Evaluation of enzyme systems and their regulation: the inapplicability of irreversible thermodynamics

The formalism called irreversible thermodynamics has been examined for its applicability to the description of enzymic reactions. The basic assumption of this formalism is that the net flux through a reaction is related to the free energy change, a relationship which is usually assumed to be linear. These assumptions are shown to be approximately true for the trivial case of an enzyme reaction within 0.8 kJ/mol (0.2 kcal/mol) of equilibrium in the absence of changes in any regulatory parameters (such as inhibitor, activator, or enzyme concentration). For all other reaction conditions the net flux is not related to the free energy change and in special cases for which an apparent relationship is seen, it is not linear. Thus, application of the formalism of irreversible thermodynamics gives rise to qualitatively and quantitatively erroneous results and conclusions. Since most regulatory enzymic reactions are far from equilibrium, and the net reaction rate of such reactions is regulated by changes in inhibitors, activators and/or enzyme concentration, the formalism of irreversible thermodynamics is in general neither applicable nor useful in understanding the behavior of biological reaction systems.