Vandercammen A, Van Schaftingen E
Laboratoire de Chimie Physiologique, International Institute of Cellular and Molecular Pathology, Brussels, Belgium.
Eur J Biochem. 1991 Sep 1;200(2):545-51. doi: 10.1111/j.1432-1033.1991.tb16217.x.
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.
当葡萄糖作为可变底物时，大鼠肝脏葡萄糖激酶（己糖激酶IV或D）的调节蛋白表现为该酶的完全竞争性抑制剂，即它使葡萄糖的半饱和浓度随其浓度呈线性增加，而不影响V（底物无限浓度时的速度）。调节蛋白的抑制作用与棕榈酰辅酶A的抑制作用与N-乙酰葡糖胺的抑制作用具有协同性，表明前两种抑制剂结合到与催化位点不同的位点。相反，调节蛋白和棕榈酰辅酶A的作用相互竞争，表明这两种抑制剂结合到同一位点。在该核苷酸浓度低于0.5 mM时，调节蛋白对Mg-ATP表现出非竞争性抑制。在较高浓度下，后者部分地通过降低对6-磷酸果糖的表观亲和力来拮抗调节蛋白的抑制作用。以下阴离子对葡萄糖表现出非竞争性抑制作用：磷酸根离子（Pi）、硫酸根离子、碘离子、溴离子、硝酸根离子、氯离子、氟离子和醋酸根离子。毫摩尔范围内浓度的Pi和硫酸根离子通过与6-磷酸果糖竞争来降低调节蛋白的抑制作用。单价阴离子也拮抗调节蛋白的抑制作用，其效力顺序为：碘离子＞溴离子＞硝酸根离子＞氯离子＞氟离子＞醋酸根离子，并且它们对6-磷酸果糖的作用是非竞争性的。来自海蟾蜍和猪肝的葡萄糖激酶与大鼠肝脏酶一样，受到调节蛋白以及微摩尔浓度的棕榈酰辅酶A的抑制。相反，这两种化合物均不抑制大鼠脑、牛心或酵母的己糖激酶，也不抑制嗜热栖热芽孢杆菌的低Km特异性葡萄糖激酶。
