Flux[lower, fluxvar, upper]
Flux[lower, fluxvar, upper, fluxval:0, obj:0]
Flux is an uninstantiated function used in a reaction to indicate that it is a flux reaction, not a kinetic rate law.
The parameters have the same meaning as the corresponding parameters in COBRA flux models.
If objective coefficients are specified the model can be optimized for the fluxes using OptimizeFluxModel