Phosphofructokinase from lycopersicon esculentum fruits-II. Changes in the regulatory properties with dissociation

Abstract

The regulatory properties of PFK. from the tomato are discussed in relation to the dissociation of the oligomeric form of the enzyme. Both the oligomeric and monomeric forms of PFK were inhibited by citrate, malate, PEP, 2-phosphoglycerate, phosphoglycolate and ammonium sulphate. PEP was the most potent inhibitor of PFK activity with 9 and 10 μn PEP causing 50%, inhibition of the oligomeric and monomeric forms of PFK respectively. The inhibition by all these metabolites of the oligomeric form of PFK was sigmoidal while their inhibition of the monomeric form was hyperbolic. The magnitude of inhibition by these metabolites is affected by the levels of Mg 2+. The oligomeric form of the enzyme is more resistant to citrate inhibition than the monomeric form. In the presence of citrate or ammonium sulphate, the kinetics of the oligomeric form of PFK with F6P yielded positive cooperativity while in their absence, the kinetics revealed negative cooperative interactions. Phosphoenolpyruvate had no effect on the nature of the kinetics with F6P. ADP is stimulatory to the oligomeric form while it is slightly inhibitory to the monomeric form. The significance of these properties and their relation with the regulation of PFK activity in vivo are discussed.