Expression, kinetics and regulatory properties of native and recombinant ADP-glucose pyrophosphorylase isoforms from chickpea

ADP-glucose pyrophosphorylase (AGPase, ATP: α-D-glucose 1-phosphate adenyltransferase EC 2.7.7.27) catalyzes the entry step to starch biosynthesis in plants. The leaf enzyme is subject to allosteric control by 3-phosphoglyceric acid (3-PGA) and inorganic phosphate (Pi), which activate and inhibit, respectively, catalytic activity. The major AGPase activity present in seed tissue from several legume plants has been reported to be insensitive to these allosteric effector molecules. In an effort to engineer starchy legume chickpea (Cicer arietinum L.), an important food crop for many Middle Eastern and Asian countries, for increased crop yields, the AGPase activities from leaves and developing seeds and a bacterial expressed recombinant leaf enzyme were studied. Our results demonstrate that the catalytic activities of all enzyme forms, including the major activity detected in developing seeds, was dependent on 3-PGA. The allosteric regulatory behavior, 3-PGA activation and Pi inhibition responses, were generally similar for the three enzyme activities although distinct differences were observed. The consequence of an allosterically regulated AGPase on starch synthesis during seed formation is discussed.