A phosphoglycerate to inorganic phosphate ratio is the major factor in controlling starch levels in chloroplasts via ADP-glucose pyrophosphorylase regulation.
Purified barley leaf ADP-glucose pyrophosphorylase, a key enzyme of the starch synthesis in the chloroplast stroma, was analysed with respect to its possible regulation by factors defining the metabolic/effector status of the chloroplast during light and dark conditions. The enzyme required 3-phosphoglyceric acid for the maximal activity and was inhibited by inorganic phosphate. The optimal pH for the enzyme was at circa 7.0, regardless of the presence or absence of 3-phosphoglyceric acid, whereas the maximal activation by 3-phosphoglyceric acid was observed at pH 8.5 and higher. Changes in the concentration of Mg2+ and dithiothreitol had little or no effect on the enzymatic activity of AGPase. It has been directly demonstrated for the first time that a 3-phosphoglyceric acid/inorganic phosphate ratio, a crucial regulatory parameter, could be directly related to a defined activation state of the enzyme, allowing the prediction of a relative AGPase activity under given conditions. The predicted changes in the enzyme activity were directly correlated with earlier reported responses of starch levels to the 3-phosphoglyceric acid/inorganic phosphate ratio in chloroplasts. Consequences of this for the starch biosynthesis are discussed.
Kleczkowski
L A
LA
Department of Plant Physiology, Umeå University, Sweden. leszek.kleczkowski@plantphys.umu.se
D002736:Chloroplasts / Q000201:enzymology; D051599:Glucose-1-Phosphate Adenylyltransferase; D005988:Glyceric Acids / Q000378:metabolism*; D001467:Hordeum / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D008274:Magnesium; D009713:Nucleotidyltransferases / Q000378:metabolism*; D010084:Oxidation-Reduction; D010710:Phosphates / Q000378:metabolism*; D013213:Starch / Q000096:biosynthesis*
D005988:Glyceric Acids; D010710:Phosphates; C005156:3-phosphoglycerate; D013213:Starch; D009713:Nucleotidyltransferases; D051599:Glucose-1-Phosphate Adenylyltransferase; D008274:Magnesium