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 Br− > NO3− > Cl − > F− 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.