Myofibril-bound muscle phosphofructokinase is less sensitive to inhibition by ATP than the free enzyme, but retains its sensitivity to stimulation by bisphosphorylated hexoses.

Phosphofructokinase activity is modulated by allosteric effectors and macromolecular interactions (e.g. binding to myofibrillar components). The aim of this study was to determine the effects of ATP and bisphosphorylated sugars upon phosphofructokinase in the presence of myofibrils. Myofibrils were prepared from resting and electrically stimulated rat muscle. Dephosphorylation of myofibrils was performed with alkaline phosphatase acid. Purified rabbit skeletal muscle phosphofructokinase was used for all experiments. Myofibrils from resting muscle showed a higher capacity to bind phosphofructokinase and a lower phosphate content than myofibrils from stimulated muscle. Dephosphorylation of myofibrils did not increase their binding capacity. Myofibrils greatly counteracted the inhibition of phosphofructokinase by high concentrations of ATP, without affecting maximum activity. In the presence of myofibrils, both glucose 1,6-bisphosphate and fructose 2,6-bisphosphate additionally activated muscle phosphofructokinase. We suggest that the binding of phosphofructokinase to myofibrils in combination with increasing glucose 1,6-bisphosphate concentration could be important in the enhancement of the glycolytic flux that takes place during muscle contraction.