The role of cations in avian liver phosphoenolpyruvate carboxykinase catalysis. Activation and regulation.

The activation of the avian liver phosphoenolpyruvate carboxykinase catalyzed reversible decarboxylation of oxalacetate by Mn2+ has been studied. The Mn2+ facilitates the interaction of oxalacetate to the enzyme. At saturating ITP and oxalacetate concentrations, Mn2+ has a Km = 2 microM. The cation Mg2+ can substitute for Mn2+ with 50% Vmax in the oxalacetate decarboxylation and 2% Vmax in the oxalacetate formation. The Km for Mg2+ is 3 orders of magnitude greater than the Km of Mn2+, however. Of the other cations tested (Ca2+, Zn2+, Fe2+, and Cd2+), Co2+ was the only other cation found to activate the enzyme in both directions. In the presence of Mg2+, the enzyme is extremely sensitive to trace metal contaminants which can cause activation. At 1 mM Mg2+, 20 microM Mn2+ causes a 15-fold activation of activity. The apparent Km for Mn2+ (2 microM) at high concentrations of Mg2+ is the identical value calculated for free Mn2+. In a mixed metal (Mg2+ and Mn2+) assay, the Km values for phosphoenolpyruvate and for oxalacetate are independent of the concentration of Mg2+ but decrease upon an increase in Mn2+. The kinetic results demonstrate two roles for the divalent cations for activity. The cation forms a metal . nucleotide complex which serves as the substrate. The results indicate that MgITP is a better substrate than MnITP, but MnIDP is a better substrate than MgIDP. The cation also binds to the enzyme to form an enzyme . metal complex which is the active form of the enzyme. This cation functions to aid in the interaction of either oxalacetate or phosphoenolpyruvate to the enzyme. The activation by micromolar amounts of Mn2+ at millimolar concentrations of Mg2+ suggests that this enzyme, and thus the pathway of gluconeogenesis, can be modulated by changes in concentration of Mn2+ within mitochondria.