Rabbit Muscle Phosphofructokinase: Studies on the Polymerization

The mode of polymerization of rabbit muscle phosphofructokinase has been investigated at 10° over a protein concentration range of 0.4 to 20 mg per ml in phosphate buffer at pH 6 and pH 8 and at a single concentration of 5 mg per ml in the region of pH 6 to 8. At pH 8 the data suggest that there are several forms of the active enzyme of molecular weight about 360,000 which differ markedly in their mode of polymerization. At least two of these forms have been separated on this basis. One form polymerizes rapidly and reversibly to higher molecular weight forms in what may be an indefinite manner. A second form may polymerize to a dimer of the active enzyme while a third form appears not to undergo any polymerization. At pH 6 and at the lowest protein concentrations studied here the enzyme exists as an inactive form of about 160,000 molecular weight. This species can also polymerize, although in a very different manner than the enzyme at pH 8. At very high enzyme concentrations, the pH 6 material, which presumably represents a protonated species of the enzyme, polymerizes to an active enzyme of which the molecular weight is 360,000 or greater. Between pH 6 and 8 the sedimentation behavior reflects the distribution of enzyme in the protonated (pH 6) and the deprotonated (pH 8) forms.