Metal-ion chelators are useful tools in characterizing activation patterns of seven-transmembrane helix receptors (7TMRs), and have been successfully employed in unraveling activation-inducedhelical movements at numerous rhodopsin-like receptors [1, 2].Moreover, some receptors of the chemokine subclass of rhodopsin-like 7TMRs have been found to respond to micromolar concentrations of metal ion chelators in the presence of zinc salts [3-5], exhibiting activity most likely through the formation of a coordination complex consisting of negatively charged amino acid sidechains, metal ion and chelator, stabilized by secondary hydrophobic interactions between the chelator scaffold and aromatic amino acid sidechains in the receptor structure.Not much is known, however, of the exact molecular interactions that occur in the binding site of a receptor when a chelator ligand accompanied by transition metal ions, such as Zn2+, are in contact with the receptor. For this purpose, we combined in vitro mutagenesis studies with molecular docking experiments in order to map the binding site of the aqueous in situ zinc complexes of 2,2:6’,2”-terpyridine (ZnTpy) and 4-chloro-2,2’:6’,2”-terpyridine (ZnClTpy), two polypyridine ligands which, in spite of only one structural difference, exhibit different behaviors at the chemokine receptor CCR5in the presence of ZnCl2[4]. Thus, ZnTpy acts an ago-allosteric modulator, both enhancing the binding of CCL3 and having activity on its own at CCR5, while ZnClTpy acts as a purely allosteric ligand, enhancing the binding of the chemokine alone [4]. We found that, aside the complete lack of activity of the aqueous complexes at the E283A mutant, F109 is crucial for the allosteric behavior of both aqueous complexes, while aromatic residues lining the major binding pocket exclusively impact the intrinsic activity of ZnTpy. We furthermore found that residues in the minor binding pocket are distinctively important for both aqueous complexes. Taken together, these findings suggest that both complexes are accommodated in the binding site of the receptor in a similar manner, an observation which was additionally supported by molecular docking studies, further indicating that the molecular outcome of receptor activation cannot be exclusively revealed from mutational analysis, and most likely involves changes in receptor conformation undetectable by mutagenesis or molecular docking studies.