Adenosine diphosphate glucose pyrophosphatase: A plastidial phosphodiesterase that prevents starch biosynthesis

A distinct phosphodiesterasic activity (EC 3.1.4) was found in both mono- a nd dicotyledonous plants that catalyzes the hydrolytic breakdown of ADPgluc ose (ADPG) to produce equimolar amounts of glucose-1-phosphate and AMP. The enzyme responsible for this activity, referred to as ADPG pyrophosphatase (AGPPase). was purified over 1.100-fold from barley leaves and subjected to biochemical characterization. The calculated K-eq' (modified equilibrium c onstant) value for the ADPG hydrolytic reaction at pH 7.0 and 25 degrees C is 110. and its standard-state free-energy change value (Delta G') is -2.9 kcal/mol (1 kcal = 4.18 kJ). Kinetic analyses showed that, although AGPPase can hydrolyze several low-molecular weight phosphodiester bond-containing compounds. ADPG proved to be the best substrate (K-m = 0.5 mM). P-i and pho sphorylated compounds such as 3-phosphoglycerate. PPi, ATP, ADP, NADP(+). a nd AMP are inhibitors of AGPPase. Subcellular localization studies revealed that AGPPase is localized exclusively in the plastidial compartment of cul tured cells of sycamore (Acer pseudoplatanus L.). whereas it occurs both in side and outside the plastid in barley endosperm. In this paper, evidence i s presented that shows that AGPPase. whose activity declines concomitantly with the accumulation of starch during development of sink organs, competes with starch synthase (ADPG:1.4-alpha-D-glucan 4-alpha-D-glucosyltransferas e; EC 2.4.1.21) for ADPG. thus markedly blocking the starch biosynthesis.