Anion-independent Iron Coordination by the Campylobacter jejuni Ferric Binding Protein

Abstract

Campylobacter jejuni, the leading cause of human gastroenteritis, expresses a ferric binding protein (cFbpA) that in many pathogenic bacteria functions to acquire iron as part of their virulence repertoire. Recombinant cFbpA is isolated with ferric iron bound from Escherichia coli. The crystal structure of cFbpA reveals unprecedented iron coordination by only five protein ligands. The histidine and one tyrosine are derived from the N-terminal domain, whereas the three remaining tyrosine ligands are from the C-terminal domain. Surprisingly, a synergistic anion present in all other characterized ferric transport proteins is not observed in the cFbpA iron-binding site, suggesting a novel role for this protein in iron uptake. Furthermore, cFbpA is shown to bind iron with high affinity similar to Neisserial FbpA and exhibits an unusual preference for ferrous iron (oxidized subsequently to the ferric form) or ferric iron chelated by oxalate. Sequence and structure analyses reveal that cFbpA is a member of a new class of ferric binding proteins that includes homologs from invasive and intracellular bacteria as well as cyanobacteria. Overall, six classes are defined based on clustering within the tree and by their putative iron coordination. The absence of a synergistic anion in the iron coordination sphere of cFbpA also suggests an alternative model of evolution for FbpA homologs involving an early ironbinding ancestor instead of a requirement for a preexisting anion-binding ancestor.

FutA2 Is a Ferric Binding Protein from Synechocystis PCC 6803

“…An exception is the Fbp from Yersinia enterocolitica, which has five protein-derived ligands and a coordinated water molecule (38). It has been suggested that the class III proteins represent the earliest forms of Fbps from which proteins with modified iron sites and different synergistic anion-binding requirements have evolved (33). FutA1 and FutA2 have been assigned to this class, and for the former this has been confirmed by the crystal structure of the protein (36).…”

“…The class III Fbp from Campylobacter jejuni (cFbpA) reportedly binds Fe(II), which is subsequently oxidized to Fe(III) (33). It has recently been suggested that both FutA1 and FutA2 favor Fe(II), with hardly any detectable Fe(III) binding and that this preference is likely to extend to all class III Fbps (36).…”

“…To date, FutA1 has not been identified in the soluble fraction of the periplasm (24,26,27). Fbps have been classified on the basis of sequence and structure, including known iron coordination environments (33). The so-called class III members seem not to possess a small molecule anionic ligand, a feature thought to be common to all other classes of Fbps (11,(33)(34)(35)(36) and mammalian transferrins (3,37).…”

“…Fbps have been classified on the basis of sequence and structure, including known iron coordination environments (33). The so-called class III members seem not to possess a small molecule anionic ligand, a feature thought to be common to all other classes of Fbps (11,(33)(34)(35)(36) and mammalian transferrins (3,37). An exception is the Fbp from Yersinia enterocolitica, which has five protein-derived ligands and a coordinated water molecule (38).…”

Ferric ion (hydr)oxo clusters in the “Venus flytrap” cleft of FbpA: Mössbauer, calorimetric and mass spectrometric studies

“…Our present investigation involves the study of multiple iron(III) binding to apo-nFbpA using iron citrates as the metal source. Based on a wild-type variant of FbpA isolated from Bordetella pertussis, [28,29] we have also studied the apo-nFbpA and H9Y-nFbpA fitted using a single-site binding model. The theoretical fit of the data suggests either that four iron ions are bound to a single-site or that four iron atoms are bound to four identical sites, and that the binding reaction is exothermic.…”