title: Effects of Mn2+ on the exchange reaction of phosphoenolpyruvate carboxykinase in the presence of high concentrations of Mg2+.
abstract: The mitochondrial phosphoenolpyruvate carboxykinase (GTP:oxaloacetate carboxy-lyase (transphosphorylating), EC 4.1.1.32), purified from chick embryo liver, was synergistically activated by a combination of Mn2+ and Mg2+ in the oxaloacetate ---- H14CO-3 exchange reaction. Increases in the Mg2+ concentration caused decreases in the K0.5 value of Mn2+ in line with the earlier finding that the enzyme was markedly activated by low Mn2+ (microM) plus high Mg2+ (mM). In the presence of 2.5 mM Mg2+, increases in the Mn2+ level first enhanced the activity of phosphoenolpyruvate carboxykinase, and then suppressed it to the maximal velocity shown in the presence of Mn2+ alone. Kinetic studies showed that high Mn2+ inhibited the activity of Mg2+ noncompetitively, and those of GTP and oxaloacetate uncompetitively. The inhibition constant for oxaloacetate (K'i = 550 microM) was lower than that of Mg2+ (Ki = K'i = 860 microM) or GTP (K'i = 1.6 mM), and was nearly equal to the apparent half-maximal inhibition concentration of Mn2+. These results suggested that Mn2+ can play two roles, of activating and suppressing phosphoenolpyruvate carboxykinase activity in the presence of high Mg2+.
mesh_terms: D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D002642:Chick Embryo; D006160:Guanosine Triphosphate / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000196:embryology / Q000201:enzymology; D008274:Magnesium / Q000378:metabolism*; D008345:Manganese / Q000378:metabolism*; D008928:Mitochondria / Q000201:enzymology; D010071:Oxaloacetates / Q000378:metabolism; D010729:Phosphoenolpyruvate Carboxykinase (GTP) / Q000378:metabolism*
chemical_list: D001639:Bicarbonates; D010071:Oxaloacetates; D008345:Manganese; D006160:Guanosine Triphosphate; D010729:Phosphoenolpyruvate Carboxykinase (GTP); D008274:Magnesium