Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA.
Phys Biol. 2010 Jan 7;7(1):016006. doi: 10.1088/1478-3975/55/1/016006.
In the bacterium Escherichia coli, the enzyme glutamine synthetase (GS) converts ammonium into the amino acid glutamine. GS is principally active when the cell is experiencing nitrogen limitation, and its activity is regulated by a bicyclic covalent modification cascade. The advantages of this bicyclic-cascade architecture are poorly understood. We analyze a simple model of the GS cascade in comparison to other regulatory schemes and conclude that the bicyclic cascade is suboptimal for maintaining metabolic homeostasis of the free glutamine pool. Instead, we argue that the lag inherent in the covalent modification of GS slows the response to an ammonium shock and thereby allows GS to transiently detoxify the cell, while maintaining homeostasis over longer times.
在细菌大肠杆菌中，酶谷氨酰胺合成酶（GS）将铵转化为氨基酸谷氨酰胺。当细胞受到氮限制时，GS 主要活跃，其活性受双环共价修饰级联调节。这种双环级联结构的优点理解得还不够透彻。我们分析了 GS 级联的一个简单模型与其他调节方案相比，并得出结论，双环级联对于维持游离谷氨酰胺池的代谢稳态是次优的。相反，我们认为 GS 的共价修饰所固有的滞后会减缓对铵冲击的反应，从而使 GS 能够在更长的时间内暂时解毒细胞，同时维持稳态。
