Glycolytic and gluconeogenic states in an enzyme system reconstituted from phosphofructokinase and fructose 1,6-bisphosphatase.

Transitions between glycolytic and gluconeogenic states have been investigated in an open and homogeneous enzyme system containing phosphofructokinase, fructose 1,6-bisphosphatase, pyruvate kinase, adenylate kinase and glucose 6-phosphate isomerase. The direction of substrate flow was found to depend on the maximum activities of phosphofructokinase and fructose 1,6-bisphosphatase as well as on the influx concentrations of fructose 6-phosphate and fructose 1,6-bisphosphate. At high and low maximum activities of phosphofructokinase and fructose 1,6-bisphosphatase unique and stable stationary states occur, whereas at intermediate enzyme concentrations sustained oscillations emerge. Stationary states with a low rate of substrate cycling demand both appropriate enzyme concentrations and an adequate substrate supply. Accordingly, transitions between efficient glycolytic and gluconeogenic states require changes of the enzyme concentrations and of the supply of substrates. Such transitions exhibit a transient oscillatory response. The sustained oscillations generated at intermediate activities of phosphofructokinase and fructose 1,6-bisphosphatase lead to a significant diminution of the rate of substrate cycling when compared with the respective steady state values. During the oscillations glycolytic and gluconeogenic states are consecutively passed through. Because of this a temporal organization of the antagonistic reactions is achieved. In our system the kinetic organization of the two opposite reactions is mainly brought about by the reciprocal allosteric effects of AMP on the activities of the two enzymes.