Intramolecular sensitization and structure of a Tb$^{3+}$/2-hydroxyquinoline conjugate in the paraoxonase 1 active site

Paraoxonase 1 (PON1) is a Ca$^{2+}$-dependent enzyme involved in oxidative stress processes and is widely studied for its protective roles in various diseases. Intermolecular sensitization of lanthanide ions was implemented by replacing Ca$^{2+}$ ions from the recombinant PON1 (rePON1) catalytic site in the presence of 2-hydroxyquinoline (2HQ) as an external antenna. Although the replacement of Ca$^{2+}$ ions with lanthanide ions indicates weaker binding affinity for the coordination of 2HQ in the protein milieu of the rePON1 active site, it results in the formation of a highly emissive supramolecular complex in the case of Tb$^{3+}$ ions. The architecture of the ternary rePON1 : Tb$^{3+}$ : 2HQ conjugate, which allows efficient terbium sensitization and its specific long-wavelength metal phosphorescence emission, was resolved by X-ray crystallography. These findings could establish a non-catalytic quantification strategy for PON1 and provide additional structural insights into lanthanide substitution in this Ca$^{2+}$-dependent enzyme.