PHOSPHOFRUCTOKINASE FROM THE HOST FRACTION OF SOYBEAN NODULES

The predominant form of phosphofructokinase (EC 2.7.1.11; PFK) in the host fraction of soybean nodules was isolated as a large aggregated pr otein with apparent molecular mass near 2,000 kDa. SDS-PAGE indicated that the enzyme was made up of three polypeptides of molecular mass of 55, 59 and 62 kDa. The soybean nodule PFK displayed typical Michaelis -Menten kinetics with respect to fructose-6-phosphate and the nucleosi de triphosphate substrate at the pH optimum (pH 8.6) and at pH 7. MgAT P was the most effective phosphoryl donor. Phosphoenolpyruvate (PEP) w as a potent inhibitor of PFK activity, and the enzyme was also strongl y inhibited by 3-phosphoglycerate, 2-phosphoglycerate, and to a lesser extent, PPi. PFK was activated by KCl, NaCl and Pi, and Pi relieved t he inhibition caused by PEP. A minor form of PFK, with apparent native molecular mass of 200 kDa, was also present in extracts from the host fraction of soybean nodules. The 200-kDa form of PFK was only weakly activated by KCl and Pi and was much less sensitive to inhibition by P EP than the predominant form of PFK in the nodules.