Physicochemical Parameters and Subunit Composition of Yeast Phosphofructokinase

The properties of proteolytically non‐modified phosphofructokinase from baker's yeast were investigated by means of hydrodynamic, electrophoretic, and chemical methods. The sedimentation coefficient shows a linear dependence on the protein concentration down to 0.01 mg/ml. The sedimentation coefficient has been determined to be S020,w= 20.81 S. At concentrations of the enzyme at less than 0.01 mg/ml a significant decrease of this value was found. The partial specific volume of the enzyme as determined by three different methods at 20°C is v2= 0.742 ml/g. From equilibrium sedimentation a molecular weight of 835000 ± 32000 was calculated for the native enzyme. Amino acid analysis of the enzyme was performed and the number of half‐cystine residues determined as 11/100000 g is in good agreement with the number of titratable –SH groups after denaturation of the enzyme. The subunit molecular weight was determined by applying equilibrium sedimentation in the presence of sodium dodecylsulphate. Taking into account that 0.37 g of dodecylsulphate is bound per g of the enzyme protein as found by equilibrium dialysis, a subunit molecular weight of 104000 could be calculated. This value is about 15% smaller than that obtained by gel electrophoresis in the presence of sodium dodecylsulphate. Evidently, yeast phosphofructokinase seems to be octameric. Limited proteolysis in the presence of proteinase B from yeast was investigated by following the alterations of the sedimentation coefficient, the molecular weight, and the subunit pattern of the enzyme. During partial proteolytic degradation the molecular weight of the native enzyme decreases by about 230000. This diminution is in accordance with the difference in the molecular weight per subunit if an octameric composition is considered.