Biosynthesis of starch in Chlorella pyrenoidosa: II. Regulation of ATP: α- d-Glucose 1-phosphate adenyl transferase (ADP-Glucose pyrophosphorylase) by inorganic phosphate and 3-phosphoglycerate

Abstract

ADP-Glucose pyrophosphorylase from Chlorella pyrenoidosa has been partially purified and its properties have been investigated. The enzyme activity was activated about 20-fold by 3-phosphoglyceric acid, the first product of the photosynthetic reduction cycle. The enzyme appears to be separate and distinct from UDP-glucose pyrophosphorylase, which was not activated by 3-phosphoglycerate. ADP-Glucose and ATP gave sigmoid-shaped rate versus concentration curves in the presence or absence of 3-phosphoglycerate. The V max of pyrophosphorolysis and synthesis of ADP-glucose was increased 7-fold and 18-fold respectively by 3-phosphoglycerate, and the K values of ADP-glucose and ATP were decreased 1.6-fold in the presence of the activator. Inorganic phosphate and ADP were inhibitors of the enzyme. The K i for phosphate and ADP were 2 × 10 −4 m and 2.5 × 10 −3 m, respectively. The inhibition by phosphate can be reversed by high concentrations of the activator, 3-phosphoglycerate. However, only partial reversal of ADP inhibition occurred with high concentrations of 3-phosphoglycerate. The significance of 3-phosphoglycerate activation and inhibition by phosphate and ADP in the regulation of starch biosynthesis in Chlorella was discussed.