Crystallographic and X-ray absorption spectroscopic characterization of Helicobacter pylori UreE bound to Ni2+ and Zn2+ reveals a role for the disordered C-terminal arm in metal trafficking

The survival and growth of the pathogen Helicobacter pylori in the gastric acidic environment is ensured by the activity of urease, an enzyme containing two essential Ni²⁺ ions in the active site. The metallo-chaperone UreE facilitates in vivo Ni²⁺ insertion into the apoenzyme. Crystals of apo-HpUreE (H. pylori UreE) and its Ni⁺- and Zn⁺-bound forms were obtained from protein solutions in the absence and presence of the metal ions. The crystal structures of the homodimeric protein, determined at 2.00 Å (apo), 1.59 Å (Ni²⁺) and 2.52 Å (Zn²⁺) resolution, show the conserved proximal and solvent-exposed His¹⁰² residues from two adjacent monomers invariably involved in metal binding. The C-terminal regions of the apoprotein are disordered in the crystal, but acquire significant ordering in the presence of the metal ions due to the binding of His¹⁵². The analysis of X-ray absorption spectral data obtained using solutions of Ni²⁺- and Zn²⁺-bound HpUreE provided accurate information of the metal-ion environment in the absence of solid-state effects. These results reveal the role of the histidine residues at the protein C-terminus in metal-ion binding, and the mutual influence of protein framework and metal-ion stereo-electronic properties in establishing co-ordination number and geometry leading to metal selectivity.