NON-CONNECTED BRANCHES OF ALTERNATIVE STATIONARY STATES IN THE 6-PHOSPHOFRUCTOKINASE/FRUCTOSE 1, 6-BISPHOSPHATASE MODEL CYCLE

Abstract

In a previous paper [26] the emergence of irreversible transitions between alternative branches of stationary states in a bistable dynamical regime of the phosphofructokinase/fructose 1,6-bisphosphatase model cycle was shown to exist in addition to reversible hysteretic transitions. In the case of irreversible transitions, all these transitions lead to only one of the branches of stationary states, and hence no hysteretic loop can take place. In the present report we demonstrate that for some parameters a situation may arise where, in a bistable dynamic regime, both limit points are not accessible by changes of the respective parameters. Then the system behaves like a monostable device although, depending upon its history, alternative stationary states related to distinct functional properties, e.g., the direction of fluxes or the energetic charge of the states, may be approached. Transitions from one branch of stationary solutions to another one would require adequate tuning of other parameters which could drive the system out of the domain in which non-connected bands of stationary states arise. In the investigated model system, the adenylate energy charge influx and the rate of the influx in the hexosemonophosphates may serve as parameters causing the emergence of non-connected bands of stationary solutions.