Equilibrium enzymes in metabolic pathways.

It is commonly believed that certain reactions in a metabolic sequence may be at or close to equilibrium because of the large excess of catalytic capacity compared to the flux through these enzyme loci. Simple algebraic manipulations can show that the equilibrium and steady state conditions are mutually exclusive. However, solution of the complete reaction schemes for model "equilibrium" reactions shows that they can remain far from equilibrium even though the ratio of enzyme flux to steady state flux through the overall pathway is high. These calculations show that a reaction's proximity to equilibrium depends on the overall flux through the enzyme locus as well as on the kinetic parameters of the other enzymes in the pathway. Thus, combinations of kinetic parameters may exist that allow certain reactions to approach equilibrium but these conditions are not universal.