Differences between Mg2+ and Transition Metal Ions in the Activation of Calcineurin

Exogenous metal ion activation of calcineurin catalyzed hydrolysis of para-nitrophenyl phosphate was kinetically characterized at 20, 25, 30, and 37°C. Analysis yielded estimates for thermodynamic parameters for the activation of calcineurin by each of the metal ions. Values for ΔGMe° were varied with the best activators resulting in more stable enzyme–metal ion complexes and with ΔGMe° dominated by the entropic component. Mg2+ was the only nontransition metal ion which supported significant activity and showed some distinct characteristics including a negative ΔSMe°, suggesting that activation by Mg2+ may have resulted in a unique enzyme–metal ion form. Circular dichroism showed that metal ions increased the α-helical content of calcineurin, but little significant differences in the spectra were identified between using activating and nonactivating metal ions. Activating Mg2+, but not nonactivating Ca2+, did cause changes in the Fourier transform infrared photoacoustic spectrum of calcineurin compared to the spectrum of calcineurin with Mn2+. Other metal ions, Co2+ and Ni2+, also caused no changes in the infrared spectrum. Possible explanations for these differences between Mg2+ and transition metal ions in the activation of calcineurin are discussed.