  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.
