Robustness in Escherichia coli glutamate and glutamine synthesis studied by a kinetic model.

LODEIRO AR; MELGAREJO AA

Montevideo, Uruguay

Congreso; 6th International Conference of Biological Physics.; 2007

International Union of Pure and Applied Physics

In this work the role of glutamine synthetase (GS) regulation and glutamate dehydrogenase (GDH) activity on metabolic control of glutamine and glutamate synthesis from ammonia and oxoglutarate in Escherichia coli was studied. Both enzymes form a linear pathway, which can have also 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 non-linear differential equations system. To simulate GS regulation by covalent modification, we introduced a relatioship that took into account the levels of oxoglutarate and glutamine as signal inputs, as well as the ultrasensitive response of enzyme adenylylation. Thus, by includiing this relationship or not we were able to model the system with or without GS regulation. On the other hand, GDH activity was changed manually. The response of the model in different stationary states, or under the influence of input exhaustion or oscillation, was analyzed in both topologies. Our results indicate a metabolic control coefficient for GDH ranging from 0.40 in the linear topology without GS regulation, to less than 0.08 in all other situiations, with a default free GDH concentration of  4 uM. Thus, in these conditions GDH seemed to have limited influence on metabolic control of this pathway, being cyclic topology the more robust. When GS was regulated, system robustness was higher in both topologies and against all perturbations. 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 k6 and k7, respectively. Effects of regulation were markedly observed only when k6 < k7. Hence, GS regulation seemed important for metabolic stability in a changing environment, depending on the cell metabolic regime.