Functional characterisation of two ferric-ion coordination modes of TtFbpA, the periplasmic subunit of an ABC-type iron transporter from Thermus thermophilus HB8
Peng Lu, Yoshitaka Moriwaki, Mimin Zhang, Yukie Katayama, Yi Lu, Ken Okamoto, Tohru Terada, Kentaro Shimizu, Mengyao Wang, Takehiro Kamiya, Toru Fujiwara, Tomiko Asakura, Michio Suzuki, Etsuro Yoshimura, Koji Nagata
The ferric ion binding protein A of Thermus thermophilus HB8 (TtFbpA) is the periplasmic subunit of an ABC-type iron transporter. Two Fe3+-bound crystal structures at pH 5.5 and pH 7.5 and one apo structure have been reported for TtFbpA. In addition to three Tyr residues, TtFbpA coordinates with Fe3+ using two monodentate HCO3− and one H2O to form a six-coordinated mode at pH 5.5 or one bidentate CO32− to form a five-coordinated mode at pH 7.5. We investigated the biological significance of these Fe3+-bound forms of TtFbpA and the synergistic anions (HCO3− and CO32−). Quantum mechanical calculations in silico indicated that only these coordination modes were plausible out of six possibilities. Comparison of the crystal structures revealed a key motif, RZX1X2L(I/V), that would couple the Fe3+ coordination mode and the TtFbpA protein conformation. Both gel filtration chromatography and isothermal titration calorimetry showed that TtFbpA could bind Fe3+ at pH 7.5 but not at pH 5.5. Isothermal titration calorimetry also revealed that the binding at pH 7.5 was a three-step endothermic reaction that required NaHCO3. These results indicate that the holo structure at pH 5.5 is unstable in solution and may correspond to a transition state of TtFbpA–Fe3+ binding at pH 7.5 because HCO3− is much more abundant than CO32− at both pH values. Reorganisation of the synergistic ions and coupled protein conformational change will occur to form the stable TtFbpA–Fe3+ complex at pH 7.5, but not at pH 5.5. Identification of such a transition state will contribute to molecular design of novel FbpA inhibitors.
Absorption, Chemical stability, Coordination chemistry, Circular dichroism, Secondary structure, Protein folding, Inorganic chemistry, Biochemistry