Probing the metal ion selectivity in methionine aminopeptidase via changes in the luminescence properties of the enzyme bound europium ion

We report herein, for the first time, that Europium ion (Eu3+) binds to the “apo” form of Escherichia coli methionine aminopeptidase (EcMetAP), and such binding results in the activation of the enzyme as well as enhancement in the luminescence intensity of the metal ion. Due to competitive displacement of the enzyme-bound Eu3+ by different metal ions, we could determine the binding affinities of both “activating” and “non-activating” metal ions for the enzyme via fluorescence spectroscopy. The experimental data revealed that among all metal ions, Fe2+ exhibited the highest binding affinity for the enzyme, supporting the notion that it serves as the physiological metal ion for the enzyme. However, the enzyme–metal binding data did not adhere to the Irving–William series. On accounting for the binding affinity vis a vis the catalytic efficiency of the enzyme for different metal ions, it appears evident that that the “coordination states” and the relative softness” of metal ions are the major determinants in facilitating the EcMetAP catalyzed reaction.

We provide evidence that the luminescence signal of Eu3+ is significantly enhanced upon binding to E. coli methionine aminopeptidase, and it serves as an excellent probe for determining the binding affinities of different metal ions to the enzyme. With kinetic data, we provide physico-chemical rationale of metal ions in activating the enzymeFigure optionsDownload as PowerPoint slideHighlights
► E. coli methionine aminopeptidase (MetAp) is activated by Eu3+.
► The luminescence signal of the enzyme-bound Eu3+ serves as a probe for determining the binding affinity of different metal ions.
► Coordination geometry and Lewis acid softness are major determinants of metal ion selectivity in MetAP catalyzed reaction.