NETWORK ORGANIZATION OF CELL-METABOLISM - MONOSACCHARIDE INTERCONVERSION

The structural properties of carbohydrate metabolism are being studied . The present contribution focuses mainly on those processes involving the transfer of carbon fragments among sugars. It is shown how enzymi c activities hx the way the system self-organizes stoichiometrically a t the steady state. It is proven that there exists a specific correspo ndence between the set of all possible enzymic activities, the activit y set, and the set of stoichiometrically compatible flux distributions through the pathway. On the one hand, there are enzymic activities th at do not allow a stoichiometrically feasible coupling at the steady s tate of the reactions involved in the conversion. On the other hand, t here are enzymic activities that are related to one or more flux distr ibutions at the steady state (i.e. with one or several rate vectors re spectively). For this latter group, it can be demonstrated that the st ructure of the system depends on other non-structural factors, such as boundary constraints and the kinetic parameters. As a consequence, it is suggested that this kind of metabolic process must be viewed as a complex reaction network instead of a sequential number of steps. Some implications of these derivations are illustrated for the particular conversion CO2 --> C-3. General remarks are also discussed within the framework of network models of cell metabolism.