Phosphofructokinase from baker's yeast: kinetic properties of a proteolytically modified enzyme

A tetrameric enzyme form of phosphofructokinase from yeast (called 12 S-enzyme), formed by limited proteolysis of the octameric enzyme in the presence of ATP and by subsequent dissociation in two half-molecules shows sigmoidal kinetics with respect to fructose 6-phosphate and inhibition by ATP. Similar to the native phosphofructokinase, the modified enzyme is also efficiently activated by AMP and fructose 2,6-bisphosphate. Both activators increase the affinity for the substrate fructose 6-phosphate and the respective maximum activity. In contrast to the native phosphofructokinase, however, both AMP and fructose 2,6-bisphosphate change the sigmoidal fructose 6-phosphate velocity curve into a hyperbolic one. AMP and fructose 2,6-bisphosphate decrease the ATP inhibition, probably by modulating the affinity of the allosteric sites to ATP.