Effects of magnesium, manganese and adenosine triphosphate ions on pyruvate carboxylase from baker's yeast.
1. Pyruvate carboxylase from baker's yeast acts with either MgATP(2-) or MnATP(2-) as substrate. The optimum pH for the enzyme reaction is 8.0 with MgATP(2-) and 7.0 with MnATP(2-). 2. When the reaction velocity is plotted against MgATP(2-) (or MnATP(2-)) concentration slightly sigmoid curves are obtained, either in the presence or in the absence of acetyl-CoA (an allosteric activator). In the presence of excess of free Mg(2+) (or Mn(2+)) the curves turn into hyperbolae, whereas in the presence of excess of free ATP(4-) the apparent sigmoidicity of the curves increases. 3. The sigmoidicity of the plots of v against MgATP(2-) (or MnATP(2-)) concentration can be explained by the inhibitory effect of free ATP(4-), the concentration of which, in the experimental conditions employed, is significant and varies according to the total concentration of the ATP-magnesium chloride (or ATP-manganese chloride) mixture. Free ATP(4-) behaves as a negative modifier of yeast pyruvate carboxylase. 4. The effect of high concentrations of Mg(2+) (or Mn(2+)) on the kinetics of yeast pyruvate carboxylase can be explained as a deinhibition with respect to ATP(4-), instead of a direct enzyme activation. 5. At pH6.5 manganese chloride is more effective than magnesium chloride as enzyme activator even in the presence of a great excess (16-fold) of the latter. This is consistent with a significant contribution of the MnATP(2-) complex to the activity of yeast pyruvate carboxylase, in medium conditions resembling those existing inside the yeast cell (pH6.25-6.75; 12mm-magnesium chloride and 0.75mm-manganese chloride). 6. The physiological significance of the enzyme inhibition by free ATP(4-) is doubtful since the Mg(2+) and Mn(2+) concentrations reported to exist inside the yeast cell are sufficient to decrease ATP(4-) concentrations to ineffective values.
D000255:Adenosine Triphosphate; D055598:Chemical Phenomena; D002621:Chemistry; D002712:Chlorides; D003065:Coenzyme A; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008025:Ligases / Q000037:antagonists & inhibitors; D008274:Magnesium; D008345:Manganese; D010071:Oxaloacetates; D011773:Pyruvates; D012440:Saccharomyces / Q000201:enzymology*; D013053:Spectrophotometry
D002712:Chlorides; D010071:Oxaloacetates; D011773:Pyruvates; D008345:Manganese; D000255:Adenosine Triphosphate; D008025:Ligases; D008274:Magnesium; D003065:Coenzyme A
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