Fructose 6-phosphate prevents the proteolyzed derivative of Escherichia coli phosphofructokinase from dissociation and inactivation.

N-terminal sequence analysis shows that the limited proteolysis of Escherichia coli phosphofructokinase results in the removal of the 40-50 C-terminal residues of each chain. When tetrameric, this proteolyzed derivative is still active albeit insensitive to allosteric effectors (Le Bras, G., and Garel, J.-R. (1982) Biochemistry 21, 6656-6660). In the absence of fructose 6-phosphate, the proteolyzed phosphofructokinase spontaneously loses its activity and dissociates into dimeric species. This inactivation/dissociation is slowed down by the binding of fructose 6-phosphate to only part of the sites; it is completely prevented by the saturation of all four fructose 6-phosphate sites. The other substrate ATP does not protect the proteolyzed phosphofructokinase against this inactivation/dissociation. This inactivation/dissociation is not due to denaturation and can be reversed in some conditions by the addition of fructose 6-phosphate. The active tetrameric structure of phosphofructokinase is stable when either the C-terminal segment is not removed or the fructose 6-phosphate sites are occupied.