Yingbin Fu

Activation of the Potato Tuber ADP-glucose Pyrophosphorylase by Thioredoxin

The potato tuber (Solanum tuberosumL.) ADP-glucose pyrophosphorylase (ADP-GlcPPase) catalyzes the first committed step in starch biosynthesis. The main type of regulation of this enzyme is allosteric, and its activity is controlled by the ratio of activator, 3-phosphoglycerate to inhibitor, Pi. It was reported (Fu, Y., Ballicora, M. A., Leykam, J. F., and Preiss, J. (1998) J. Biol. Chem. 273, 25045–25052) that the enzyme was activated by reduction of the Cys12disulfide linkage present in the catalytic subunits. In this study, both reduced thioredoxin f and m from spinach (Spinacia oleracea) leaves reduced and activated the enzyme at low concentrations (10 μm) of activator (3-phosphoglycerate). Fifty percent activation was at 4.5 and 8.7 μm for reduced thioredoxin f andm, respectively, and 2 orders of magnitude lower than for dithiothreitol. The activation was reversed by oxidized thioredoxin. Cys12 is conserved in the ADP-GlcPPases from plant leaves and other tissues except for the monocot endosperm enzymes. We postulate that in photosynthetic tissues, reduction could play a role in the fine regulation of the ADP-GlcPPase mediated by the ferredoxin-thioredoxin system. This is the first time that a covalent mechanism of regulation is postulated in the synthesis of starch.

Mechanism of Reductive Activation of Potato Tuber ADP-glucose Pyrophosphorylase

The potato tuber (Solanum tuberosumL.) ADP-glucose pyrophosphorylase activity is activated by a incubation with ADP-glucose and dithiothreitol or by ATP, glucose- 1-phosphate, Ca2+, and dithiothreitol. The activation was accompanied by the appearance of new sulfhydryl groups as determined with 5,5′-dithiobis(2-nitrobenzoic acid). By analyzing the activated and nonactivated enzymes on SDS-polyacrylamide gel electrophoresis under nonreducing conditions, it was found that an intermolecular disulfide bridge between the small subunits of the potato tuber enzyme was reduced during the activation. Further experiments showed that the activation was mediated via a slow reduction and subsequent rapid conformational change induced by ADP-glucose. The activation process could be reversed by oxidation with 5,5′-dithiobis(2-nitrobenzoic acid). Incubation with ADP-glucose and dithiothreitol could reactivate the oxidized enzyme. Chemical modification experiments with [14C]iodoacetic acid and 4-vinylpyridine determined that the intermolecular disulfide bridge was located between Cys12 of the small subunits of the potato tuber enzyme. Mutation of Cys12 in the small subunit into either Ala or Ser eliminated the requirement of DTT on the activation and prevented the formation of the intermolecular disulfide of the potato tuber enzyme. The mutants had instantaneous activation rates as the wild-type in the reduced state. A two-step activation model is proposed.

Adenosine 5'-diphosphate-glucose pyrophosphorylase from potato tuber. Significance of the N terminus of the small subunit for catalytic properties and heat stability.

cDNAs encoding the large subunit and a possibly truncated small subunit of the potato tuber (Solanum tuberosum L.) adenosine 5[prime]-diphosphate-glucose pyrophosphorylase have been expressed in Escherichia coli (A.A. Iglesias, G.F. Barry, C. Meyer, L. Bloksberg, P.A. Nakata, T. Greene, M.J. Laughlin, T.W. Okita, G.M. Kishore, J. Preiss, J Biol Chem [1993] 268: 1081–1086). However, some properties of the transgenic enzyme were different from those reported for the enzyme from potato tuber. In this work, extension of the cDNA was performed to elongate the N terminus of the truncated small subunit by 10 amino acids. This extension is based on the almost complete conservation seen at the N-terminal sequence for the potato tuber and the spinach leaf small subunits. Expressing the extended cDNA in E. coli along with the large subunit cDNA yielded a transgenic heterotetrameric enzyme with similar properties to the purified potato tuber enzyme. It was also found that the extended small subunit expressed by itself exhibited high enzyme activity, with lower affinity for activator 3-phosphoglycerate and higher sensitivity toward inorganic phosphate inhibition. It is proposed that a major function of the large subunit of adenosine 5[prime]-diphosphate-glucose pyrophosphorylases from higher plants is to modulate the regulatory properties of the native heterotetrameric enzyme, and the small subunits major function is catalysis.

Regulation of Higher Plant Leaf and Reserve Tissue Starch Synthesis

ADP-glucose is the glucosyl donor in the synthesis of starch and its synthesis is catalyzed by ADP-glucose pyrophosphorylase (reaction I, E.C. 2.7.7.27; ATP: α-D-glucose-1 -phosphate adenylyltransferase). Reaction II, catalyzed by starch synthase (E.C. 2.4.1.21) ADP-glucose; 1,4-α-D-glucan 4-α-glucosyltransferase, transfers the glucosyl residue from the sugar nucleotide to elongate the 1,4-α-D-glucan chain. Reaction III is catalyzed by branching enzyme (E.C. 2.4.1.18) 1,4-α-D-glucan 6-α-(1,4-α-glucano)-transferase and forms the branched chains found in amylopectin.