Ultrasensitive Behavior in the Synthesis of Storage Polysaccharides in Cyanobacteria

D.F. GOMEZ CASATI, S. CORTASSA, M.A. AON, A.A. IGLESIAS

PLANTA

Springer-Verlag

Año: 2003 vol. 216 p. 969 - 975

0032-0935

AbstractThe glycogen synthetic pathway operates ultrasensitively as a function of the ADPglucose pyrophosphorylase  (ADPGlcPPase) allosteric effectors, 3-phosphoglycerate and Pi, in permeabilized cells of the cyanobacterium Anabaena PCC 7120. In vitro data previously showed that the ultrasensitive behavior of ADPGlcPPase depends upon cross-talk between the twoallosteric effectors, the enzyme?s response being additionally modulated by molecular crowding [D.F. Go´ mez Casatiet al. (2000) Biochem J 350:139?147]. In the present work we show, experimentally and with a mathematical model, that a-1,4-glucan synthesis is also ultrasensitive in cells due to the propagation of the switch-like behavior of ADPGlcPPase to the synthetic pathway. Amplifications of up to 20-fold in storagepolysaccharide synthesis can be achieved with a modest 6.7-fold increase in 3-phosphoglycerate in the presence of 5 mM Pi in contrast to the 30-fold necessary in its absence. This is the first time that this phenomenon has been reported to occur in the glycogen synthetic pathway of a photosynthetic prokaryote. The implications of the results for plant cell physiology during light?dark transitions are discussed.Keywords ADPglucose pyrophosphorylase ÆAnabaena Æ Glycogen/starch synthesis Æ Molecularcrowding Æ Permeabilized cyanobacteria ÆUltrasensitivity (of enzymes)