The above expressions measure how much enzymes formula_42 and formula_43 control the steady state concentration of intermediate formula_44. We can also consider how the steady state reaction rates formula_40 and formula_41 are affected by perturbations in formula_42 and formula_43. This is often of importance to metabolic engineers who are interested in increasing rates of production. At steady state the reaction rates are often called the fluxes and abbreviated to formula_103 and formula_104. For a linear pathway such this example, both fluxes are equal at steady state so that the flux through the pathway is simply referred to as formula_105. Expressing the change in flux as a result of a perturbations in formula_42 and taking the limit as before we obtain: