Abstract

This paper describes the kinetics of phosphofructokinase from the liver fluke Fasciola hepatica. Activation of the enzyme was accompanied by a marked increase in its affinity for both fructose-6-P and ATP. Results with the activated enzyme showed that each of the three ligands (fructose-6-P, ATP and cyclic 3',5'-AMP) could alter the saturation function of the other. The enzyme was strongly inhibited by ATP in the presence of low fructose-6-P concentrations. Both cyclic 3',5'-AMP and fructose-6-P could increase the concentrations of ATP required to produce 50% inhibition. Possible interaction among the substrates and the effector ligands on the enzyme was considered in terms of the Monod-Changeux-Wyman model for allosteric proteins. Saturation curves for fructose-6-P indicated cooperative homotropic interactions between multiple binding sites for this substrate. ATP at high concentration increased the homotropic interactions while cyclic 3',5'-AMP had the opposite effect. The saturation function of cyclic 3',5'-AMP was also shown to be influenced by both enzyme substrates. Fructose-6-P at high concentration caused a decrease in the half-saturation point for cyclic 3',5'-AMP while ATP had the opposite effect. Saturation curves for Mg⁺⁺ were also altered by a change in concentration of fructose-6-P, ATP or cyclic 3',5'-AMP. The possible role of these kinetic properties on the fine regulation of activated phosphofructokinase was discussed.