Competitive inhibition of liver glucokinase by its regulatory protein.
The regulatory protein of rat liver glucokinase (hexokinase IV or D) behaved as a fully competitive inhibitor of this enzyme when glucose was the variable substrate, i.e. it increased the half-saturating concentration of glucose as a linear function of its concentration without affecting V (velocity at infinite concentration of substrate). The inhibition by the regulatory protein and that by palmitoyl-CoA were synergistic with that by N-acetyl-glucosamine, indicating that the two former inhibitors bind to a site distinct from the catalytic site. In contrast, the effects of the regulatory protein and palmitoyl-CoA were competitive with each other, indicating that these two inhibitors bind to the same site. The regulatory protein exerted a non-competitive inhibition with respect to Mg-ATP at concentrations of this nucleotide less than 0.5 mM. At higher concentrations, the latter antagonized the inhibition by the regulatory protein partly by decreasing the apparent affinity for fructose 6-phosphate. The following anions inhibited glucokinase non-competitively with respect to glucose: Pi, sulfate, I-, Br-, No3-, Cl-, F- and acetate. Pi and sulfate, at concentrations in the millimolar range, decreased the inhibition by the regulatory protein by competing with fructose 6-phosphate. Monovalent anions also antagonized the inhibition by the regulatory protein with the following order of potency: I- greater than Br- greater than NO3- greater than Cl- greater than F- greater than acetate and their effect was non-competitive with respect to fructose 6-phosphate. Glucokinase from Buffo marinus and pig liver were, like the rat liver enzyme, inhibited by the regulatory protein, as well as by palmitoyl-CoA at micromolar concentrations. In contrast, neither compound inhibited hexokinases from rat brain, beef heart or yeast, or the low-Km specific glucokinase from Bacillus stearothermophilus.
Vandercammen
A
A
Laboratoire de Chimie Physiologique, International Institute of Cellular and Molecular Pathology, Brussels, Belgium.
Van Schaftingen
E
E
D000085:Acetates / Q000494:pharmacology; D019342:Acetic Acid; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D002846:Chromatography, Affinity; D005941:Glucokinase / Q000037:antagonists & inhibitors* / Q000302:isolation & purification / Q000378:metabolism; D006593:Hexokinase / Q000037:antagonists & inhibitors; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D010171:Palmitoyl Coenzyme A / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D012965:Sodium Chloride / Q000494:pharmacology
D000085:Acetates; D010171:Palmitoyl Coenzyme A; D012965:Sodium Chloride; D011189:Potassium Chloride; D000255:Adenosine Triphosphate; D006593:Hexokinase; D005941:Glucokinase; D019342:Acetic Acid