Thermodynamic validity criterion for the irreversible Michaelis-Menten equation.

Enzyme kinetics is very often characterised by the irreversible Michaelis-Menten (MM) equation. However, in open chemical reaction networks such as metabolic pathways, this approach can lead to significant kinetic and thermodynamic inconsistencies. Based on recent developments in nonequilibrium chemical thermodynamics, we present a validity criterion solely expressed in terms of the equilibrium constant of the enzyme-catalysed reaction. When satisfied, it guarantees the ability of the irreversible MM equation to generate kinetic and thermodynamic data that are quantitatively reliable for reasonable ranges of concentrations. Our validity criterion is thus a precious tool to ensure reliable kinetic and thermodynamic modelling of pathways. We also show that it correctly identifies the so-called irreversible enzymatic reactions in glycolysis and Krebs cycle.