[1] Binding of metal ions via chelation is usually achieved via histidines or cysteines.
In other cases a coordinated metal cofactor is used in the active site of an enzyme to assist catalysis.
Poly-histidine tags (of six or more consecutive His residues) are utilized for protein purification by binding to columns with nickel or cobalt, with micromolar affinity.
[3] Furthermore, histidine-rich low-complexity regions are found in metal-binding and especially nickel-cobalt binding proteins.
[4] These histidine-rich low complexity regions have an average length of 36 residues, of which 53% histidine, 23% aspartate, 9% glutamate.