Differences between Mg(2+) 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 degrees C. Analysis yielded estimates for thermodynamic parameters for the activation of calcineurin by each of the metal ions. Values for DeltaG(Me)( degrees ) were varied with the best activators resulting in more stable enzyme-metal ion complexes and with DeltaG(Me)( degrees ) dominated by the entropic component. Mg(2+) was the only nontransition metal ion which supported significant activity and showed some distinct characteristics including a negative DeltaS(Me)( degrees ), suggesting that activation by Mg(2+) may have resulted in a unique enzyme-metal ion form. Circular dichroism showed that metal ions increased the alpha-helical content of calcineurin, but little significant differences in the spectra were identified between using activating and nonactivating metal ions. Activating Mg(2+), but not nonactivating Ca(2+), did cause changes in the Fourier transform infrared photoacoustic spectrum of calcineurin compared to the spectrum of calcineurin with Mn(2+). Other metal ions, Co(2+) and Ni(2+), also caused no changes in the infrared spectrum. Possible explanations for these differences between Mg(2+) and transition metal ions in the activation of calcineurin are discussed.