The ferric binding protein belongs to the substrate-binding protein super-family and transports ferric ions across the periplasmic space in gram negative bacteria. This process involves the binding and release of ferric ions through conformational changes of the ferric binding protein, and the assistance of a synergistic anion. Here we report the crystal structure of Thermus thermophilus HB8's (TtFbpA) ferric binding protein A in four different forms, which represent the apo state (apo-TtFbpA), the carbonate-bound state (TtFbpACO(3)),and the iron- and carbonate-bound state (TtFbpAFeCO(3)). The ferric ion in TtFbpAFeCO(3) is bound by three tyrosine residues from TtFbpA and one synergistic carbonate ion. Structural comparisons among the three different states reveal the molecular mechanisms of iron-binding by TtFbpA. Our results, together with previous studies on other bacterial periplasmic ferric binding proteins, provide a complete understanding of the structural basis for iron binding and release in the periplasm of gram-negative bacteria. (C) 2013 Elsevier Inc. All rights reserved.