Effect of Magnesium Ion (Mg^2+) and the Magnesium Adenosine Triphosphate Ion (MgATP^2-) on Pigeon Liver Pyruvate Carboxylase1

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Abstract
Kinetic methods have been used to determine whether Mg^2+ and MgATP^2- play an important role in regulating pigeon liver pyruvate carboxylase [pyruvate: C0(2) ligase (ADP), EC 6.4.1.1]. Mg^2+ not only forms a complex with ATP^4- (MgATP^2-) but is also required for the enzyme activation (and probably for the binding of MgATP^2- to this enzyme). Contrary to the results of other investigators, the MgATP^2- complex was not found to activate pigeon liver pyruvate carboxylase. We could not demonstrate homotropic cooperativity with MgATP^2-. Excess Mg^2+ induced allosteric stimulation of the enzymatic activity at different concentrations of MgATP^2-. With different Mg^2+ concentrations, changes also occurred in the apparent Km, V(max) and R(s) values. Without excess of Mg^2+ (heterotropic effector) only about 2% of the total enzymic activity available could be demonstrated in the presence of MgATP^2-. It is concluded that Mg^2+ exhibits a homotropic cooperative effect and is required for the activation of this enzyme. Mg^2+ may bind either to a specific effector site, at the active site, or at the binding site for MgATP^2- which is capable of functioning as an effector site and in this way facilitates the carboxylation of pyruvate.

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