ENZYME MEMORY .2. KINETICS AND THERMODYNAMICS OF SLOW CONFORMATION CHANGES OF WHEAT-GERM HEXOKINASE-LI
ENZYME MEMORY .2. KINETICS AND THERMODYNAMICS OF SLOW CONFORMATION CHANGES OF WHEAT-GERM HEXOKINASE-LI
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DOI：
10.1111/j.1432-1033.1977.tb11916.x
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发表时间：
1977-01-01
期刊：
影响因子：
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通讯作者：
MEUNIER, JC
中科院分区：
文献类型：
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作者：
BUC, J;RICARD, J;MEUNIER, JC
当温度从2-35度跃升。将C施加于小麦胚芽己糖激酶LI溶液中，酶经历了一个极其缓慢的构象变化，随后监测蛋白质色氨酸残基的荧光发射。这个过程发生在一个可检测的步骤中，虽然它涉及到多肽链的有限展开，但与酶的任何失活无关。这种缓慢构象变化的弛豫时间的倒数（.lambda.2）随着葡萄糖浓度的增加而略有增加。.lambda的图。2对葡萄糖浓度呈向下弯曲。如果反应混合物中存在G-6-P，则弛豫时间的倒数。2、随着配体浓度的增加而降低。衰减。lambda。2关于G-6-P是双曲的。这种动力学行为与酶记忆中的构象转变是相同的。从动力学研究中得到了控制酶的构象转变的速率常数值。**图形**。菱形构象（结合G-6-P的构象状态）与圆形构象（优先结合葡萄糖的构象状态）相互转化的值为10-3 s-1，而逆向速率常数k''-5的值约为5倍。10 - 6 s - 1。因此，构象的变化几乎是不可逆的；为35.度。在没有配体的情况下，99.5%的游离酶呈环状构象。对菱形构象转化为圆形构象的热力学研究表明，激活熵。这个过程的能量是高度负的（-42卡或-175焦耳/开尔文）。显然，在构象变化过程中必须达到的过渡态必须具有高度折叠的结构。这种情况清楚地区分了酶记忆的构象转变和热变性过程。
When a temperature-jump from 2-35.degree. C is applied to wheat-germ hexokinase LI in solution, the enzyme undergoes an extremely slow conformation change which was followed by monitoring the fluorescence emission of tryptophanyl residues of the protein. This process occurs in 1 detectable step and is not associated with any inactivation of the enzyme, although it involves a limited unfolding of the polypeptide chain. The reciprocal of the relaxation time of this slow conformation change (.lambda.2) slightly increases with glucose concentration. A plot of .lambda.2 against glucose concentration exhibits a downward curvature. If G-6-P is present in the reaction mixture, the reciprocal of the relaxation time, .lambda.2, decreases when the ligand concentration is increased. The decay of .lambda.2 with respect to G-6-P is hyperbolic. This kinetic behavior is identical to that expected for the conformational transition involved in enzyme memory. From this kinetic study the values of the rate constants governing the conformational transition of the enzyme were obtained. The forward rate constant .**GRAPHIC**. of the interconversion of the rhombus conformation (the conformation state that binds G-6-P) to the circle conformation (the conformation state that binds glucose preferentially) has a value of 10-3 s-1, while the backward rate constant k''-5 has a value of about 5 .times. 10-6 s-1. The conformation change is thus almost irreversible; at 35.degree. C and in the absence of any ligand, 99.5% of the free enzyme is under the circle conformation. A thermodynamic study of the conversion of the rhombus to the circle conformation has shown that the entropy of activation .**GRAPHIC**. of this process is highly negative (-42 cal or -175 J K[Joules per degree Kelvin]-1 mol-1). Apparently the transition state which has to be reached during the conformation change must have a highly folded structure. This situation clearly distinguish the conformational transition of enzyme memory from a thermal denaturation process.