Structure and metal binding properties of ZnuA, a periplasmic zinc transporter from Escherichia coli

ZnuA is the periplasmic Zn²⁺-binding protein associated with the high-affinity ATP-binding cassette ZnuABC transporter from Escherichia coli. Although several structures of ZnuA and its homologs have been determined, details regarding metal ion stoichiometry, affinity, and specificity as well as the mechanism of metal uptake and transfer remain unclear. The crystal structures of E. coli ZnuA (Eco-ZnuA) in the apo, Zn²⁺-bound, and Co²⁺-bound forms have been determined. ZnZnuA binds at least two metal ions. The first, observed previously in other structures, is coordinated tetrahedrally by Glu59, His60, His143, and His207. Replacement of Zn ²⁺ with Co²⁺ results in almost identical coordination geometry at this site. The second metal binding site involves His224 and several yet to be identified residues from the His-rich loop that is unique to Zn ²⁺ periplasmic metal binding receptors. Electron paramagnetic resonance and X-ray absorption spectroscopic data on CoZnuA provide additional insight into possible residues involved in this second site. The second site is also detected by metal analysis and circular dichroism (CD) titrations. Eco-ZnuA binds Zn²⁺ (estimated K _d < 20 nM), Co²⁺, Ni²⁺, Cu²⁺, Cu⁺, and Cd²⁺, but not Mn²⁺. Finally, conformational changes upon metal binding observed in the crystal structures together with fluorescence and CD data indicate that only Zn²⁺ substantially stabilizes ZnuA and might facilitate recognition of ZnuB and subsequent metal transfer.