Adpglucose Pyrophosphorylase

Studies conducted to determine regulatory enzymes involved in starch synthesis in plants have shown that AGPase is the key regulatory enzyme. Determination of regulatory enzymes is traditionally based on the identification of non-equilibrium steps in any given metabolic pathway [9]. In a metabolic process at equilibrium the forward and reverse reaction rates are equal, and is said to have no net flux in either direction. A reaction in equilibrium is most stable as there is no free energy change. When a reaction is favored in one direction, due to the regulatory role of an enzyme, the equilibrium is lost and free energy is exchanged. The reaction will progress until a large proportion of the reactants are converted into products. The flux, or rate of flow, is zero at equilibrium and unity when it proceeds exclusively in one direction.

Metabolic processes seldom attain equilibrium except when dead. Metabolic processes instead are usually found in a steady-state condition, i.e., in a state where the substrate is continually supplied and the products continually removed, so that the concentrations of the intermediates of the pathway remain constant despite a constant flux through the pathway. Hence, usually a regulatory enzyme produces a flux control coefficient closer to unity and other enzymes exhibit a flux control coefficient closer to zero.

Plant AGPase, a heterotetrameric enzyme composed of two small and two large subunits, is allosterically regulated in photosynthetic tissues. AGPase is activated by 3-phosphoglyceric acid (3-PGA) and is inhibited by inorganic phosphate (Pi) [10]. This ensures that there is a great degree of coordination between carbon assimilation, sucrose synthesis and starch synthesis. The flux control coefficient for starch synthesis in leaves of Arabidopsis thaliana by AGPase is measured to be as high as 0.6, confirming the view that AGPase is the key regulatory enzyme in starch synthesis [11],

In non-photosynthetic tissues, however, the regulatory role of AGPase has been somewhat inconclusive. Sensitivity to 3-PGA and Pi by tuber expressed AGPase has been demonstrated [12], Contrary to this, AGPase from barley endosperm seems to be insensitive to 3-PGA and Pi [13, 14]. Also, within one day of detaching growing potato tubers from the mother plant, starch synthesis is inhibited despite the unchanged levels of AGPase and 3-PGA [15]. In addition, the flux control coefficient of AGPase on the starch synthesis in pea embryos and isolated amyloplasts are determined to be 0.1 [16], suggesting that AGPase has a somewhat low level of control for starch synthesis in sink tissues.

Sucrose

Fructose

UDPglucose

Fructose

UDPglucose

Fructose 6-P

Glucose 1-P <-►ADPglucose
0 0

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