It is shown by analysis of a numerical example that kinetic barrier diagrams [Südi (1991) Biochem. J. 276; 265-268] are also useful for displaying the phenomenological resistance of an enzymic turnover to net chemical fluxes observed under steady-state conditions. Most importantly, a new additivity rule is revealed by net flux profiles, which refers to limiting (‘ideal’) conditions. The rule states that the net fluxes that one obtains under initial steady-state conditions are strictly identical with the overall flux in the corresponding equilibrium system. This finding amounts to defining the relation between unidirectional fluxes and net fluxes, and explains why the total ‘one-way flux resistance’ to the overall reaction at chemical equilibrium on the one hand, and the ‘net flux resistance’ to both initial steady-state reactions on the other hand, have to be equal. The conclusions are claimed to be generally valid for consecutive chemical reactions.

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