Robustness in Escherichia coli Glutamate and Glutamine Synthesis Studied by a Kinetic Model
Robustness in Escherichia coli Glutamate and Glutamine Synthesis Studied by a Kinetic Model
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DOI：
10.1007/s10867-008-9109-9
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发表时间：
2008-04-01
影响因子：
1.8
通讯作者：
Melgarejo, Augusto
中科院分区：
文献类型：
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作者：
Lodeiro, Anibal;Melgarejo, Augusto
大肠杆菌对氨和氧戊二酸合成谷氨酰胺和谷氨酸的代谢控制是严格而复杂的。本研究探讨了谷氨酰胺合成酶（GS）和谷氨酸脱氢酶（GDH）调控在该控制中的作用。这两种酶形成线性途径，如果包括谷氨酸-氧戊二酸氨基转移酶（GOGAT）活性，也可以具有环状拓扑结构。我们利用耦合非线性微分方程系统模拟了线性或循环拓扑结构中的代谢途径。为了通过共价修饰模拟GS调控，我们引入了一个关系，考虑了氧戊二酸和谷氨酰胺作为信号输入的水平，以及酶腺苷化的超敏感反应。因此，通过包括或不包括这种关系，我们能够在有或没有GS调节的情况下对系统进行建模。此外，GS和GDH活性被人工改变。在两种路径拓扑下，分析了模型在不同稳态、n输入耗尽或振荡影响下的响应。我们的研究结果表明，在有GS调节的线性途径中，GDH的代谢控制系数为0.94，而在没有调节的循环途径中，GDH的代谢控制系数为0.24，默认GDH浓度为8 μ m。因此，在这些条件下，GDH似乎在线性途径中具有高度的控制作用，而在循环途径中影响有限。当GS受到调节时，系统对n输入扰动的响应更加敏感，特别是在循环途径中。此外，我们发现调节对扰动的影响取决于谷氨酰胺和谷氨酸输出一阶动力学常数的相对值，我们分别将其命名为k(6)和k(7)。调节的效果呈指数增长，因子约为2，线性增加(k（7)-k(6)）。这些趋势是持续的，但在高GS浓度下差异较小。因此，根据细胞的代谢状态，在不断变化的环境中，GS调节似乎对代谢稳定很重要。
Metabolic control of glutamine and glutamate synthesis from ammonia and oxoglutarate in Escherichia coli is tight and complex. In this work, the role of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) regulation in this control was studied. Both enzymes form a linear pathway, which can also have a cyclic topology if glutamate-oxoglutarate amino transferase (GOGAT) activity is included. We modelled the metabolic pathways in the linear or cyclic topologies using a coupled nonlinear differential equations system. To simulate GS regulation by covalent modification, we introduced a relationship that took into account the levels of oxoglutarate and glutamine as signal inputs, as well as the ultrasensitive response of enzyme adenylylation. Thus, by including this relationship or not, we were able to model the system with or without GS regulation. In addition, GS and GDH activities were changed manually. The response of the model in different stationary states, or under the influence of N-input exhaustion or oscillation, was analyzed in both pathway topologies. Our results indicate a metabolic control coefficient for GDH ranging from 0.94 in the linear pathway with GS regulation to 0.24 in the cyclic pathway without regulation, employing a default GDH concentration of 8 mu M. Thus, in these conditions, GDH seemed to have a high degree of control in the linear pathway while having limited influence in the cyclic one. When GS was regulated, system responses to N-input perturbations were more sensitive, especially in the cyclic pathway. Furthermore, we found that effects of regulation against perturbations depended on the relative values of the glutamine and glutamate output first-order kinetic constants, which we named k (6) and k (7), respectively. Effects of regulation grew exponentially with a factor around 2, with linear increases of (k(7)-k(6)). These trends were sustained but with lower differences at higher GS concentration. Hence, GS regulation seemed important for metabolic stability in a changing environment, depending on the cell's metabolic status.