One of the better understood structural correlates of Fe3+ binding by the transferrins is the conformational shift demonstrated by both lobes. FbpA, a prokaryotic protein involved in periplasmic iron transport, has previously been shown to be structurally and functionally homologous to the transferrins. Similar to each individual lobe of the transferrins, it is hypothesized that FbpA exists in two distinct conformations depending on whether metal is bound. Evidence for these changes is provided by the differential susceptibility of FbpA to trypsin digestion. Binding of Fe3+ by FbpA significantly decreases the ability of trypsin to digest wild-type protein. Construction of a null binding mutant, Tyr195Ile, confirms that protein "locked" in the apo-conformation is similarly susceptible to trypsin. This mutant also marks the initial characterization of an FbpA molecule unable to bind iron, suggesting that the Tyr195 residue is directly involved in iron binding. Other FbpA mutants which do bind iron show moderate resistance to digestion which suggests that they remain in the holo-protein conformation when binding Fe3+. The conformational states of FbpA may have important implications in protein-protein recognition during transport of Fe3+ between membranes, and may explain how these proteins function in the context of periplasm-to-cytosol Fe3+ transport.