| ADP-glucose pyrophosphorylase (AGPase) catalyzes the first committed step of starch biosynthesis in higher plants. The enzyme is allosterically activated by 3-phosphoglyceric acid (3-PGA) and inhibited by inorganic phosphate (Pi). Activity of AGPase is also controlled by redox regulation, a mechanism which tunes its activity in response to fluctuating light and sugar levels. The plant AGPases are composed of pairs of large subunits (LSs) and small subunits (SSs) which collectively comprise its heterotetrameric structure. Current evidence suggests that the SS has a dominant role in the enzyme catalysis while both the SS and the LS influence the allosteric regulatory properties of the enzyme. There are multiple isoforms of the enzyme depending on the tissue and intracellular localization. In cereal endosperm major AGPase activity is cytosolic in addition to the minor contribution from the amyloplast isoform. LS missense mutants, EM540, EM715 and EM817, of the rice endosperm cytosolic AGPase were isolated which had lower seed weights than the LS null mutants. EM540/817 and EM715 had T139I and A171V mutations in the LS, respectively. To investigate the effects of these mutations recombinant wild type and mutant AGPases as well as SS homotetrameric enzyme were expressed in Escherichia coli, purified to near homogeneity and assessed for their kinetic properties. Kinetic analysis showed that the lower seed weights of the LS missense mutants compared to the null mutants are due to the poorer allosteric inhibitory properties of the mutant enzymes than the SS homotetrameric enzyme. In a second study, activity of the wild type enzyme was demonstrated to be controlled by redox regulation through modification of the LS N-terminal. To examine the roles of cysteine residues (C12, C47 and C58) located at this region several mutant combinations were generated and kinetically characterized. The results showed that the wild type AGPase is more active and it is greater than 3-fold more responsive to 3-PGA when reduced. In addition, the LS residues, C47 and C58, are essential for proper 3-PGA response of the enzyme. Collectively, the results provide important insights about the control of starch metabolism in rice endosperm. |
