Kishore N, Tewari Y B, Akers D L, Goldberg R N, Miles E W
Biotechnology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Biophys Chem. 1998 Jul 27;73(3):265-80. doi: 10.1016/s0301-4622(98)00151-3.
Microcalorimetry and high-performance liquid chromatography have been used to conduct a thermodynamic investigation of the following reactions catalyzed by the tryptophan synthase alpha 2 beta 2 complex (EC 4.2.1.20) and its subunits: indole(aq) + L-serine(aq) = L-tryptophan(aq) + H2O(1); L-serine(aq) = pyruvate(aq) + ammonia(aq); indole(aq) + D-glyceraldehyde 3-phosphate(aq) = 1-(indol-3-yl)glycerol 3-phosphate(aq); L-serine(aq) + 1-(indol-3-yl)glycerol 3-phosphate(aq) = L-tryptophan(aq) + D-glyceraldehyde 3-phosphate(aq) + H2O(1). The calorimetric measurements led to standard molar enthalpy changes for all four of these reactions. Direct measurements yielded an apparent equilibrium constant for the third reaction; equilibrium constants for the remaining three reactions were obtained by using thermochemical cycle calculations. The results of the calorimetric and equilibrium measurements were analyzed in terms of a chemical equilibrium model that accounted for the multiplicity of the ionic states of the reactants and products. Thermodynamic quantities for chemical reference reactions involving specific ionic forms have been obtained. These quantities permit the calculation of the position of equilibrium of the above four reactions as a function of temperature, pH, and ionic strength. Values of the apparent equilibrium constants and standard transformed Gibbs free energy changes delta r G'(m) degree under approximately physiological conditions are given. Le Châtelier's principle provides an explanation as to why, in the metabolic pathway leading to the synthesis of L-tryptophan, the third reaction proceeds in the direction of formation of indole and D-glyceraldehyde 3-phosphate even though the apparent equilibrium constant greatly favors the formation of 1-(indol-3-yl)glycerol 3-phosphate.
微量量热法和高效液相色谱法已被用于对色氨酸合酶α2β2复合物（EC 4.2.1.20）及其亚基催化的以下反应进行热力学研究：吲哚（水溶液）+L-丝氨酸（水溶液）=L-色氨酸（水溶液）+H2O（液）；L-丝氨酸（水溶液）=丙酮酸（水溶液）+氨（水溶液）；吲哚（水溶液）+D-甘油醛3-磷酸（水溶液）=1-（吲哚-3-基）甘油3-磷酸（水溶液）；L-丝氨酸（水溶液）+1-（吲哚-3-基）甘油3-磷酸（水溶液）=L-色氨酸（水溶液）+D-甘油醛3-磷酸（水溶液）+H2O（液）。量热测量得出了这四个反应的标准摩尔焓变。直接测量得到了第三个反应的表观平衡常数；其余三个反应的平衡常数通过热化学循环计算获得。根据一个考虑了反应物和产物离子状态多样性的化学平衡模型，对量热和平衡测量结果进行了分析。获得了涉及特定离子形式的化学参考反应的热力学量。这些量允许计算上述四个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化吉布斯自由能变化ΔrG'(m)°的值。勒夏特列原理解释了为什么在导致L-色氨酸合成的代谢途径中，尽管表观平衡常数极大地有利于1-（吲哚-3-基）甘油3-磷酸的形成，但第三个反应仍朝着吲哚和D-甘油醛3-磷酸形成的方向进行。
