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Molecular Pharmacology, Vol 3, 177-187, Copyright © 1967 by the American Society for Pharmacology and Experimental Therapeutics

Phosphofructokinase from the Liver Fluke Fasciola hepatica

II. Kinetic Properties of the Enzyme

DEBORAH BENNETT STONE 1 and TAG E. MANSOUR 1

1 Department of Pharmacology, Stanford University School of Medicine, Palo Alto, California 94304

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.

Note:
ACKNOWLEDGMENTS This work was supported by Research Grant AI04214 from the National Institute of Allergy and Infectious Diseases, U. S. Public Health Service; and a Research Career Development Award GM-K3-3848 from the Division of General Medical Sciences, U. S. Public Health Service.

Submitted on October 25, 1966




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