On the functional organization in a biological structure: the example of enzyme organization.

In this paper, we have considered how the spatial localization of enzymatic reactions, ranging from the elementary type (one step) to that of a metabolic pathway in 2 different phases, may affect the stability of metabolite concentrations. The spatial localization of molecules in the reactions involves: (1) the confinement of some enzymes to cellular substructures (organelles, membranes, cytoskeleton, multienzyme complexes); (2) exchanges of metabolites between cellular substructures (local phase) and cytosol. This organization may be called as structural. Under these conditions, we have studied the dynamical behaviour of the metabolic pathway investigating the velocity of convergence towards the reference steady-state after perturbation of metabolite concentrations. This type of stability may be called as functional stability. We show that an increase in exchanges by diffusion of metabolites between the local phase and cytosol from one hand, or a decrease in the local phase volume on the other hand, result in an increase of the functional stability around the steady-state. This is verified for one step of the pathway as well as for the entire pathway or when the pathway is present in the local phase and in the cytosol.