The ferric ion-binding protein A (FbpA), a member of transferrin superfamily, is a periplasmic iron transporter employed by many Gram-negative pathogens. Our experiments indicated copper(II) could bind with Neisseria gonorrhoeae FbpA (NgFbpA), and the binding constant reached up to (8.7 ± 0.2) × 108 M−1 via UV-vis titration. The crystal structure of recombinant Cu–NgFbpA at 2.1 Å revealed that the oxo-Cu2 clusters (dinuclear centres) assembled in the iron binding cleft and were bound to the two adjacent tyrosine residues (Y195 and Y196) of the protein, two Cu ions coordinated with two tyrosines, Y195 and Y196, respectively, which was different from the binding model of Fe ion with FbpA, in which Y195 and Y196 coordinated together with one Fe ion. While this was similar to the binding of Zr and Hf ion clusters, Y195 and Y196 coordinated with two metal ions and the μ-oxo-bridges linking the metal ions. Structural superimposition demonstrated that oxo-Cu2–NgFbpA still keeping an open conformation, similar to the apo-form of NgFbpA. The structure presented additional information towards an understanding of the function of FbpA, and provided a detailed binding model for FbpA protein with the possible metal ions in a biological system.
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