Ultrasensitive glycogen synthesis in Cyanobacteria

Cyanobacter ADPglucose pyrophosphorylase exhibits a ultrasensitive response in activity towards its allosteric effector 3-phosphoglycerate, elicited b y orthophosphate and polyethyleneglycol-induced molecular crowding, The ult rasensitive response was observed either when the enzyme operates in the ze ro or first order region for its physiological substrates, The ultrasensiti vity exhibited maximal amplification factors of 15-19-fold with respect to 1% of the maximal system velocity. Only a 2.4-3.8-fold increase in 3PGA con centration was necessary to augment the flux from 10% to 90% through AGPase as compared with 200-fold required for the control. The results are discus sed in terms of finely tuned regulatory mechanisms of polysaccharide synthe sis in oxygenic photosynthetic organisms. (C) 1999 Federation of European B iochemical Societies.