This is an three-step enzymatic model, consisting of three reactions with each reaction controlled by an enzyme. This is an open system so that flux J3 represents either substrate S2 being consumed or leaving the system where these reactions take place.
The three reaction fluxes are given by:
J1: Xo -> S1: e1*(k1*Xo - k2*S1)
J2: S1 -> S2: e2*(k3*S1 - k4*S2)
J3: S2 -> : e3* k5*S2
Where e1, e2, e3 represent enzyme concentrations, k1, k2, k3, k4, k4 are reaction rate constants and S1, S2 are substrate concentrations. Xo represents a boundary species which feeds into the system at a fixed concentration.