Novel anion-independent iron coordination by members of a third class of bacterial periplasmic ferric ion-binding proteins.

The uptake of the element iron is vital for the survival of most organisms. Numerous pathogenic Gram-negative bacteria utilize a periplasm-to-cytosol ATP-binding cassette transport pathway to transport this essential atom in to the cell. In this study, we investigated the Yersinia enterocolitica (YfuA) and Serratia marcescens (SfuA) iron-binding periplasmic proteins. We have determined the 1.8-angstroms structures of iron-loaded (YfuA) and iron-free (SfuA) forms of this class of proteins. Although the sequence of these proteins varies considerably from the other members of the transferrin structural superfamily, they adopt the same three-dimensional fold. The iron-loaded YfuA structure illustrates the unique nature of this new class of proteins in that they are able to octahedrally coordinate the ferric ion in the absence of a bound anion. The iron-free SfuA structure contains a bound citrate anion in the iron-binding cleft that tethers the N- and C-terminal domains of the apo protein and stabilizes the partially open structure.