Modification of yeast phosphofructokinase with pyridoxal 5'-phosphate

Modification of yeast phosphofructokinase (E.C. 2.7.1.11) with pyridoxal 5'-phosphate leads to a decreased enzyme activity. ATP at higher concentrations protects the enzyme against inactivation, while fructose 6-phosphate has no effect. At relatively low concentrations of pyridoxal phosphate the inhibition of activity by ATP was decreased or even abolished. AMP still activates the modified enzyme pointing to a separate binding site in the regulatory centre. The dissociation of the phosphopyridoxyl enzyme in products with apparent sedimentation coefficients of about 4-6 S gives evidence that lysine might be involved in the stabilization of the quaternary structure of yeast phosphofructokinase. The incorporation of pyridoxal phosphate in the 17 S form of yeast phosphofructokinase as obtained by a partial proteolysis does not show differences from the 20 S, proteolytically unmodified enzyme.