Studies of the kinetic mechanism of maize endosperm ADP-glucose pyrophosphorylase uncovered complex regulatory properties.
ADP-glucose pyrophosphorylase catalyzes the synthesis of ADP-glucose (ADP-Glc) from Glc-1-phosphate (G-1-P) and ATP. Kinetic studies were performed to define the nature of the reaction, both in the presence and absence of allosteric effector molecules. When 3-phosphoglycerate (3-PGA), the putative physiological activator, was present at a saturating level, initial velocity studies were consistent with a Theorell-Chance BiBi mechanism and product inhibition data supported sequential binding of ATP and G-1-P, followed by ordered release of pyrophosphate and ADP-Glc. A sequential mechanism was also followed when 3-PGA was absent, but product inhibition patterns changed dramatically. In the presence of 3-PGA, ADP-Glc is a competitive inhibitor with respect to ATP. In the absence of 3-PGA--with or without 5.0 mm inorganic phosphate--ADP-Glc actually stimulated catalytic activity, acting as a feedback product activator. By contrast, the other product, pyrophosphate, is a potent inhibitor in the absence of 3-PGA. In the presence of subsaturating levels of allosteric effectors, G-1-P serves not only as a substrate but also as an activator. Finally, in the absence of 3-PGA, inorganic phosphate, a classic inhibitor or antiactivator of the enzyme, stimulates enzyme activity at low substrate by lowering the K(M) values for both substrates.
Boehlein
Susan K
SK
Program in Plant Molecular and Cellular Biology and Horticultural Sciences, University of Florida, Gainesville, Florida 32611, USA.
Shaw
Janine R
JR
Stewart
Jon D
JD
Hannah
L Curtis
LC
D011756:Diphosphates / Q000378:metabolism; D056625:Endosperm / Q000201:enzymology*; D051599:Glucose-1-Phosphate Adenylyltransferase / Q000378:metabolism*; D005988:Glyceric Acids / Q000378:metabolism; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000378:metabolism*; D013379:Substrate Specificity; D003313:Zea mays / Q000201:enzymology*
D011756:Diphosphates; D005988:Glyceric Acids; D010940:Plant Proteins; C005156:3-phosphoglycerate; D051599:Glucose-1-Phosphate Adenylyltransferase