Crystallographic and biochemical analyses of the metal-free Haemophilus influenzae Fe3+-binding protein.
The crystal structure of the iron-free (apo) form of the Haemophilus influenzae Fe(3+)-binding protein (hFbp) has been determined to 1.75 A resolution. Information from this structure complements that derived from the holo structure with respect to the delineation of the process of iron binding and release. A 21 degrees rotation separates the two structural domains when the apo form is compared with the holo conformer, indicating that upon release of iron, the protein undergoes a change in conformation by bending about the central beta-sheet hinge. A surprising finding in the apo-hFbp structure was that the ternary binding site anion, observed in the crystals as phosphate, remained bound. In solution, apo-hFbp bound phosphate with an affinity K(d) of 2.3 x 10(-3) M. The presence of this ternary binding site anion appears to arrange the C-terminal iron-binding residues conducive to complementary binding to Fe(3+), while residues in the N-terminal binding domain must undergo induced fit to accommodate the Fe(3+) ligand. These observations suggest a binding process, the first step of which is the binding of a synergistic anion such as phosphate to the C-terminal domain. Next, iron binds to the preordered half-site on the C-terminal domain. Finally, the presence of iron organizes the N-terminal half-site and closes the interdomain hinge. The use of the synergistic anion and this iron binding process results in an extremely high affinity of the Fe(3+)-binding proteins for Fe(3+) (nFbp K'(eff) = 2.4 x 10(18) M(-1)). This high-affinity ligand binding process is unique among the family of bacterial periplasmic binding proteins and has interesting implications in the mechanism of iron removal from the Fe(3+)-binding proteins during FbpABC-mediated iron transport across the cytoplasmic membrane.
Bruns
C M
CM
Department of Molecular Biology MB8, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA.
Anderson
D S
DS
Vaughan
K G
KG
Williams
P A
PA
Nowalk
A J
AJ
McRee
D E
DE
Mietzner
T A
TA
D000838:Anions / Q000378:metabolism; D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D018360:Crystallography, X-Ray; D006193:Haemophilus influenzae / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism; D006868:Hydrolysis; D007501:Iron / Q000378:metabolism*; D033862:Iron-Binding Proteins; D033902:Periplasmic Binding Proteins; D010710:Phosphates / Q000378:metabolism; D014357:Trypsin / Q000494:pharmacology
D000838:Anions; D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D033862:Iron-Binding Proteins; D033902:Periplasmic Binding Proteins; D010710:Phosphates; D007501:Iron; D014357:Trypsin