Regulation of ADP-Glucose Pyrophosphorylase from Chlorella vulgaris

ADP-glucose pyrophosphorylase was partially purified from Chlorella vulgaris 11h. 3-Phosphoglycerate activated the enzyme by lowering the Michaelis constant for glucose-1-phosphate (from 0.97 to 0.36 millimolar in the presence of 2 millimolar phosphoglycerate) and ATP (from 0.23 to 0.10 millimolar), as well as increasing the V(max). Saturation curves for 3-phosphoglycerate were hyperbolic and the activator concentration at half V(max) value for 3-phosphoglycerate was 0.41 millimolar either in the presence or absence of phosphate. Phosphate inhibited the enzyme in a competitive manner with respect to glucose-1-phosphate, but did not affect the Michaelis constant value for ATP. 3-Phosphoglycerate changed neither the inhibitor concentration at half V(max) value of 1.0 millimolar for phosphate nor the hyperbolic inhibition kinetics for phosphate. The enzyme required divalent cations for its activity. The activation curves for Mn(2+) and Mg(2+) were highly sigmoidal. The activator concentration at half V(max) values for Mn(2+) and Mg(2+) were 2.8 and 3.7 millimolar, respectively. With optimal cations, the Michaelis constant values for ATP-Mn and ATP-Mg were 0.1 and 0.4 millimolar, respectively.