Systems Biology - Methods - Fluxomics
From Keaslingwiki
A metabolic flux is the number of moles of a particular metabolite that are consumed or produced via a particular reaction per unit cell mass per unit time. The basis of the mathematical representation is a series of simultaneous steady-state mole balances on cellular metabolites. This set of balances can be represented in matrix notation: S . v =b
Where S is the stoichiometric matrix containing the stoichiometric coefficients for reactions in the network. The vector v contains the unknown intracellular fluxes for each reaction (mol vol-1 time-1), and b is the vector of net rates of formation of metabolites (mol vol-1 time-1). The system is assumed to be at steady state, implying that there is no net accumulation of intracellular metabolites, and therefore the entries of b are zero for intracellular metabolites. Writing a balance equation for each metabolite in the network results is a system of n equations (number of metabolites) in m unknowns (fluxes). In any branched metabolic network, the number of fluxes is greater than the number of metabolites, which leaves us with an underdetermined system. Therefore instead of calculating an exact solution, we can only determine the possible set of phenotypic solutions in which the cell can operate. Isotopic label tracing is a powerful experimental technique that can be combined with stoichiometric analysis, providing the additional information required to quantify metabolic fluxes in underdetermined systems. The calculation of intracellular fluxes, using isotopomer analysis is based on the fact that when cells are fed a growth substrate with certain carbon positions labeled with 13C, the distribution of this label in the intracellular metabolites can precisely be determined, based on the known biochemistry of the pathways. Furthermore, due to the high connectivity of metabolic networks, this distribution will depend strongly upon the values of the intracellular fluxes. By measuring the label distribution in the metabolites, these fluxes can therefore be calculated exactly.
