ON THE FUNCTIONAL-ORGANIZATION IN A BIOLO GICAL STRUCTURE - THE EXAMPLE OF ENZYME ORGANIZATION

In this paper, we have considered how the spatial localization of enzy matic reactions; ranging from the elementary type (one step) to that o f 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 cellul ar substructures (organelles, membranes, cytoskeleton, multienzyme com plexes); (2) exchanges of metabolites between cellular sub-structures (local phase) and cytosol. This organization may be called as structur al. Under these conditions, we have studied the dynamical behaviour of the metabolic pathway investigating the velocity of convergence towar ds the reference steady-state after perturbation of metabolite concent rations. This type of stability may be called as functional stability. We show that an increase in exchanges by diffusion of metabolites bet ween the local phase and cytosol from one hand, or a decrease in the l ocal phase volume on the other hand, result in an increase of the func tional stability around the steady-state. This is verified for one ste p of the pathway as well as for the entire pathway or when the pathway is present in the local phase and in the cytosol.