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Calprotectin (CP) is an abundant metal-chelating protein involved in host defense, and the ability of human CP to bind Fe(ii) in a calcium-dependent manner was recently discovered. In the present study, near-infrared magnetic circular dichroism spectroscopy is employed to investigate the nature of Fe(ii) coordination at the two transition-metal-binding sites of CP that are a His 3 Asp motif (site 1) and a His 6 motif (site 2). Upon the addition of sub-stoichiometric Fe(ii), a six-coordinate (6C) Fe(ii) center associated with site 2 is preferentially formed in the presence of excess Ca(ii). This site exhibits an exceptionally large ligand field (10 D  q = 11 045 cm -1 ) for a non-heme Fe(ii) protein. Analysis of CP variants lacking residues of the His 6 motif supports that CP coordinates Fe(ii) at site 2 by employing six His ligands. In the presence of greater than one equiv. of Fe(ii) or upon mutation of the His 6 motif, the metal ion also binds at site 1 of CP to form a five-coordinate (5C) Fe(ii)-His 3 Asp motif that was previously unidentified in this system. Notably, the introduction of His-to-Ala mutations at the His 6 motif results in a mixture of 6C (site 2) and 5C (site 1) signals in the presence of sub-stoichiometric Fe(ii). These results are consistent with a reduced Fe(ii)-binding affinity of site 2 as more weakly coordinating water-derived ligands complete the 6C site. In the absence of Ca(ii), both sites 1 and 2 are occupied upon addition of sub-stoichiometric Fe(ii), and a stronger ligand field is observed for the 5C site. These spectroscopic studies provide further evaluation of a unique non-heme Fe(ii)-His 6 site for metalloproteins and support the notion that Ca(ii) ions influence the Fe(ii)-binding properties of CP.

Magnetic circular dichroism studies of iron(ii) binding to human calprotectin