E275 and F276 in β12-β13 Loop of Protein Phosphatase-1 Resist Mn 2+ -Mediated Activation

Studies on the activation of isocitrate dehydrogenase kinase/phosphatase (AceK) by Mn2+ and Mg2

Isocitrate dehydrogenase kinase/phosphatase (AceK) is a bifunctional enzyme with both kinase and phosphatase activities that are activated by Mg²⁺. We have studied the interactions of Mn²⁺and Mg²⁺ with AceK using isothermal titration calorimetry (ITC) combined with molecular docking simulations and show for the first time that Mn²⁺ also activates the enzyme activities. However, Mn²⁺ and Mg²⁺ exert their effects by different mechanisms. Although they have similar binding constants (of 1.11 × 10⁵ and 0.98 × 10⁵ M^-1, respectively) for AceK and induce conformational changes of the enzyme, they do not compete for the same binding site. Instead Mn²⁺ appears to bind to the regulatory domain of AceK, and its effect is transmitted to the active site of the enzyme by the conformational change that it induces. The information in this study should be very useful for understanding the molecular mechanism underlying the interaction between AceK and metal ions, especially Mn²⁺ and Mg²⁺.