New insights into the thermostability of bacterial ferredoxins: High-resolution crystal structure of the seven-iron ferredoxin from Thermus thermophilus

The crystal structure of the seven-iron ferredoxin from Thermus thermophilus (FdTt) has been determined at 1.64 Å resolution, allowing us to unveil the common mechanisms of thermostabilization within "bacterial-type" ferredoxins. FdTt and other homologous thermophilic seven-iron ferredoxins are smaller than their mesophilic counterparts. Thermostabilizing features are optimized in a minimal structural and functional unit, with an extensive cross-linking of secondary structure elements mediated by improved polar and hydrophobic interactions. Most of the potentially stabilizing features are focused on the vicinity of the functional [3Fe-4S] cluster. The structural [4Fe-4S] cluster is shielded in thermophilic FdTt by an increased number of polar interactions involving the two N-terminal residues. Comparisons with the hyperthermostable ferredoxin from Thermotoga maritima reveal that (1) a reduction in the number of non-glycine residues in strained conformations, (2) improved polar interactions within the common iron-sulfur cluster binding (βαβ)₂ motif, and (3) an optimized charge distribution at the protein surface, constitute a common strategy for increasing the thermal stability of these ferredoxins.