Regulation of Starch Biosynthesis in Plant Leaves: Activation and Inhibition of ADPglucose Pyrophosphorylase

Abstract

Abstract. The ADPglucose pyrophosphorylases of 7 plant-leaf tissues were partially purified and characterized. In all cases the enzymes showed stability to heat treatment at 650 for 5 minutes in the presence of 0.02 M phosphate buffer, pH 7.0. The leaf ADPglucose pyrophosphorylases were activated 5 to 15-fold by 3-phosphoglycerate. Fructose-6-phosphate and fruotose 1, 6-diphosphate stimulated ADPglucose pyrophosphorylase to lesser extents.The AO,5 (conc of activator required to give 50 %o of the observed maximal activation) of 3-phosphoglycerate for the barley enzyme was 7 X 10-6 M while for the sorghum enzyme it was 3.7 X 10-4 M. Inorganic phosphate proved to be an effective inhibitor of ADPglucose synthesis. The 10.5 (conc of inhibitor that gave 50 % inhibition of activity for the various leaf enzymes varied from 2 X 10-M (barley) to 1.9 X 10-4 M (sorghum). This inhibition was reversed or antagonized by the activator 3-phosphoglycerate. These results form the basis for an hypothesis of the regulation of leaf starch biosynthesis.The hiosynithesis of the a-1,4 gluco,side linkage of starch from UDPglucose3 and ADPglucose in many pliawt exltracts have been the subject of many reporits (1, 2, 5, 8-16, 29--36) since the initiall obseirvatilons by Lelloir'is group (7,26,38). In alil extradts the rate of transfer of glucose from ADPglucose (reacltion 1) to the polysaicdharide prime,r was many-fold fa!ster than the rate of transfer from uridine dipho,sphalte gluco,se (UDPglucose) (re,action 2).

Insensitivity of Barley Endosperm ADP-Glucose Pyrophosphorylase to 3-Phosphoglycerate and Orthophosphate Regulation

“…After 10 to 15 min at 37OC, the reactions were stopped by boiling for 30 to 40 s. ADP-glucose was adsorbed onto DE81 paper (Whatman Intemational, Maidstone, England), and the adsorbed 14C-radioactivity was counted by liquid scintillation spectroscopy (see Sanwal et al [1968] for other technical details). Under these assay conditions, the activity was linear with respect to time and amount of extract added.…”

“…The higher plant enzyme, which is composed of two different subunit types (More11 et al, 1987;Okita et al, 1990), is characterized by a potent activation by 3-PGA and inhibition by Pi (Sanwal et al, 1968;Preiss, 1991). The activation (3-PGA) and inhibition (Pi) constants for AGP are usually on the order of micromolar (Sanwal et al, 1968;Sowokinos, 1981;Sowokinos and Preiss, 1982;Plaxton and Preiss, 1987), and the ratio of the two effectors is believed to play a key * Corresponding author; fax 47-9-941465. role in the control of starch biosynthesis (Neuhaus and Stitt, 1990;Preiss, 1991). The fine regulation by 3-PGA and Pi has been demonstrated for AGP from both photosynthetic and nonphotosynthetic plant tissues (Sanwal et al, 1968;Sowokinos, 1981;Sowokinos and Preiss, 1982;Plaxton and Preiss, 1987;Preiss, 1991).…”

Zinc Nutrition and Starch Metabolism in Phaseolus vulgaris L.

“…Frydman and Cardini (6) pointed out that the two forms may be modifications of the same enzyme and that, particularly in leaves, the two forms had the same substrate specificity for ADP glucose. The soluble form of starch synthetase represented at least 80% of the total starch synthetase activity in the leaves of rice, tobacco, tomato, kidney beans, sorghum, barley, and sugar beet (16).…”

Modulation of Allosteric Regulation by E38K and G101N Mutations in the Potato Tuber ADP-glucose Pyrophosphorylase

“…[9][10][11][12][13] The specific phosphate ester molecule is evolutionarily adapted to controlling carbon flux in the major carbon metabolic pathway. The results obtained here show the possibility of extending the specificity of the allosteric effectors of plant AGPase activity, and in turn of increasing the net catalytic activity of AGPase for enhanced starch production.…”

“…Bacterial AGPases are activated by pyruvate (PYR), fructose 6-phosphate (F6P), and fructose 1,6-bisphosphate (F16BP), and are inhibited by AMP. 9,10) On the other hand, the activator and the inhibitor of higher plant AGPases are 3-phosphoglycerate (3-PGA) and orthophosphate (Pi) respectively. [11][12][13] In contrast to bacterial AGPase, a homotetrameric protein, the higher plant AGPase is composed of two small subunits (SSs) and two large subunits (LSs), which form a heterotetramer.…”