Slow binding modulation of paraoxonase 1 activity with terbium ions

Paraoxonase 1 (PON1) is a metalloenzyme that requires calcium ions at both catalytic and structural binding sites to hydrolyze the substrates. The enzyme is efficiently inhibited by several metal ions, especially transition metals, which tend to bind non-specifically to oxygen, nitrogen, and sulfur ligands of amino acid residues on the PON1 surface. In contrast, several lanthanide ions can specifically replace isomorphous Ca²⁺ ions from many protein binding sites, making them among the most potent metal inhibitors of PON1. However, the exact kinetic effects of lanthanides on PON1 activity are not well understood. Therefore, we investigated the inhibitory effects of Tb³⁺ ions on recombinant PON1 (rePON1) to elucidate how Tb³⁺ binding modulates its enzymatic activity. Our results reveal that Tb³⁺ functions as a slow-binding, reversible inhibitor of rePON1's lactonase activity through a sequential, two-step mechanism involving both metal-binding sites.