Modeling the role of covalent enzyme modification in Escherichia coli nitrogen metabolism. Author Philip Kidd, Ned Wingreen Publication Year 2010 Type Journal Article Abstract 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. Keywords Escherichia coli, Nitrogen, Models, Biological, Models, Chemical, Homeostasis, Glutamate-Ammonia Ligase Journal Phys Biol Volume 7 Issue 1 Pages 016006 Date Published 03/2010 Alternate Journal Phys Biol Google ScholarBibTeXEndNote X3 XML