Several functions of HRG have been described to be mediated by Zn II . In the physiological context, free Zn II is cytotoxic at low concentrations, thus plasma Zn II levels are tightly regulated, with the majority of the 15-20 M Zn II present in plasma bound to (mostly) albumin and small molecules, keeping the free Zn II concentration in the nanomolar range. Within the organism, increased Zn II levels can arise following release from activated platelets or damaged cells, reaching transient localised Zn II concentrations estimated to be up to 50 M allowing binding to effector proteins such as HRG. Zn II binding to HRG was demonstrated to exert a regulatory effect on HRG binding to other targets and ligands by modifying respective affinities. The large number of histidine residues makes the HRR the primary suspect for binding the divalent metal ions. Early binding studies reported sites accommodating approximately 10 to 20 divalent metal ions, and a more recent study confirmed the binding of 10 Zn II ions using isothermal titration calorimetry (ITC). This number is consistent with approximately 12 tandem repeats of the consensus sequence G[H/P][H/P]PH present within the HRR in HRG (13 repeats in rabbit, see Figure ), which can function as high-affinity metal ion binding sites by offering coordination by multiple histidine residues. However, a detailed understanding of the local environment of the metal ion binding sites is lacking.