Holo-Ni2+ Helicobacter pylori NikR contains four square-planar nickel-binding sites at physiological pH.

@article{Benini2011HoloNi2HP,
  title={Holo-Ni2+ Helicobacter pylori NikR contains four square-planar nickel-binding sites at physiological pH.},
  author={Stefano Benini and Michele Cianci and Stefano Ciurli},
  journal={Dalton transactions},
  year={2011},
  volume={40 31},
  pages={7831-3}
}
The crystal structure of Helicobacter pylori holo-NikR, a Ni(2+)-dependent transcription factor, determined at pH 7.3, shows four square-planar nickel-binding sites, involving one cysteinate and three histidine ligands. This observation reconciles previous inconsistencies among calorimetric data, structural information at non-physiological pH, and computational studies.