Regulation in metabolic systems under homeostatic flux control

The general properties of metabolic systems under homeostatic flux control are analyzed. It is shown that the main characteristic point for an enzyme in such a system is a sharp transition from limitation outside the system t o limitation by some enzyme inside the system. A method for the quantitativ e treatment of the experimental dependence of metabolic flux on enzyme cont ent is presented. The conception of "nonlimiting," "near-limiting," and "li miting" enzymes is developed for these systems. It is pointed out that reac tions close to a thermodynamic equilibrium under normal conditions can cons iderably limit the homeostatic fluxes, The rules for regulation of fluxes i n such systems are illustrated by the data obtained for transgenic plants w ith reduced activities of some Calvin-cycle enzymes and further examples. A comparison is made between the developed quantitative description of metab olic fluxes under homeostatic flux control and the methods of metabolic con trol analysis. (C) 2000 Academic Press.