Characterizing intrinsic stability of different pathway systems.

<p><b>a)</b> For different pathways, two measures of intrinsic stability are presented. First, in dark blue, is the fraction of unconstrained, random parameter sets which reach a productive steady state. Second, in light blue, is the fraction of EMRA-determined parameter sets constrained to a steady state which are also stable. The intrinsic stability of pathways differs greatly between pathways, and also depending on which measure is used. Thus, a rational method of pathway balancing would be useful. (SD < 2% for all systems, n = 3 x 1000 parameter sets). Since phoshoketolase has two activities, cleaving either F6P (called Fpk) or X5P (called Xpk), we investigated used a ratio of Fpk/Xpk activites, 1:3. <b>b)</b> A representation of how steady state is not always stable. After perturbation from a constrained steady state, the fraction of parameter sets which retain stability tends to decrease, and steady state flux may change. Eventually, a parameter set may become unstable after perturbation.