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首页> 外文期刊>Biophysical Chemistry: An International Journal Devoted to the Physical Chemistry of Biological Phenomena >Thermodynamics of reactions catalyzed by anthranilate synthase.
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Thermodynamics of reactions catalyzed by anthranilate synthase.

机译：邻氨基苯甲酸合酶催化反应的热力学。

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Microcalorimetry and high performance liquid chromatography have been used to conduct a thermodynamic investigation of reactions catalyzed by anthranilate synthase, the enzyme located at the first step in the biosynthetic pathway leading from chorismate to tryptophan. One of the overall biochemical reactions catalyzed by anthranilate synthase is: chorismate(aq) + ammonia(aq) = anthranilate(aq) + pyruvate(aq) + H2O(l). This reaction can be divided into two partial reactions involving the intermediate 2-amino-4-deoxyisochorismate (ADIC): chorismate(aq) + ammonia(aq) = ADIC(aq) + H2O(l) and ADIC(aq) = anthranilate(aq) + pyruvate(aq). The native anthranilate synthase and a mutant form of it that is deficient in ADIC lyase activity but has ADIC synthase activity were used to study the overall ammonia-dependent reaction and the first of the above two partial reactions, respectively. Microcalorimetric measurements were performed on the overall reaction at a temperature of 298.15 K and pH 7.79. Equilibrium measurements were performed on the first partial (ADIC synthase) reaction at temperatures ranging from 288.15 to 302.65 K, and at pH values from 7.76 to 8.08. The results of the equilibrium and calorimetric measurements were analyzed in terms of a chemical equilibrium model that accounts for the multiplicity of ionic states of the reactants and products. These calculations gave thermodynamic quantities at the temperature 298.15 K and an ionic strength of zero for chemical reference reactions involving specific ionic forms. For the reaction: chorismate2-(aq) + NH4+(aq) = anthranilate-(aq) + pyruvate-(aq) + H+(aq) + H2O(l), delta rHmo = -(116.3 +/- 5.4) kJ mol-1. For the reaction: chorismate2-(aq) + NH4+(aq) = ADIC-(aq) + H2O(l), K = (20.3 +/- 4.5) and delta rHmo = (7.5 +/- 0.6) kJ mol-1. Thermodynamic cycle calculations were used to calculate thermodynamic quantities for three additional reactions that are pertinent to this branch point of the chorismate pathway. The quantities obtained in this study permit the calculation of the position of equilibrium of these reactions as a function of temperature, pH, and ionic strength. Values of the apparent equilibrium constants and the standard transformed Gibbs energy changes delta rG'mo under approximately physiological conditions are given.
机译：微量量热法和高效液相色谱法已用于对邻氨基苯甲酸合酶催化的反应进行热力学研究，邻氨基苯甲酸合酶是从分支酸到色氨酸的生物合成途径的第一步。邻氨基苯甲酸合酶催化的总体生化反应之一是：分支酸（aq）+氨（aq）=邻氨基苯甲酸（aq）+丙酮酸（aq）+ H2O（1）。该反应可分为两个部分反应，涉及中间的2-氨基-4-脱氧异丁酸酯（ADIC）：分支酸酯（aq）+氨（aq）= ADIC（aq）+ H2O（l）和ADIC（aq）=邻氨基苯甲酸酯（ aq）+丙酮酸（aq）。天然邻氨基苯甲酸合酶及其缺乏ADIC裂解酶活性但具有ADIC合酶活性的突变形式分别用于研究总体氨依赖性反应和上述两个部分反应中的第一个反应。在总反应温度为298.15K和pH为7.79下进行微量量热法测量。在第一部分（ADIC合酶）反应中，在288.15至302.65 K的温度以及7.76至8.08的pH值下进行平衡测量。根据化学平衡模型分析了平衡和量热测量的结果，该模型考虑了反应物和产物的离子态的多样性。这些计算得出涉及特定离子形式的化学参考反应在298.15 K温度下的热力学量和零离子强度。对于该反应：分支酸2-（水溶液）+ NH4 +（水溶液）=邻氨基苯甲酸酯-（水溶液）+丙酮酸-（水溶液）+ H +（水溶液）+ H2O（1），δrHmo =-（116.3 +/- 5.4）kJ mol -1。对于该反应：分支酸2-（水溶液）+ NH4 +（水溶液）= ADIC-（水溶液）+ H2O（1），K =（20.3 +/- 4.5）和ΔrHmo=（7.5 +/- 0.6）kJ mol-1 。使用热力学循环计算来计算与分支酸盐途径的该分支点有关的另外三个反应的热力学量。在这项研究中获得的数量允许计算这些反应的平衡位置随温度，pH和离子强度的变化。给出了在近似生理条件下的表观平衡常数和标准转换的吉布斯能量变化δrG'mo的值。

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