Homeostatic regulation upon changes of enzyme activities in the Calvin cycle as an example for general mechanisms of flux control. What can we expectfrom transgenic plants?

In order to explain the mechanisms of Calvin-cycle regulation, 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 sy stem can be the value of a sharp transition from some constant homeostatic flux to a limitation by this enzyme. A special method for the quantitative treatment of the experimental dependence of a metabolic flux such as photos ynthesis on enzyme content is developed. It is pointed out that reactions c lose to a thermodynamic equilibrium under normal conditions can considerabl y limit the homeostatic fluxes with a decrease of the enzyme content. Calvi n-cycle enzymes are classified as non-limiting, near-limiting and limiting. The deduced rules for the regulation of a homeostatic metabolic pathway ar e used to explain the data obtained for transgenic plants with reduced acti vities of Calvin-cycle enzymes. The role of compensating mechanisms that ma intain the photosynthesis rate constant upon the changes of enzyme contents is analyzed for the Calvin cycle. The developed analysis explains the shar p transitions between limiting and non-limiting conditions that can be seen in transgenic plants with reduced content of some Calvin-cycle enzymes, an d the limiting role of such reversible enzymes as aldolase, transketolase a nd others. The attempt is made to predict the properties of plants with inc reased enzyme contents in the Calvin cycle.