text: Existing metabolic networks control the movement of molecules through their enzymatic steps by regulating enzymes that catalyze irreversible reactions.  The movement of molecules through reversible steps is generally unregulated by enzymes, but rather regulated by the concentration of products and reactants. Irreversible reactions at regulated steps of a pathway have a negative free energy change, thereby promoting spontaneous reactions in one direction only.  Reversible reactions have no or very small free energy change.  As a result, the movement of molecules through a metabolic network is governed by simple chemical equilibria (at reversible steps), with specific key enzymes that are subject to regulation (at irreversible steps). This enzymatic regulation may be indirect, in the case of an enzyme being regulated by some cell signalling mechanism (like phosphorylation), or it may be direct, as in the case of allosteric regulation, where metabolites from a different portion of a metabolic network bind directly to and affect the catalytic function of other enzymes in order to maintain homeostasis.
  A result that may seem at first counter intuitive, is that regulated steps tends to have small flux control coefficients. The reason is that these steps are part of a control system that stabilizes fluxes, hence a perturbation in the activity of a regulated step will inevitably trigger the control system to resist the perturbation, hence the flux control coefficients will tend to be small. This explains why, for example, that phosphofructokinase in glycolysis has such as small flux control coefficient.