxin from hyperthermophilic bacterium Aquifex aeolicus.
abstract: Aquifex aeolicus is the only hyperthermophile that is known to contain a plant- and mammalian-type [2Fe-2S] ferredoxin (Aae Fd1). This unique protein contains two cysteines, in addition to the four that act as ligands of the [2Fe-2S] cluster, which form a disulfide bridge. We have investigated the stability of Aae Fd1 with (wild-type) and without (C87A variant) the disulfide bond, with respect to pH, thermal and chemical perturbation, and compared the results to those for the mesophilic [2Fe-2S] ferredoxin from spinach. Unfolding reactions of all three proteins are irreversible due to cluster decomposition in the unfolded state. Wild-type and C87A Aae Fd1 proteins are extremely stable: unfolding at 20 degrees C requires high concentrations of the chemical denaturant and long incubation times. Moreover, their thermal-unfolding midpoints are 40-50 degrees higher than that for spinach ferredoxin (pH 7). The stability of the Aae Fd1 protein is significantly lower at pH 2.5 than pH 7 and 10, suggesting that ionic interactions play a role in structural integrity. Interestingly, the iron-sulfur cluster in C87A Aae Fd1 rearranges into a transient species with absorption bands at 520 and 610 nm, presumably a linear three-iron cluster, in the high-pH unfolded state.
authors.affiliation: Department of Chemistry, Tulane University, 6823 St. Charles Avenue, New Orleans, LA 70118-5698, USA.
authors.forename: Catherine L
authors.initials: CL
authors.lastname: Higgins
authors.forename: Jacques
authors.initials: J
authors.lastname: Meyer
authors.forename: Pernilla
authors.initials: P
authors.lastname: Wittung-Stafshede
mesh_terms: D001419:Bacteria; D005288:Ferredoxins / Q000737:chemistry*; D019791:Guanidine; D006361:Heating; D006863:Hydrogen-Ion Concentration; D011489:Protein Denaturation; D017510:Protein Folding
chemical_list: C081714:2Fe-2S ferredoxin; D005288:Ferredoxins; D019791:Guanidine