SUPPRESSION OF KINETIC COOPERATIVITY OF HEXOKINASE-D (GLUCOKINASE) BY COMPETITIVE INHIBITORS - A SLOW TRANSITION MODEL
SUPPRESSION OF KINETIC COOPERATIVITY OF HEXOKINASE-D (GLUCOKINASE) BY COMPETITIVE INHIBITORS - A SLOW TRANSITION MODEL
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DOI：
10.1111/j.1432-1033.1984.tb08536.x
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发表时间：
1984-01-01
期刊：
影响因子：
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通讯作者：
NIEMEYER, H
中科院分区：
文献类型：
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作者：
CARDENAS, ML;RABAJILLE, E;NIEMEYER, H
己糖激酶D（葡萄糖激酶）[大鼠肝脏]与甘露糖表现出正协同作用，h [Hill系数]值（1.5-1.6）与葡萄糖相同，但K0.5值更高（pH 8.0时为8 mM， pH 7.5时为12 mM）。相反，果糖和2-脱氧葡萄糖表现出米夏埃尔动力学。甘露糖、果糖、2-脱氧葡萄糖和n -乙酰氨基葡萄糖是葡萄糖磷酸化的竞争性抑制剂，降低了与葡萄糖的协同性。将h值降至1.0的相对效率依次为果糖>甘露糖> 2-脱氧葡萄糖> n -乙酰氨基葡萄糖。半乳糖，既不是底物也不是抑制剂，不能改变协同性。在非常低的葡萄糖浓度（< 0.5 K0.5）下，n -乙酰氨基葡萄糖或甘露糖对葡萄糖磷酸化的竞争性抑制是协同的，这表明抑制剂与多种酶形式相互作用。这些和先前报道的结果是在一个缓慢转变模型的基础上进行讨论的，该模型假设己糖激酶D在没有配体的情况下主要以一种构象状态（E1）存在，并且葡萄糖（或甘露糖）的结合诱导了向EII的构象转变。这种新构象对糖底物具有更高的亲和力，并且比EI具有更高的催化活性。随着糖浓度的增加，两种酶形态之间的稳态分布会发生变化，从而产生协同性。抑制剂通过诱导或捕获EII构象来抑制与葡萄糖的协同作用。此外，该模型假定己糖激酶D与不同糖底物（与葡萄糖和甘露糖合作，与2-脱氧葡萄糖和果糖合作）的不同动力学行为是糖底物诱导的构象转变速度不同的结果。
Hexokinase D (glucokinase) [rat liver] displays positive cooperativity with mannose with the same h [Hill coefficient] values (1.5-1.6) as with glucose but with higher K0.5 values (8 mM at pH 8.0 and 12 mM at pH 7.5). In contrast, fructose and 2-deoxyglucose exhibit Michaelian kinetics. Mannose, fructose, 2-deoxyglucose and N-acetylglucosamine acted as competitive inhibitors of glucose phosphorylation and decreased the cooperativity with glucose. Their relative efficiency for reducing the value of h to 1.0 was fructose > mannose > 2-deoxyglucose > N-acetylglucosamine. Galactose, which is not a substrate nor an inhibitor, was unable to change the cooperativity. The competitive inhibition of glucose phosphorylation by N-acetylglucosamine or mannose was cooperative at very low glucose concentrations (< 0.5 K0.5), suggesting the interaction of the inhibitors with more than one enzyme form. These and previously reported results are discussed on the basis of a slow transition model, which assumes that hexokinase D exists mainly in one conformation state (E1) in the absence of ligands and that the binding of glucose (or mannose) induces a conformational transition to EII. This new conformation would have a higher affinity for the sugar substrates and a higher catalytic activity than EI. Cooperativity would emerge from shifts of the steady-state distribution between the 2 enzyme forms as the sugar concentration increase. The inhibitors would suppress cooperativity with glucose by inducing or trapping the EII conformation. In addition, the model postulates that the different kinetic behavior of hexokinase D with the different sugar substrates, cooperative with glucose and mannose and Michaelian with 2-deoxyglucose and fructose, is the consequence of differences in the velocities of the conformational transitions induced by the sugar substrates.